Inquiry Activity

If you look closely at the top of what appears to be a leaf in the center of this photograph, you can see a head. This walkingleaf insect is a superb example of camouflage.

Inquiry Activity

Do lima beans show variation?

Procedure ~ 1. Count out 1 0 lima beans and measure the length of

each in millimeters. Record your results in a data table.

2. Combine your data with the data of two other classmates. Place all the data on one graph. Plot the length on the x-axis and the number of beans of each length on the y-axis.

Think About It 1. Analyzing Data Calculate the average length of

the beans. Are most lima beans close to the average length?

2. Predicting How do you think a graph of data from the entire class would be different from your graph of data?

15-1 The Puzzle of Life's Diversity

Nature presents scientists with a puzzle. Humans share the Earth with millions of other kinds of organisms of every

imaginable shape, size, and habitat. This variety ofliving things is called biological diversity. How did all these different organisms arise? How are they related? These questions make up the puzzle oflife's diversity.

What scientific explanation can account for the diversity of life? The answer is a collection of scientific facts, observations, and hypotheses known as evolutionary theory. Evolution, or change over time, is the process by which modern organisms have descended from ancient organisms. A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

Voyage of the Beagle The individual who contributed more to our understanding of evolution than anyone was Charles Darwin. Darwin was born in England on February 12, 1809-the same day as Abraham Lincoln. Shortly after completing his college studies, Darwin joined the crew of the H.M.S. Beagle. In 1831, he set sail from England for a voyage around the world. His route is shown in Figure 15-1. Although no one knew it at the time, this was to be one of the most important voyages in the history of science.

During his travels, Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time. That hypothesis, now supported by a huge body of evidence, has become the theory of evolution.

Galapagos Islands

Guide for Reading ..............................

~ Key Concepts • What was Charles Darwin's

contribution to science? • What pattern did Darwin

observe among organisms of the Galapagos Islands?

Vocabulary evolution theory fossil

Reading Strategy: Using Visuals Before you read, examine Figure 15-1. Find the British Isles, where Darwin's journey began, then trace his route. Write a statement describing his travels.

'Y Figure 15-1 On a five-year voyage on the Beagle, Charles Darwin visited several continents and many remote islands. ~ Darwin's observations led to a revolutionary theory about the way life changes over time.

Pacific Ocean

H.M.S. Beagle

Darwin's Theo1~y of Evolution 369

Figure 15-2 Many of the fossils that Darwin discovered resembled living organisms but were not identical to them. The glyptodon, an extinct animal known only from fossil remains, is an ancient relative of the armadillo of South America. Comparing and Contrasting What are some similarities and differences between these two types of animals?

370 Chapter 15

Wherever the ship anchored, Darwin went ashore to collect plant and animal specimens that he added to an ever-growing collection. At sea, he studied his specimens, read the latest scientific books, and filled many notebooks with his observations and thoughts. Darwin was well educated and had a strong interest in natural history. His curiosity and analytical nature were ultimately the keys to his success as a scientist. During his travels, Darwin came to view every new finding as a piece in an extraordinary puzzle: a scientific explanation for the diversity of life on this planet.

Darwin's Observations Darwin knew a great deal about the plants and animals of his native country. But he saw far more diversity during his travels. For example, during a single day in a Brazilian forest, Darwin collected 68 different beetle species-despite the fact that he was not even searching for beetles! He began to realize that an enormous number of species inhabit the Earth.

Patterns of Diversity Darwin was intrigued by the fact that so many plants and animals seemed remarkably well suited to whatever environment they inhabited. He was impressed by the many ways in which organisms survived and produced offspring. He wondered if there was some process that led to such a variety of ways of reproducing.

Darwin was also puzzled by where different species livedand did not live. He visited Argentina and Australia, for example, which had similar grassland ecosystems. Yet, those grasslands were inhabited by very different animals. Also, neither Argentina nor Australia was home to the sorts of animals that lived in European grasslands. For Darwin, these patterns posed challenging questions. Why were there no rabbits in Australia, despite the presence of habitats that seemed perfect for them? Similarly, why were there no kangaroos in England?

Pinta

Marchena . • Tower

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• Santa Fe

Floreana.

Living Organisms and Fossils Darwin soon realized that living animals represented just part of the puzzle posed by the natural world. In many places during his voyage, Darwin collected the preserved remains of ancient organisms, called fossils. Some of those fossils resembled organisms that were still alive, as shown in Figure 15-2. Others looked completely unlike any creature he had ever seen. As Darwin studied fossils, new questions arose. Why had so many of these species disappeared? How were they related to living species?

The Galapagos Islands Of all the Beagle's ports of call, the one that influenced Darwin the most was a group of small islands located 1000 km west of South America. These are the Galapagos Islands. Darwin noted that although they were close together, the islands had very different climates. The smallest, lowest islands were hot, dry, and nearly barren. Hood Island, for example, had sparse vegetation. The higher islands had greater rainfall and a different assortment of plants and animals. Isabela Island had rich vegetation.

Darwin was fascinated in particular by the land tortoises and marine iguanas in the Galapagos. He learned that the giant tortoises varied in predictable ways from one island to another, as shown in Figure 15-3. The shape of a tortoise's shell could be used to identify which island a particular tortoise inhabited. Darwin later admitted in his notes that he "did not for some time pay sufficient attention to this statement."

CHECKPOINT How did the fossils Darwin observed compare with the living organisms he studied?

Hood

A Figure 15-3 <> Darwin observed that the characteristics of many animals and plants varied noticeably among the different Galapagos Islands. Among the tortoises, the shape of the shell corresponds to different habitats. The Hood Island tortoise (right) has a long neck and a shell that is curved and open around the neck and legs, allowing the tortoise to reach the sparse vegetation on Hood Island. The tortoise from lsabela Island (lower left) has a dome-shaped shell and a shorter neck. Vegetation on this island is more abundant and closer to the ground. The tortoise from Pinta Island has a shell that is intermediate between these two forms.

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Darwin's Theory of Evolution 371

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.& Figure 15-4 Darwin's notebooks and some of the finch specimens he collected have been preserved for today's scientists to study. Inferring What might modern scientists learn from examining evidence collected by earlier investigators?

Darwin also saw several types of small, ordinarylooking brown birds hopping around, looking for seeds. As an eager naturalist, he collected many specimens, several of which are shown in Figure 15-4. However, he did not find them particularly unusual or important. As Darwin examined the birds, he noted that they had differently shaped beaks. He thought that some of the birds were wrens, some were warblers, and some were blackbirds. But he came to no other conclusions-at first .

The Journey Home While heading home, Darwin spent a great deal of time thinking about his findings. Examining different mockingbirds from the Galapagos, Darwin noticed that individual birds collected from the island of Floreana looked different from those collected on James Island. They also looked different from individuals collected on other islands. Darwin also remembered that the tortoises differed from island to island. Although Darwin did not immediately understand the reason for these patterns of diversity, he had stumbled across an important finding. ~Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galapagos. After returning to England, Darwin began to wonder if animals living on different islands had once been members of the same species. According to this hypothesis, these separate species would have evolved from an original South American ancestor species after becoming isolated from one another. Was this possible? If so, it would turn people's view of the natural world upside down.

1. Key Concept What did 4. What is a fossil?

Biotic and Abiotic Factors In Chapter 51 you learned that both biotic and abiotic factors affect ecosystems. Distinguish between these two factors, give some examples of each, and explain how they might have affected the tortoises that Darwin observed on the Galapagos Islands.

Darwin's travels reveal to him about the number and variety of living species?

2. ~Key Concept How did tortoises and birds differ among the islands of the Galapagos?

3. What is evolution? Why is evolution referred to as a theory?

372 Chapter 15

5. Critical Thinking Inferring Darwin found fossils of many organisms that were different from any living species. How would this finding have affected his understanding of life's diversity?

15-2 Ideas That Shaped Darwin's Thinking

I f Darwin had lived a century earlier, he might have done little more than think about the questions rai ed during his trav

els. But Darwin's voyage came during one of the most exciting periods in the history of Western science. Explorers were traversing the globe, and great thinkers were beginning to challenge established views about the natural world. Darwin was powerfully influenced by the work of these scientists, especially those who were studying the history of Earth. In turn, he himself greatly changed the thinking of many scientists and nonscientists. Some people, however, found Darwin's ideas too shocking to accept. To understand how radical Darwin's thoughts appeared, you must understand a few things about the world in which he lived.

Most Europeans in Darwin's day believed that the Earth and all its forms oflife had been created only a few thousand years ago. Since that original creation, they concluded, neither the planet nor its living species had changed. A robin, for example, has always looked and behaved as robins had in the past. Rocks and major geological features were thought to have been produced suddenly by catastrophic events that humans rarely, if ever, witnessed.

By the time Darwin set sail, numerous discoveries had turned up important pieces of evidence. A rich fossil record, including the example in Figure 15-5, was challenging that traditional view oflife. In light of such evidence, some scientists even adjusted their beliefs to include not one but several periods of creation. Each of these periods, they contended, was preceded by a catastrophic event that killed off many forms of life. At first, Darwin may have accepted these beliefs. But he began to realize that much of what he had observed did not fit neatly into this view of unchanging life. Slowly, after studying many scientific theories of his time, Darwin began to change his thinking dramatically.

.... Figure 15-5 This engraving, made around 1850, shows the fossil remains of a giant sloth from South America. During the 1800s, explorers were finding the remains of numerous animal types that had no living representatives. Inferring What did such fossil evidence indicate about life in the past?

Gulde for ·Reading

Key Concepts • How did Hu tton and Lyell

describe geological change? • According to Lamarck, how

did species evolve? • What was Malthus's theory of

population growth?

Reading Strategy: Finding Main Ideas As you read about the individuals who influenced Darwin's thinking, write a sentence briefly describing what Darwin learned from each one.


£ Figure 15-6 These huge rocks, which are composed of sandstone, show distinct layers that were laid down over millions of years. ~ Hutton and Lyell cited geological features such as these rocks as evidence that Earth is many millions of years old.

An Ancient, Changing Earth During the eighteenth and nineteenth centuries, scientists examined Earth in great detail. They gathered information suggesting that Earth was very old and had changed slowly over time. Two scientists who formed important theories based on this evidence were James Hutton and Charles Lyell.

Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present.

Hutton and Geological Change In 1795, the geologist James Hutton published a detailed

hypothesis about the geological forces that have shaped Earth. Hutton proposed that layers of rock, such as those shown in Figure 15-6, form very slowly. Also, some rocks are moved up by forces beneath Earth's surface. Others are buried, and still others are pushed up from the sea floor to form mountain ranges. The resulting rocks, mountains, and valleys are then shaped by a variety of natural forces-including rain, wind, heat, and cold temperatures. Most of these geological processes operate extremely slowly, often over millions of years. Hutton, therefore, proposed that Earth had to be much more than a few thousand years old.

Origins of Evolutionary Thought The groundwork for the modern theory of evolution was laid during the 7 700s and 7 BOOs. Charles Darwin developed the central idea of evolution by natural selection, but others before and during his time also built essential parts of the theory.

1785 James Hutton Hutton proposes that Earth is shaped by geological forces that took plC!ce over extremely long periods of time. He estimates Earth to be millions-not . thousands-of years old.

1798 Thomas Malthus In his Essay on the Principle of Population, Malthus predicts that the human population will grow faster than the space and food supplies needed to sustain it.

1809 Jean-Baptiste Lamarck Lamarck publishes his hypotheses of the inheritance of acquired traits. The ideas are flawed, but he is one of the first to propose a mechanism explaining how organisms change over time.

'?.----~· ---- --, '<: ---------~--

.. - ·----374 Chapter 15

Lyell's Principles of Geology Just before the Beagle set sail, Darwin had been given the first volume of geologist Charles Lyell's book Principles of Geology. Lyell stressed that scientists must explain past events in terms of processes that they can actually observe, since processes that shaped the Earth millions of years earlier continue in the present. Volcanoes release hot lava and gases now, just as they did on an ancient Earth. Erosion continues to carve out canyons, just as it did in the past.

Lyell's work explained how awesome geological features could be built up or torn down over long periods of time. Lyell helped Darwin appreciate the significance of geological phenomena that he had observed. Darwin had witnessed a spectacular volcanic eruption. Darwin wrote about an earthquake that had lifted a stretch ofrocky shoreline-with mussels and other animals attached to it-more than 3 meters above its previous position. He noted that fossils of marine animals were displaced many feet above sea level. Darwin then understood how geological processes could have raised these rocks from the sea floor to a mountaintop.

This understanding of geology influenced Darwin in two ways. First, Darwin asked himself: If the Earth could change over time, might life change as well? Second, he realized that it would have taken many, many years for life to change in the way he suggested. This would have been possible only if the Earth were extremely old.

CHECKPOINT What are some ways the Earth has changed over time?

1831 Charles Darwin Darwin sets sail on the H.M.S. Beagle, a voyage that will provide him with vast amounts of evidence leading to his theory of evolution.

1833 Charles Lyell In the second and final volume of Principles of Geology, Lyell explains that processes occurring now have shaped Earth's geological features over long periods of time.

1858 Alfred Wallace Wallace writes to Darwin, speculating on evolution by natural selection, based on his studies of the distribution of plants and animals.

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Use the library or the Internet to find out more about Darwin and Wallace. Write a dialogue between these two men, where the conversation shows the similarities in their careers and theories.

1859 Darwin publishes On the Origin of Species.

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..A. Figure 15-7 ~Lamarck proposed that the selective use or disuse of an organ led to a change in that organ that was then passed on to offspring. This proposed mechanism is shown here applied to fiddler crabs. (1) The male crab uses its small front claw to attract mates and ward off predators. (2) Because the front claw has been used repeatedly, it becomes larger. (3) The acquired characteristic, a larger claw, is then passed on to the crab's offspring. Lamarck's explanation, proposed in 1809, was found to be incorrect.

376 Chapter 15

Lamarck's Evolution Hypotheses The French naturalist Jean-Baptiste Lamarck was among the first scientists to recognize that living things have changed over time-and that all species were descended from other species. He also realized that organisms were somehow adapted to their environments. In 1809, the year that Darwin was born, Lamarck published his hypotheses. ~Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. Over time, this process led to change in a species.

Tendency Toward Perfection Lamarck proposed that all organisms have an innate tendency toward complexity and perfection. As a result, they are continually changing and acquiring features that help them live more successfully in their environments. In Lamarck's view, for instance, the ancestors of birds acquired an urge to fly. Over many generations, birds kept trying to fly, and their wings increased in size and became more suited to flying.

Use and Disuse Because of this tendency toward perfection, Lamarck proposed that organisms could alter the size or shape of particular organs by using their bodies in new ways. For example, by trying to use their front limbs for flying, birds could eventually transform those limbs into wings. Conversely, if a winged animal did not use its wings-an example of disuse-the wings would decrease in size over generations and finally disappear.

Inheritance of Acquired Traits Like many biologists of his time, Lamarck thought that acquired characteristics could be inherited. For example, if during its lifetime an animal somehow altered a body structure, leading to longer legs or fluffier feathers, it would pass that change on to its offspring. By this reasoning, if you spent much of your life lifting weights to build muscles, your children would inherit big muscles, too.

Evaluating Lamarck's Hypotheses Lamarck's hypotheses of evolution, illustrated in Figure 15-7, are incorrect in several ways. Lamarck, like Darwin, did not know how traits are inherited. He did not know that an organism's behavior has no effect on its heritable characteristics. However, Lamarck was one of the first to develop a scientific hypothesis of evolution and to realize that organisms are adapted to their environments. He paved the way for the work oflater biologists.

Population Growth Another important influence on Darwin came from the English economist Thomas Malthus. In 1798, Malthus published a book in which he noted that babies were being born faster than people were dying. Malthus reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone. The only forces he observed that worked against this growth were war, famine, and disease. Conditions in certain parts of nineteenth-century England, illustrated in Figure 15-8, reinforced Malthus's somewhat pessimistic view of the human condition.

When Darwin read Malthus's work, he realized that this reasoning applied even more strongly to plants and animals than it did to humans. Why? Because humans produce far fewer offspring than most other species do. A mature maple tree can produce thousands of seeds in a single summer, and one oyster can produce millions of eggs each year. If all the offspring of almost any species survived for several generations, they would overrun the world.

Obviously, this has not happened, because continents are not covered with maple trees, and oceans are not filled with oysters. The overwhelming majority of a species' offspring die. Further, only a few of those offspring that survive succeed in reproducing. What causes the death of so many individuals? What factor or factors determine which ones survive and reproduce, and which do not? Answers to these questions became central to Darwin's explanation of evolutionary change.

.... Figure 15-8 ~Malthus reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient food and living space for everyone. He supported his theory with the evidence he observed in the streets of London.

1. · Key Concept What two ideas from geology were important to Darwin's thinking?

2. Key Concept According to Lamarck, how did organisms acquire traits?

3. Key Concept According to Malthus, what factors limited population growth?

4. How did Lyell's Principles of Geology influence Darwin?

5. Critical Thinking Evaluating Evaluate the strengths and weaknesses of Lamarck's hypotheses of evolution. How did they contribute to scientific thought? Why have they been rejected?

Creative Writing Imagine that you are Thomas Malthus. Write an article that would appear in a newspaper of the time that explains your ideas. Explain the impact of a growing population on society and the environment.


15-3 Darwin Presents His Case

Guide for Reading llf'.l----------------K e y Concepts • How is natural variation used

in artificial selection? • How is natural selection

related to a species' fitness? • What evidence of evolution

did Darwin present?

Vocabulary artificial selection struggle for existence fitness adaptation survival of the fittest natural selection descent with modification common descent homologous structure vestigial organ

Reading Strategy: Building Vocabulary As you read, write a phrase or sentence in your own words to define each highlighted, boldface term.

378 Chapter 15

llThen Darwin retw·ned to England in 1836, he brought back V V specimens from around the world. Subsequent findings

about these specimens soon had the scientific community abuzz. Darwin learned that his Galapagos mockingbirds actually belonged to three separate species found nowhere else in the world! Even more surprising, the brown birds that Darwin had thought to be wrens, warblers, and blackbirds were all finches. They, too, were found nowhere else. The same was true of the Galapagos tortoises, the marine iguanas, and many plants that Darwin had collected on the islands. Each island species looked a great deal like a similar species on the South American mainland. Yet, the island species were clearly different from the mainland species and from one another.

Publication of On the Origin of Species Darwin began filling notebooks with his ideas about species diversity and the process that would later be called evolution. However, he did not rush out to publish his thoughts. Recall that Darwin's ideas challenged fundamental scientific beliefs of his day. Darwin was not only stunned by his discoveries, he was disturbed by them. Years later, he wrote, "It was evident that such facts as these ... could be explained on the supposition that species gradually became modified, and the subject haunted me." Although he discussed his work with friends, he shelved his manuscript for years and told his wife to publish it in case he died.

In 1858, Darwin received a short essay from Alfred Russel Wallace, a fellow naturalist who had been doing field work in Malaysia. That essay summarized the thoughts on evolutionary change that Darwin had been mulling over for almost 25 years! Suddenly, Darwin had an incentive to publish his own work. At a scientific meeting later that year, Wallace's essay was presented together with some of Darwin's work.

~ Figure 15-9 Each zebra inherits genes that give it a distinctive pattern of stripes. Those visibly different patterns are an example of natural variation in a species. Formulating Hypotheses What might be some genetic variations that are not visible?

Eighteen months later, in 1859, Darwin published the results of his work, On the Origin of Species. In his book, he proposed a mechanism for evolution that he called natural selection. He then presented evidence that evolution has been taking place for millions of years-and continues in all living things. Darwin's work caused a sensation. Many people considered his arguments to be brilliant, while others strongly opposed his message. But what did Darwin actually say?

What event motivated Darwin to publish his ideas?

Inherited Variation and Artificial Selection One of Darwin's most important insights was that members of each species vary from one another in important ways. Observations during his travels and conversations with plant and animal breeders convinced him that variation existed both in nature and on farms. For example, some plants in a species bear larger fruit than others. Some cows give more milk than others. From breeders, Darwin learned that some of this was heritable variation-differences that are passed from parents to offspring. Darwin had no idea of how heredity worked. Today, we know that heritable variation in organisms is caused by variations in their genes. We also know that genetic variation is found in wild species as well as in domesticated plants and animals.

Darwin argued that this variation mattered. This was a revolutionary idea, because in Darwin's day, variations were thought to be unimportant, minor defocts. But Darwin noted that plant and animal breeders used heritable variation-what we now call genetic variation-to improve crops and livestock. They would select for breeding only the largest hogs, the fastest horses, or the cows that produced the most milk. Darwin termed this process artificial selection. In artificial selection, nature provided the variation, and humans selected those variations that they found useful. Artificial selection has produced many diverse domestic animals and crop plants, including the plants shown in Figure 15-10, by selectively breeding for different traits.

~ Figure 15-10 ~ In artificial selection, humans select from among the naturally occurring genetic variations in a species. From a single ancestral plant, breeders selecting for enlarged flower buds, leaf buds, leaves, or stems have produced all these plants.

Brussels sprouts

Quick Lab

New vegetables from old? Materials various Brassico (cabbage family) vegetables

Procedure ~ Examine each of the vegetables and compare them. Determine which organ of the ancestral plant breeders may have chosen to produce each vegetable.

Analyze and Conclude Formulating Hypotheses Choose one of the vegetables. Explain how breeders might have produced that variety from the ancestral plant, shown below.

species


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T Figure 15-11 Survival of the fittest can take many different forms. For one species, it may be an ability to run fast, whereas for another species, it may be behavioral tactics that it uses to outsmart predators. For the porcupine, sharp quills make a powerful, hungry predator back away from an attack. Inferring What other types of characteristics might increase chances of survival?

Evolution by Natural Selection Darwin's next insight was to compare processes in nature to artificial selection. By doing so, he developed a scientific hypothesis to explain how evolution occurs. This is where Darwin made his greatest contribution-and his strongest break with the past.

The Struggle for Existence Darwin was convinced that a process like artificial selection worked in nature. But how? He recalled Malthus's work on population growth. Darwin realized that high birth rates and a shortage of life's basic needs would eventually force organisms into a competition for resources. The struggle for existence means that members of each species compete regularly to obtain food, living space, and other necessities of life. In this struggle, the predators that are faster or have a particular way of ensnaring other organisms can catch more prey. Those prey that are faster, better camouflaged, or better protected, such as the porcupine shown in Figure 15-11, can avoid being caught. This struggle for existence was central to Darwin's theory of evolution.

Survival of the Fittest A key factor in the struggle for existence, Darwin observed, was how well suited an organism is to its environment. Darwin called the ability of an individual to survive and reproduce in its specific environment fitness. Darwin proposed that fitness is the result of adaptations. An adaptation is any inherited characteristic that increases an organism's chance of survival. Successful adaptations, Darwin concluded, enable organisms to become better suited to their environment and thus better able to survive and reproduce. Adaptations can be anatomical, or structural, characteristics, such as a porcupine's sharp quills. Adaptations also include an organism's physiological processes, or functions, such as the way in which a plant performs photosynthesis. More complex features, such as behavior in which some animals live and hunt in groups, can also be adaptations.

The concept of fitness, Darwin argued, was central to the process of evolution by natural selection. Generation after generation, individuals compete to survive and produce offspring. The baby birds in Figure lS-12, for example, compete for food and space while in the nest. Because each individual differs from other members of its species, each has unique advantages and disadvantages. Individuals with characteristics that are not well suited to their environment-that is, with low levels of fitness-either die or leave few offspring. Individuals that are better suited to their environment-that is, with adaptations that enable fitness-survive and reproduce most successfully. Darwin called this process surviVal of the fittest.

Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as natural 'selection. In both artificial selection and natural selection, only certain individuals of a population produce new individuals. However, in natural selection, the traits being selected-and therefore increasing over time-contribute to an organism's fitness in its environment. Natural selection also takes place without human control or direction. · Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species' fitness in its environment. Natural selection cannot be seen directly; it can only be observed as changes in a population over many successive generations.

CHECKPOINT What did Darwin mean when he described certain organisms as "more fit" than others?

Descent With Modification Darwin proposed that over long periods, natural selection produces organisms that have different structures, establish different niches, or occupy different habitats. As a result, species today look different from their ancestors. Each living species has descended, with changes, from other species over time. He referred to this principle as descent with modification.

~ Figure 1 S-12 Each of these baby tanagers has its own set of inherited traits that affect its survival. A stronger bird may take food from a weaker sibling. A faster bird may escape predators more easily. Only those birds that survive and reproduce have the chance to pass their traits to the next generation. Over time, natural selection results in changes in the inherited characteristics of a population.

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Darwin's Tfleory of Evolution 381

T Figure 15-13 ~Darwin argued that the fossil record provided evidence that living things have been evolving for millions of years. Often, the fossil record includes a variety of different extinct organisms that are related to one another and to living species. The four fossil organisms shown here are cephalopods, a group that includes squid, octopi, and the chambered nautilus. The fossil record contains more than 7500 species of cephalopods, which vary, as these fossils show, from species with short, straight shells, to species with longer, coiled shells. Darwin and his colleagues noticed that the sizes, shapes, and varieties of related organisms preserved in the fossil record changed over time.

382 Chapter 15

Descent with modification also implies that all living organisms are related to one another. Look back in time, and you will find common ancestors shared by tigers, panthers, and cheetahs. Look farther back, and you will find ancestors that these felines share with horses, dogs, and bats. Farther back still are the common ancestors of mammals, birds, alligators, and fishes. If we look far enough back, the logic concludes, we could find the common ancestors of all living things. This is the principle known as common descent. According to this principle, all species-living and extinct-were derived from common ancestors. Therefore, a single "tree of life" links all living things.

Evidence of Evolution With this unified, dynamic theory of life, Darwin could finally explain many of the observations he had made during his travels aboard the Beagle. ~ Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, and similarities in early development, or embryology.

The Fossil Record By Darwin's time, scientists knew that fossils were the remains of ancient life, and that different layers ofrock had been formed at different times during Earth's history. Darwin saw fossils as a record of the history of life on Earth. Darwin, like Lyell, proposed that Earth was many millions-rather than thousands-of years old. During this long time, Darwin proposed, countless species had come into being, lived for a time, and then vanished. By comparing fossils from older rock layers with fossils from younger layers, scientists could document the fact that life on Earth has changed over time as shown in Figure 15-13.

Since Darwin's time, the number of known fossil forms has grown enormously. Researchers have discovered many hundreds of transitional fossils that document various intermediate stages in the evolution of modern species from organisms that are now extinct. Gaps remain, of course, in the fossil records of many species, although a lot of them shrink each year as new fossils are discovered. These gaps do not indicate weakness in the theory of evolution itself. Rather, they point out uncertainties in our understanding of exactly how some species evolved.

Geographic Distribution of Living Species Remember tbat many paTts of the biological puzzle that Darwin saw on his Beagle voyage involved living organisms. After Darwin discovered that those little brown birds he collected in the Galapagos were all finches, he began to wonder how they came to be similar, yet distinctly different from one another. Each species was slightly different from every other species. They were also slightly different from the most similar species on the mainland of South America. Could the island birds have changed over time, as populations in difforent places adapted to different local environments? Darwin struggled with this question for a long time. He finally decided that all these birds could have descended with modification from a common mainland ancestor.

There were other parts to the living puzzle as well. Recall that Darwin found entirely different species of animals on the continents of South America and Australia. Yet, when he looked at similar environments on those continents, he sometimes saw different animals that had similar anatomies and behaviors. Darwin's theory of descent with modification made scientific sense of this part of the puzzle as well. Species now living on different continents, as shown in Figure 15-14, had each descended from different·ancestors. However, because some animals on each continent were living under similar ecological conditions, they were exposed to similar pressures of natural selection. Because of these similar selection pressures, different animals ended up evolving certain striking features in common.

CHECKPOINT How can two species that look very different from each other be more closely related than two other species that look similar to each other?

· .. '

D Beaver • Muskrat D Beaver and

Muskrat

• coypu D Capybara

• Coypu and Capybara

.A Figure 15-14 The existence of similar but unrelated species was a puzzle to Darwin. Later, he realized that similar animals in different locations were the product of different lines of evolutionary descent. Here, the beaver and the capybara are similar species that inhabit similar environments of North America and South America . The South American coypu also shares many characteristics with the North American muskrat. Interpreting Graphics Which animal has a larger geographical range, the coypu or the muskrat?


Turtle

A Figure 15-15 The limbs of these four modern vertebrates are homologous structures. They provide evidence of a common ancestor whose bones may have resembled those of the ancient fish shown here. Notice that the same colors are used to show related structures. ~Homologous structures are one type of evidence for the evolution of living things.

Word Origins

-.-----------------Homologous, from the Greek words homos, meaning "same," and legein, meaning "say," describes similar body structures that come from a common ancestor. If the word morphe means "shape," what are homomorphic structures?

384 Chapter 15

Alligator

Ancient, lobefinned fish

Bird Mammal

Homologous Body Structures Further evidence of evolution can be found in living animals. By Darwin's time, researchers had noticed striking anatomical similarities among the body parts of animals with backbones. For example, the limbs of reptiles, birds, and mammals-arms, wings, legs, and flippers-vary greatly in form and function. Yet, they are all constructed from the same basic bones, as shown in Figure 15-15.

Each of these limbs has adapted in ways that enable organisms to survive in different environments. Despite these different functions, however, these limb bones all develop from the same clumps of cells in embryos. Structures that have different mature forms but develop from the same embryonic tissues are called homologous (hoh-MAHL-uh-guhs) structures. Homologous structures provide strong evidence that all four-limbed vertebrates have descended, with modifications, from common ancestors.

There is still more information to be gathered from homologous structures. If we compare the front limbs, we can see that all bird wings are more similar to one another than any of them are to bat wings. Other bones in bird skeletons most closely resemble the homologous bones of certain reptiles-including crocodiles and extinct reptiles such as dinosaurs. The bones that support the wings of bats, by contrast, are more similar to the front limbs of humans, whales, and other mammals than they are to those of birds. These similarities and differences help biologists group animals according to how recently they last shared a common ancestor.

Not all homologous structures serve important functions. The organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs in other species. These vestigial organs may resemble miniature legs, tails, or other structures. The legs of the skinks shown in Figure 15-16 are an example of vestigial organs. Why would an organism possess organs with little or no function? One possibility is that the presence of a vestigial organ may not affect an organism's ability to survive and reproduce. In that case, natural selection would not cause the elimination of that organ.

Homologies also appear in other aspects of plant and animal anatomy and physiology. Certain groups of plants and algae, for example, share homologous variations in stem, leaf, root, and flower structures, and in the way they carry out photosynthesis. Mammals share many homologies that distinguish them from other vertebrates. Dolphins may look something like fishes, but homologies show that they are mammals. For example, like other mammals, they have lungs rather than gills and obtain oxygen from air rather than water.

Similarities in Embryology The early stages, or embryos, of many animals with backbones are very similar. This does not mean that a human embryo is ever identical to a fish or a bird embryo. However, as you can see in Figure 15-17, many embryos look especially similar during early stages of development. What do these similarities mean?

There have, in the past, been incorrect explanations for these similarities. Also, the biologist Ernst Haeckel fudged some of his drawings to make the earliest stages of some embryos seem more similar than they actually are! Errors aside, however, it is clear that the same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates. These common cells and tissues, growing in similar ways, produce the homologous structures discussed earlier.

What are homologous structures?

Chicken Rat

Figure 15-16 These three animals are skinks, a type of lizard. In some species of skinks, legs have become vestigial. They are so reduced that they no longer function in walking. In humans, the appendix is an example of a vestigial organ because it carries out no function in digestion. Inferring How might vestigial organs provide clues to an animal's evolutionary history?

Figure 15-17 In their early stages of development, chickens, turtles, and rats look similar, providing evidence that they shared a common ancestry. Inferring How could a study of these embryos help show the relationships among animals with backbones?

Darwin's Theory of Evolution 3P ·

.6. Figure 15-18 Darwin's On the Origin of Species presented a revolutionary view of the living world. Many scientists agree with Darwin's statement that "There is a grandeur in this view of life, ... that ... from so simple a beginning, endless forms so beautiful and wonderful have been and are being evolved." Applying Concepts New species are continually being discovered. How could you use Darwin's theory to learn more about these new species?

1. • Key Concept How is artificial selection dependent on variation in nature?

2. · Key Concept The theory of evolution by natural selection explains, in scientific terms, how living things evolve over time. What is being selected in this process?

\ 386 CMptec 15

Summary of Darwin's Theory Darwin's theory of evolution can be summarized as follows:

• Individual organisms differ, and some of this variation is heritable.

• Organisms produce more offspring than can survive, and many that do survive do not reproduce.

• Because more organisms are produced than can survive, they compete for limited resources.

• Each unique organism has different advantages and disadvantages in the struggle for existence. Individuals best suited to their environment survive and reproduce most successfully. These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring. This process of natural selection causes species to change over time.

• Species alive today are descended with modification from ancestral species that lived in the distant past. This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.

Strengths and Weaknesses of Evolutionary Theory Scientific advances in many fields of biology, along with geology and physics, have confirmed and expanded most of Darwin's hypotheses. Today, evolutionary theory offers vital insights to all biological and biomedical sciences-from infectious-disease research to ecology. In fact, evolution is often called the "grand unifying theory of the life sciences."

Like any scientific theory, evolutionary theory continues to change as new data are gathered and new ways of thinking arise. As you will see shortly, researchers still debate such important questions as precisely how new species arise and why species become extinct. There is also uncertainty about how life began.

3. Key Concept What types of evidence did Darwin use to support his theory of change over time?

4. Critical Thinking Evaluating Use scientific evidence to evaluate Darwin's theory of evolution by natural selection.

Newspaper Article Write a newspaper article about the meeting in which Darwin's and Wallace's hypotheses of evolution were first presented. Explain the theory of evolution by natural selection for an audience who knows nothing about the subject.

Exploration

Modeling Adaptation In this game, three families land on an alien planet. At home, the Hunter family survived by hunting in the cold north. The Seeder family farmed the temperate zone. The Fisher family lived on a tropical island. In this investigation, you will model how well each family survives in a new environment.

Problem How do organisms survive in new habitats?

Material •coin

Skills Using Models, Using Tables and Graphs, Calculating

Procedure

0 Work in groups of three, with each member playing a Hunter, Seeder, or Fisher.

6 Flip a coin. Record the result as 1 for heads, 0 for tails. Toss the coin three more times to produce a series of four 1 s and Os. This 4-digit number is the code for your new habitat.

€) If the first digit in your code is 1, you live in a hot area. If it is 0, the climate is cold . If the second digit is 1, the climate is wet. If it is 0, it's dry. If the third digit is 1, you have a dry cave to live in. If it is 0, you sleep under the stars. If the last digit is 1, there is enough food. If it is 0, food is scarce. Record a description of your habitat.

0 Find your family in the table below. Then, record each number in your row that falls under a heading that describes your habitat (hot or cold and so forth). Record the total of these 4 numbers. This total represents the energy you have accumulated from your food.

0 Subtract 8 from your total to model the energy you must use to survive. If you don't have enough energy to do this, you're out of the game. The player with the most energy wins . Record the score and habitat of each family.

0 Predicting Record a prediction of what would happen if you reversed each player's habitat code by changing all the 1 s to Os and the Os to 1 s.

f) Reverse your habitat code as described in step 6. Play a second round with these conditions.

Analyze and Conclude 1. Comparing and Contrasting In which habitat

were you most successful? Was it similar to your home environment?

2. Using Models The numbers in the table are different for each family. How did this fact help you model the survival of different organisms?

3. Drawing Conclusions Is one habitat best for all players? Explain in terms of adaptation.

Go Furth '

Applying Concepts Revise the game to reflect the different conditions of summer and winter. Then, demonstrate your game to the class.

Energy Points for Survival

Temperature Water Shelter Food

Cold Hot Dry Wet None Cave Scarce Plenty

Hunter 8 -2 0 4 -6 7 -5 8

Seeder 0 3 2 2 -1 2 -2 6

Fisher -5 8 -2 5 0 1 -1 4


15- 1 The Puzzle of Life's Diversity Key Concepts

• During his travels, Charles Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.

• Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galapagos.

Vocabulary evolution, p. 369 theory, p. 369 fossil, p. 371

15-2 Ideas That Shaped Darwin's Thinking

Key Concepts

• Hutton and Lyell helped scientists realize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present.

• Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These traits could then be passed on to their offspring. Over time, this process led to change in a species.

• Malthus reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.

15-3 Darwin Presents His Case <> Key Concepts

• In artificial selection, nature provides the variation among different organisms, and humans select those variations that they find useful.

• Over time, natural selection results in changes in the inherited characteristics of a population . These changes increase a species' fitness in its environment.

• Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, and similarities in early development, or embryology.

Vocabulary artificial selection, p. 379 struggle for existence, p. 380 fitness, p. 380 adaptation, p. 380 survival of the fittest, p. 381 natural selection, p. 381 descent with modification, p. 381 common descent, p. 382 homologous structure, p. 384 vestigial organ, p. 384

Thinking Visually Use the information in this chapter to complete the table below.

Evidence of Evolution

Type of Evidence Example What Evidence Reveals "

The fossil record 2

Geographic distribution 3 4 of living species

Homologous 5 6 body structures

Similarities in embryological 7 8 development

388 Chapter 15

Reviewing Content Choose the letter that best answers the question or completes the statement.

1. Who observed variations in the characteristics of animals and plants on the different islands of the Galapagos? a. James Hutton c. Charles Darwin b. Charles Lyell d. Thomas Malthus

2. In addition to observing living organisms, Darwin studied the preserved remains of ancient organisms, called a. fossils. c. homologous structures. b. adaptations. d. vestigial organs.

3. Which of the following ideas proposed by Lamarck was later found to be incorrect? a. Acquired characteristics can be inherited. b. All species were descended from other species. c. Living things change over time. d. Organisms are adapted to their environments.

4. Differences among individuals of a species are referred to as a. natural variation. c. natural selection. b. fitness. d. adaptation.

5. Which would an animal breeder use to produce cows that give more milk? a. overproduction b. genetic isolation c. acquired characteristics d. artificial selection

6. An inherited characteristic that increases an organism's ability to survive and reproduce in its specific environment is called a(an) a. vestigial organ. c. speciation. b. adaptation. d. radiation.

7. The concept that each living species has descended, with changes, from other species over time is referred to as a. descent with modification. b. artificial selection. c. theory of acquired characteristics. d. natural selection.

8. Fitness is a result of a. adaptations. c. common descent. b. homologies. d. variation.

9. Structures that have different mature forms but develop from the same embryonic tissue are a. vestigial organs. b. adaptations. c. homologous structures. d. fossils.

Interactive textbook with assessment at PHSchool.com

10. The diagram below best illustrates a. Lamarck's theory of evolution. b. Darwin's theory of evolution. c. Malthus's principles. d. Lyell's theory about past changes.

Understanding Concepts 11. Explain what is meant by the term evolution, and

give an example.

12. Describe three of Darwin's observations about animals in South America and on the Galapagos Islands.

13. How did the visit to the Galapagos Islands affect Darwin's thoughts on evolution?

14. How did Hutton's and Lyell's views of Earth differ from that of most people of their time?

15. Explain Lamarck's principle of use and disuse.

16. How does natural variation affect evolution?

17. What is artifical selection? How did this concept influence Darwin's thinking?

18. Distinguish between fitness and adaptation. Give an example of each.

19. How is the process of survival of the fittest related to a population's environment?

20. How does Darwin's principle of descent with modification explain the characterisitics of today's species?

21. What does fossil evidence show about evolution?

22. What evidence of evolution can be found in the geographic distribution of living animals? Give an example.

23. What is a vestigial organ? Give an example.

24. How do scientists use similarities in embryology as evidence for evolution?

25. Summarize the main ideas in Darwin's theory.


Critical Thinking 26. Inferring How does the process of natural

selection account for the diversity of organisms that Darwin observed on the Galapagos Islands?

27. Applying Concepts Explain how natural selection might have produced the modern giraffe from short-necked ancestors.

28. Formulating Hypotheses DDT is an insecticide that was first used in the 1940s to kill mosquitoes and stop the spread of malaria. At first, it was very effective. However, over a period of years, people began to notice that it was becoming less and less effective. A possible explanation for this was that the insects were becoming resistant to the DDT. Explain how the resistance may have evolved.

29. Predicting Although wild turkeys can fly, domesticated turkeys cannot. Suppose that a population of domesticated turkeys escaped from a farm into a new environment. Give examples of environmental conditions that might determine whether that population would survive over time.

30. Making Judgments Is protecting an endangered species upsetting the process of natural selection? Explain your answer.

31. Applying Concepts Charles Darwin discovered that different types of tortoises lived on the different Galapagos Islands. Two of those types are shown below. Darwin learned that each type of tortoise had adaptations that enabled it to feed on the vegetation that was characteristic of its particular island. Use Figure 1 S-3 and what you learned about Darwin's theory to explain how the different types of tortoises may have evolved.

32. Inferring Many species of birds build nests in which they lay eggs and raise the newly hatched birds. How might nest-building behavior be an adaptation that helps ensure reproductive fitness?

390 Chapter 15

33. Applying Concepts A whale flipper and a human arm are considered homologous. Do you think a whale might have vestigial hip and leg bones? Explain.

34. Inferring In all animals with backbones, oxygen is carried in blood by a molecule called hemoglobin. What might this physiological similarity indicate about the evolutionary history of animals with backbones?

35. Connecting Concepts Refer back to Chapter 11 to refresh what you learned about Mendel. If Mendel and Darwin had met, how might Mendel have helped Darwin develop his theory? Include parts of Mendel's theory in your answer.

v\lvriting in Science Write a paragraph in which you explain Darwin's concept of the struggle for existence. The paragraph should include specific examples to clarify the meaning of the concept. (Hints: Think of a few examples that illustrate the concept, and then choose the best two to write about. When you write your paragraph, begin it with a sentence that expresses the main idea.)

Performance-Based Assessment Alternative Scenarios Select an adaptation of a plant or animal. Write a scenario explaining how the trait might have evolved according to Lamarck, then write a second scenario using Darwin's ideas. Present your essay, and challenge classmates to identify the theory on which each scenario is based.

r ...... G_ o __ n_1t...,·:;!~hoo1.com For: An interactive self-test Visit: PHSchool.com Web Code: cba-5150

Test-Taking Tip If you have trouble answering

a question, make a mark beside it and go on. (Do not write in this book.) You may find information in later questions that will allow you to eliminate some answer choices in your unanswered question.

Directions: Choose the letter that best answers the question or completes the statement.

1. Which scientist formulated the theory of evolution through natural selection? (A) Charles Darwin (D) Jean-Baptiste Lamarck (B) James Hutton (E) Gregor Mendel (C) Charles Lyell

2. The ability of an individual organism to survive and reproduce in its natural environment is called (A) natural selection. (B) evolution. (C) adaptation. (D) descent with modification. (E) fitness.

3. The French scientist Jean-Baptiste Lamarck proposed which of the following theories?

I. All organisms have a common ancestor. II. Species change over time.

III. A single organism can acquire traits over its lifetime that are then passed to its offspring.

(A) I only (D) II and III only (B) II only (E) I, II, and III (C) I and II only

4. Which of the following is an important concept in Darwin's theory of evolution by natural selection? I. Struggle for existence

II. Survival of the fittest III. Descent with modification (A) I only (D) II and III only (B) II only (E) I, II, and III (C) I and II only

5. Which of the following does NOT provide evidence that living things have been evolving for millions of years? (A) fossil record (B) natural variation within a species (C) geographical distribution of living things (D) homologous structures of living organisms (E) similarities in embryological development

Standardized Test Prep

6. A farmer's use of the best livestock for breeding is an example of (A) natural selection. (D) common descent. (B) artificial selection. (E) extinction. (C) fitness.

7. Lyell's Principles of Geology influenced Darwin because it explained how (A) organisms change over time. (B) adaptations occur. (C) the surface of the Earth changes over time. (D) the Galapagos Islands were formed. (E) volcanoes occur.

8. A bird's wings are homologous to a(an) (A) fish's tailfin. (B) alligator's claws. (C) dog's front legs. (D) mosquito's wings. (E) mosquito's front legs.

Questions 9-7 0 The birds shown below are two species of the 1 3 species of finches Darwin found on the Galapagos Islands.

Woodpecker finch Large ground finch

9. What process produced the two different types of beaks shown? (A) artificial selection (B) natural selection (C) geographical distribution (D) inheritance of acquired traits (E) disuse of the beak

1 O. The large ground finch obtains food by cracking seeds. Its short, strong beak is an example of (A) the struggle for existence. (B) the tendency toward perfection. (C) the inheritance of acquired traits. (D) an adaptation. (E) a vestigial organ.


Inquiry Activity

Does sexual reproduction change genotype ratios? Procedure 1. Put 33 red and 67 black beads in a large paper

cup to represent two alleles of a certain gene in a population.

2. To model the genotype of an offspring, remove two beads. Record the genotype. Return the beads.

3. Repeat step 2 for a total of 10 offspring. Add your data to the class total.

This group of ladybug beetles illustrates a population with a number of inherited traits. Darwin recognized such variation as the raw material for evolution.

Think About It 1. Calculating What was the genotype ratio of the

offspring?

2. Comparing and Contrasting Was the genotype ratio the same as the 1 : 2 : 1 genotype ratio for a cross between two heterozygotes (Ao x Ao)? Explain .

3. Predicting If you repeated this activity over and over, would you expect the genotype ratios to change? Explain.

16-1 Genes and Variation

A s Darwin develop d his theory of evolution, he worked under a serious handicap. He didn't know how heredity worked!

Although Mendel's work on inheritance in peas was published during Darwin's lifetime, its importance wasn't recognized for decades. This lack of knowledge left two big gaps in Darwin's thinking. First, he had no idea how heritable traits, such as those shown in Figure 16-1, pass from one generation to the next. Second, although variation in heritable traits was central to Darwin's theory, he had no idea how that variation appeared.

Evolutionary biologists connected Mendel's work to Darwin's during the 1930s. By then, biologists understood that genes control heritable traits. They soon realized that changes in genes produce heritable variation on which natural selection can operate. Genes became the focus of new hypotheses and experiments aimed at understanding evolutionary change. Another revolution in evolutionary thought began with Watson and Crick's studies on DNA Their model of the DNA molecule helped evolutionary biologists because it demonstrated the molecular nature of mutation and genetic variation.

Today, molecular techniques are used to test hypotheses about how heritable variation appears and how natural selection operates on that variation. As you will learn in this chapter, fitness, adaptation, species, and evolutionary change are now defined in genetic terms. We understand how evolution works better than Darwin ever could, beginning with heritable variation.

How Common Is Genetic Variation? We now know that many genes have at least two forms, or alleles. Animals such as horses, dogs, and mice often have several alleles for traits such as body size or coat color. Plants, such as peas, often have several alleles for flower color. All organisms have additional genetic variation that is "invisible" because it involves small differences in biochemical processes. In addition, an individual organism is heterozygous for many genes. An insect may be heterozygous for as many as 15 percent of its genes. Individual fishes, reptiles, and mammals are typically heterozygous for between 4 and 8 percent of their genes.

Guide for Reading

Key Concepts • Whal are the main sources

of heritable variation in a population?

• How is evolution defined in genetic terms?

• What determines the numbers of phenotypes for a given trait?

Vocabulary gene pool relative frequency single-gene trait polygenic trait

Reading Strategy: Building Vocabulary Before you read, make a list of the vocabulary terms above. As you read, take notes about the meaning of each term.

T Figure 16-1 There are two main sources of genetic variation: mutations and the gene shuffling that results from sexual reproduction. Each of these babies has inherited a collection of traits. Some, such as hair color, are visible, while others, such as the ability to resist certain diseases, are not.

•

Sample Population

48% heterozygous

~black

Frequency of Alleles

allele for black fur

.._ Figure 16-2 When scientists determine whether a population is evolving, they may look at the sum of the population's alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. Calculating Here, in a total of 50 alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles?

( GO n!1~~ • cz For: Links on INKS

population genetics Visit: www.Scilinks.org Web Code: cbn-5161

394 Chapter 16

Variation and Gene Pools Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. Because members of a population interbreed, they share a common group of genes called a gene pool. A gene pool consists of all genes, including all the different alleles, that are present in a population.

The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage. For example, in the mouse population in Figure 16-2, the relative frequency of the dominant B allele (black fur) is 40 percent, and the relative frequency of the recessive b allele (brown fur) is 60 percent. The relative frequency of an allele has nothing to do with whether the allele is dominant or recessive. In this particular mouse population, the recessive allele occurs more frequently than the dominant allele.

Gene pools are important to evolutionary theory, because evolution involves changes in populations over time. In genetic terms, evolution is any change in the relative frequency of alleles in a population. For example, ifthe relative frequency of the B allele in the mouse population changed over time to 30 percent, the population is evolving .

CHECKPOINT What is a gene pool?

Sources of Genetic Variation Biologists can now explain how variation is produced. The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

Mutations A mutation is any change in a sequence of DNA. Mutations can occur because of mistakes in the replication of DNA or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism's phenotype. For example, a DNA codon altered from GGA to GGU will still code for the same amino acid, glycine. That mutation has no effect on phenotype. Many mutations do produce changes in phenotype, however. Some can affect an organism's fitness, or its ability to survive and reproduce in its environment. Other mutations may have no effect on fitness.

Gene Shuffling Mutations are not the only source of heritable variation. You do not look exactly like your biological parents, even though they provided you with all your genes. You probably look even less like any brothers or sisters you may have. Yet, no matter how you feel about your relatives, mutant genes are not primarily what makes them so different from you.

Most heritable differences are due to gene shuffling that occurs during the production of gametes. Recall that each chromosome of a homologous pair moves independently during meiosis. As a result, the 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes!

Another process, crossing-over, also occurs during meiosis. Crossing-over further increases the number of different genotypes that can appear in offspring. Recall that a genotype is an organism's genetic makeup. When alleles are recombined during sexual reproduction, they can produce dramatically different phenotypes. Thus, sexual reproduction is a major source of variation within many populations.

Sexual reproduction can produce many different phenotypes, but it does not change the relative frequency of alleles in a population. To understand why, compare a population's gene pool to a deck of playing cards. Each card represents an allele found in the population. The exchange of genes during gene shuffling is similar to shuffling a deck of cards. Shuffling leads to different types of hands, but it can never change the relative numbers of aces, kings, or queens in the deck. The probability of drawing an ace off the top of the deck will always be 4 in 52, or one thirteenth (4/52 = 1/13). No matter how many times you shuffle the deck, this probability will remain the same. Similarly, sexual reproduction produces many different combinations of genes, but in itself it does not alter the relative frequencies of each type of allele in a population.

CHECl<POINT What are the sources of heritable variation?

Single-Gene and Polygenic Traits Heritable variation can be expressed in a variety of ways.

The number of phenotypes produced for a given trait depends on how many genes control the trait. Among humans, a widow's peak-a downward dip in the center of the hairline-is a single-gene trait. It is controlled by a single gene that has two alleles. The allele for a widow's peak is dominant over the allele for a hairline with no peak. As a result, variation in this gene leads to only two distinct phenotypes, as shown in Figure 16-3.

As you can see, the frequency of phenotypes Cl) Q.

~ c Cl)

100

80

~ 60 .-. o~ ~ ........ 40 c Cl) :i CT

20

Word Origins

Gene comes from the Greek word gignesthai, meaning "to be born," and refers to factors that produce an organism. The prefix polycomes from the Greek word polys, meaning "many," so polygenic means "having many genes." The prefix mono- means "one." What do you think the term monogenic means?

Figure 16-3 In humans, a single gene with two alleles controls whether a person has a widow's peak (left) or does not have a widow's peak (right). As a result, only two phenotypes are possible.

The number of phenotypes a given trait has is determined by how many genes control the trait.

Single-Gene Trait

caused by this single gene is represented on the bar graph. This graph shows that the presence of a widow's peak may be less common in a population than the absence of a widow's peak, even though the allele for a widow's peak is the dominant form. In real populations, phenotypic ratios are determined by the frequency of alleles in the population as well as by whether the alleles are in the dominant or recessive form. Allele frequencies may not match Mendelian ratios.

I!! u. 0 -1-----------........ ~~ Widow's peak No widow's peak

Phenotype

Evolution of Populations 395

Figure 16-4 The graph below shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait. The photograph shows the actual distribution of heights of a group of young men. Using Tables and Graphs What does the shape of the graph indicate about height in humans?

~ ~ c Q) .c a. 0 ~ c Q) :J C" e

LL

-- Phenotype (height)_____....

Many traits are controlled by two or more genes and are, therefore, called polygenic traits. Each gene of a polygenic trait often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and phenotypes.

1. ~Key Concept In genetic terms, what indicates that evolution is occurring in a population?

2. ~ Key Concept What two processes can lead to inherited variation in populations?

3. ~Key Concept How does the range of phenotypes differ between single-gene traits and polygenic traits?

396 Chapter 16

Height in humans is one example of a polygenic trait. You can sample phenotypic variation in this trait by measuring the height of all the students in your class. You can then calculate the average height of this group. Many students will be just a little taller or shorter than average. Some of your classmates, however, will be very tall or very short. If you graph the number of individuals of each height, you may get a graph similar to the one in Figure 16-4. The symmetrical bell-like shape of this curve is typical ofpolygenic traits. A bell-shaped curve is also called a normal distribution.

14. What is a gene pool? How are

allele frequencies related to gene pools?

5. Critical Thinking Evaluating Evaluate the significance of mutations to the process of biological evolution. (Hint: How does mutation affect genetic variation?)

-

Genetic Variation How does the process known as independent assortment relate to the genetic variation that results from sexual reproduction? Hint: Refer to Chapter 11.

16-2 Evolution as Genetic Change

A genetic view of evolution offers a new way to look at key evolutionary concepts. i ach time an organism reproduces,

it passes copies of its genes to its offspring. We can therefore view evolutionary fitness as an organism's success in passing genes to the next generation. In the same way, we can view an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individual's ability to pass along its genes.

Natural selection never acts directly on genes. Why? Because it is an entire organism-not a single gene-that either survives and reproduces or dies without reproducing. Natural selection, therefore, can only affect which individuals survive and reproduce and which do not. If an individual dies without reproducing, the individual does not contribute its alleles to the population's gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.

Now recall that evolution is any change over time in the relative frequencies of alleles in a population. This reminds us that it is populations, not individual organisms, that can evolve over time. Let us see how this can happen in different situations.

Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to

changes in allele frequencies and thus to evolution. Imagine that a hypothetical population of lizards, shown in Figure 16-5, is normally brown, but experiences mutations that produce red and black forms. What happens to those new alleles? If red lizards are more visible to predators, they might be less likely to survive and reproduce, and the allele for red coloring might not become common.

Guide for Reading ....... ._.._.._.._.._.._ ... Key Concepts • How does natural selection

affect single-gene and polygenic traits?

• What is genetic drift? • What five conditions are

needed to maintain genetic equilibrium?

Vocabulary directional selection stabilizing selection disruptive selection genetic drift founder effect Hardy-Weinberg principle genetic equilibrium

Reading Strategy: Outlining Before you read, use the headings to make an outline. As you read, add a sentence after each heading to provide key information.

T Figure 16-5 ~ Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.

Effect of Color Mutations on Lizard Survival

Initial Population Generation 10 Generation 20 Generation 30

80% 80% 70%

10% 0% 0%

10% 20% 30%


.... Figure 16-6 Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. In this example, a population of seed-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low. The dotted line shows the original distribution of beak sizes. The solid line shows how the distribution of beak sizes would change as a result of selection.

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Black lizards, on the other hand, might absorb more sunlight and warm up faster on cold days. If high body temperature allows them to move faster to feed and to avoid predators, they might produce more offspring than brown forms. The allele for black color might then increase in relative frequency. If a color change has no effect on fitness, the allele that produces it would not be under pressure from natural selection.

Natural Selection on Polygenic Traits When traits are controlled by more than one gene, the effects of natural selection are more complex. As you learned earlier, the action of multiple alleles on traits such as height produces a range of phenotypes that often fit a bell curve. The fitness of individuals close to one another on the curve will not be very different. But fitness can vary a great deal from one end of such a curve to the other. And where fitness varies, natural selection can act. · Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.

Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. The range of phenotypes shifts as some individuals fail to survive and reproduce while others succeed. To understand this, consider how limited resources, such as food, can affect the long-term survival of individuals and the evolution of populations.

Among seed-eating birds such as Darwin's finches, for example, birds with bigger, thicker beaks can feed more easily on larger, harder, thicker-shelled seeds. Suppose a food shortage causes the supply of small and medium-sized seeds to run low, leaving only larger seeds. Birds whose beaks enable them to open those larger seeds will have better access to food. Birds with the big-beak adaptation would therefore have higher fitness than small-beaked birds. The average beak size of the population would probably increase, as shown in Figure 16-6.

~-..&. ...

~ ;.--:, ~ Directional Selection

"' "E ·- c: al 0 ...... ·-0 .....

!ti .. -llJ :l .c 0. E o :l c.. z .5

Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. This situation keeps the center of the curve at its current position, but it narrows the overall graph.

As shown in Figure 16-7, the mass of human infants at birth is under the influence of stabilizing selection. Human babies born much smaller than average are likely to be less healthy and thus less likely to survive. Babies that are much larger than average are likely to have difficulty being born. The fitness of these larger or smaller individuals is, therefore, lower than that of more average-sized individuals.

Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. In such situations, selection acts most strongly against individuals of an intermediate type. If the pressure of natural selection is strong enough and lasts long enough, this situation can cause the single curve to split into two. In other words, selection creates two distinct phenotypes.

For example, suppose a population of birds lives in an area where medium-sized seeds become less common and large and small seeds become more common. Birds with unusually small or large beaks would have higher fitness. As shown in Figure 16-8, the population might split into two subgroups: one that eats small seeds and one that eats large seeds.

CHECKPOINT How do stabilizing selection and disruptive selection differ?

-~-~ -~-· ~ Disruptive Selection ...

I/I

'E ·- c: mo -·-0 n; ~1 E cf :J c: z ·-

Birth Mass

Selection against both extremes keeps curve narrow and in

',same place.

A Figure 16-7 Stabilizing selection takes place when individuals near the center of a curve have higher fitness than individuals at either end. This example shows that human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.

..... Figure 16-8 ~When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. In this example, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds.


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active ar For: Genetic Drift activity Visit: PHSchool.com Web Code: cbp-5162

Figure 16-9 4> In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.

Sample of Original Population

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Genetic Drift Natural selection is not the only source of evolutionary change. In small populations, an allele can become more or less common simply by chance. Recall that genetics is controlled by the laws of probability. These laws can be used to predict the overall results of genetic crosses in large populations. However, the smaller a population is, the farther the results may be from what the laws of probability predict. This kind ofrandom change in allele frequency is called genetic drift. How does genetic drift take place? In small populations, individu-als that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Genetic drift may occur when a small group of individuals colonizes a new habitat. These individuals may carry alleles in different relative frequencies than did the larger population from which they came. If so, the population that they found will be genetically different from the parent population. Here, however, the cause is not natural selection but simply chancespecifically, the chance that particular alleles were in one or more of the founding individuals, as shown in Figure 16-9. A situation in which allele frequencies change as a result of the migration of a small subgroup of a population is known as the founder effect. One example of the founder effect is the evolution of several hundred species of fruit flies found on different Hawaiian Islands. All of those species descended from the same original mainland population. Those species in different habitats on different islands now have allele frequencies that are different from those of the original species.

CHECKPOINT What is genetic drift?

Descendants

Founding Population A

Founding Population B

Can the environment affect survival?

Materials scissors, construction paper (several colors), transparent tape, 15-cm ruler, watch with a second hand

Procedure it 4. Record how many shapes of each color you can

count from your desk in 5 seconds.

1. Predicting Predict what would happen to a population of butterflies that includes some individuals that are easy for predators to see and some that blend in with the environment.

5. Exchange your observations with your classmates to determine the class total for each color.

Analyze and Conclude

2. Choose three different-colored sheets of construction paper. Cut out a butterfly shape from each sheet, 5 x 1 0 cm in size, as shown. CAUTION:

1. Analyzing Data According to your class data, which colors of butterfly are easiest to see? Which color of butterfly would be most easily caught by a predator?

Be careful with scissors. 3. Tape your butterflies to different-colored surfaces.

Then, return to your seat.

2. Inferring What will happen to the butterfly population after many generations if predators consume most of the easy-to-see butterflies?

Evolution Versus Genetic Equilibrium To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. So biologists ask: Are there any conditions under which evolution will not occur? Is there any way to recognize when that is the case? The answers to those questions are provided by the Hardy-Weinberg principle, named after two researchers who independently proposed it in 1908.

The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. The situation in which allele frequencies remain constant is called genetic equilibrium. If the allele frequencies do not change, the population will not evolve.

Under what conditions does the Hardy-Weinberg principle hold? · Five conditions are required to maintain genetic equilibrium from generation to generation: (1) There must be random mating; (2) the population must be very large; and (3) there can be no movement into or out of the population, (4) no mutations, and (5) no natural selection.

In some populations, these conditions may be met or nearly met for long periods of time. If, however, the conditions are not met, the genetic equilibrium will be disrupted, and the population will evolve.

---Evolution of Populations 401

A Figure 16-10 c=i one of the five conditions that are needed to maintain genetic equilibrium from one generation to the next is large population size. The allele frequencies of large populations, such as this group of birds, are less likely to be changed through the process of genetic drift.

1. · Key Concept Describe how natural selection can affect traits controlled by single genes.

2. Key Concept Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes?

3. Key Concept How does genetic drift lead to a change in a population's gene pool?

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Random Mating All members of the population must have an equal opportunity to produce offspring. Random mating ensures that each individual has an equal chance of passing on its alleles to offspring.

In natural populations, however, mating is rarely completely random. Many species, including lions and wolves, select mates based on particular heritable traits, such as size or strength. Such nonrandom mating means that the genes for those traits are not in equilibrium but are under strong selection pressure.

Large Population A large population size is also important in maintaining genetic equilibrium. That is because genetic drift has less effect on large populations, such as the population of birds shown in Figure 16-10, than on small ones.

No Movement Into or Out of the Population Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. In genetic terms, the population's gene pool must be kept together and kept separate from the gene pools of other populations.

No Mutations If genes mutate from one form into another, new alleles may be introduced into the population, and allele frequencies will change.

No Natural Selection All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. In other words, there can be no natural selection operating on the population.

4. · Key Concept What is the Hardy-Weinberg principle?

5. Crltlcal Thinking Comparing and Contrasting How are directional selection and disruptive selection similar? How are they different?

Using Models Demonstrate natural selection on polygenic traits by cutting a sheet of paper into squares of five different sizes to represent sizes in a population. Use the squares to model directional, stabilizing, and disruptive selection. Then, think of an alternative way to model one type of selection. Decide which model works best, and give your reasons.

Should the Use of Antibiotics Be Restricted?

N atural selection i everywhere. One dramatic example of evolution in action poses a serious

threat to public health. Many kinds of diseasecausing bacteria are evolving resistance to antibiotics-drugs intended to kill them or interfere with their growth.

Antibiotics are one of medicine's greatest weapons against bacterial diseases. When antibiotics were discovered, they were called "magic bullets" and "wonder drugs" because they were so effective. They have made diseases like pneumonia much less of a threat than they were about sixty years ago. However, people may be overusing antibiotics. Doctors sometimes prescribe them for diseases for which they are not effective. Commercial feed for chickens and other farm animals is laced with antibiotics to prevent infection.

This wide use has caused many bacteriaincluding Mycobacterium tuberculosis, which causes tuberculosis-to evolve resistance to antibiotics. This resistance is a prime example of the evolution of a genetically controlled physiological trait. Resistance evolved because bacterial populations contained a few individuals with genes that enabled them to destroy, inactivate, or eliminate antibiotics. Descendants of those physiologically similar individuals survived and reproduced, and became today's resistant strains. Once-powerful antibiotics are now useless against resistant bacteria. Given this risk, should government agencies restrict the use of antibiotics?

The Viewpoints

Antibiotic Use Should Be Restricted The danger of an incurable bacterial epidemic is so high that action must be taken on a national level as soon as possible. Doctors overuse antibiotics in humans because patients demand them. The livestock industry likes using antibiotics in animal feeds and will not change their practice unless forced to do so.

Antibiotic Use Should Not Be Restricted Researchers are coming up with new drugs all the time. These drugs can be reserved for human use only. Doctors need to be able to prescribe antibiotics as they choose, and our food supply depends on the use of antibiotics in agriculture. The medical profession and the livestock industry need the freedom to find solutions that work best for them.

Research and Decide 1. Analyzing the Viewpoints To make an

informed decision, learn more about this issue by consulting library and Internet resources. Then, list the advantages and disadvantages of restricting the use of antibiotics.

2. Forming Your Opinion Should antibiotics be restricted? Are there some situations in which such regulations would be more appropriate than others?

cGo nline '-------PHSchool.com For: Links from the authors Visit: PHSchool.com Web Code: cbe-5162

16- 3 The Process of Speciation

Guide for Reading

.. ---------------~ Key Concepts • What factors are involved in

the formation of new species? • Describe the process of

speciation in the Galapagos finches.

Vocabulary speciation reproductive isolation behavioral isolation geographic isolation temporal isolation

Reading Strategy: Using Visuals Before you read, preview Figure 16-16. As you read about speciation of Darwin's finches, notice what happens at each step in the diagram.

404 Chapter 16

F actors such as natural selection and chance events can change the relative frequencies of alleles in a population .

But how do these changes lead to the formation of new species, or speciation?

Recall that biologists define a species as a group of organisms that breed with one another and produce fertile offspring. This means that individuals in the same species share a common gene pool. Because a population of individuals has a shared gene pool, a genetic change that occurs in one individual can spread through the population as that individual and its offspring reproduce. If a genetic change increases :fitness, that allele will eventually be found in many individuals of that population.

Isolating Mechanisms Given this genetic definition of species, what must happen for a species to evolve into two new species? The gene pools of two populations must become separated for them to become new species. · As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. At that point, the populations have separate gene pools. They respond to natural selection or genetic drift as separate units. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation.

Behavioral Isolation One type of isolating mechanism, behavioral isolation, occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. For example, the eastern and western meadowlarks shown in Figure

16-11 are very similar birds whose habitats overlap in the center of the United States. Members of the two species will not mate with each other, however, partly because they use different songs to attract mates. Eastern meadowlarks will not respond to western meadowlark songs, and vice versa.

Figure 16-11 The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs. Applying Concepts What type of reproductive isolation does this situation illustrate?

-

Geographic Isolation With geographic isolation, two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. The Abert squirrel in Figure 16-12, for example, lives in the Southwest. About 10,000 years ago, the Colorado River split the species into two separate populations. Two separate gene pools formed. Genetic changes that appeared in one group were not passed to the other. Natural selection worked separately on each group and led to the formation of a distinct subspecies, the Kaibab squirrel. The Abert and Kaibab squirrels have very similar anatomical and physiological characteristics, indicating that they are closely related. However, the Kaibab squirrel differs from the Abert squirrel in significant ways, such as fur coloring.

Geographic barriers do not guarantee the formation of new species, however. Separate lakes may be linked for a time during a :flood, or a land bridge may temporarily form between islands, enabling separated populations to mix. If two formerly separated populations can still interbreed, they remain a single species. Also, any potential geographic barrier may separate certain types of organisms but not others. A large river will keep squirrels and other small rodents apart, but it does not necessarily isolate bird populations.

Temporal Isolation A third isolating mechanism is temporal isolation, in which two or more species reproduce at different times. For example, three similar species of orchid all live in the same rain forest. Each species releases pollen only on a single day. Because the three species release pollen on different days, they cannot pollinate one another.

CHECKPOINT How can temporal isolation lead to speciation?

-

UTAH

Figure 16-12 ~When two populations of a species become reproductively isolated, new species can develop. The Kaibab squirrel evolved from the Abert squirrel. The Kaibab squirrels were isolated from the main population by the Colorado River.


Galapagos Islands Finches

Shape of Head and Beak

Common Vegetarian Large Woodpecker Cactus ground Sharp-beaked Large ground Name of tree finch insectivorous finch finch ground finch finch Finch Species tree finch

Main Food Fruits Insects Insects Cacti Seeds Seeds

Feeding Parrotlike Grasping Uses cactus Large crushing Pointed Large Adaptation beak beak spines beak crushing beak crushing beak

Habitat Trees Trees Trees Ground Ground Ground

A Figure 16-13 Detailed genetic studies have shown that these finches evolved from a species with a more-or-less general-purpose beak. Formulating Hypotheses Suggest how one of these beaks could have resulted from natural selection.

Go For: Links on

speciation Visit: www.Scilinks.org Web Code: cbn-5163

406 Chapter 16

Testing Natural Selection in Nature Now that you know the basic mechanisms of evolutionary change, you might wonder if these processes can be observed in nature. The answer is yes. In fact, some of the most important studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galapagos Islands.

Those finch species looked so different from one another that when Darwin first saw them, he did not realize they were all finches. He thought they were blackbirds, warblers, and other kinds of birds. The species he examined differed greatly in the sizes and shapes of their beaks and in their feeding habits, as shown in Figure 16-13. Some species fed on small seeds, while others ate large seeds with thick shells. One species used cactus spines to pry insects from dead wood. One species, not shown here, even pecked at the tails of large sea birds and drank their blood!

Once Darwin discovered that these birds were all finches, he hypothesized that they had descended from a common ancestor. Over time, he proposed, natural selection shaped the beaks of different bird populations as they adapted to eat different foods.

That was a reasonable hypothesis. But was there any way to test it? No one thought so, until the work of Peter and Rosemary Grant from Princeton University proved otherwise. For more than twenty years, the Grants, shown in Figure 16-14, have been collaborating to band and measure finches on the Galapagos Islands. They realized that Darwin's hypothesis relied on two testable assumptions. First, in order for beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection. Second, differences in beak size and shape must produce differences in fitness that cause natural selection to occur.

The Grants tested these hypotheses on the medium ground finch on Daphne Major, one of the Galapagos islands. This island is large enough to support good-sized finch populations, yet small enough to enable the Grants to catch and identify nearly every bird belonging to the species under study.

Variation The Grants first identified and measured as many individual birds as possible on the island. They recorded which birds were still living and which had died, which had succeeded in breeding and which had not. For each individual, they also recorded anatomical characteristics such as wing length, leg length, beak length, beak depth, beak color, feather colors, and total mass. Many of these characteristics appeared in bell-shaped distributions typical of polygenic traits. These data indicate that there is great variation of heritable traits among the Galapagos finches.

Natural Selection Other researchers who had visited the Galapagos did not see the different finches competing or eating different foods. During the rainy season, when these researchers visited, there is plenty of food. Under these conditions, finches often eat the most available type of food. During dry-season drought, however, some foods become scarce, and others disappear altogether. At that time, differences in beak size can mean the difference between life and death. To survive, birds become feeding specialists. Each species selects the type of food its beak handles best. Birds with big, heavy beaks, for example, select big, thick seeds that no other species can crack open.

The Grants' most interesting discovery was that individual birds with different-sized beaks had different chances of survival during a drought. When food for the finches was scarce, individuals with the largest beaks were more likely to survive, as shown in Figure 16-15. Beak size also plays a role in mating behavior, because big-beaked birds tend to mate with other big-beaked birds. The Grants observed that average beak size in that finch population increased dramatically over time. This change in beak size is an example of directional selection operating on an anatomical trait.

By documenting natural selection in the wild, the Grants provided evidence of the process of evolution: The next generation of finches had larger beaks than did the generation before selection had occurred. An important result of this work was their finding that natural selection takes place frequently-and sometimes very rapidly. Changes in the food supply on the Galapagos caused measurable fluctuations in the finch populations over a period of only decades. This is markedly different from the slow, gradual evolution that Darwin envisioned.

CHECKPOINT What type of natural selection did the Grants observe in the Galapagos?

Figure 16-14 Peter and Rosemary Grant have demonstrated that natural selection is still a

force in the evolution of the Galapagos finches. Applying Concepts How does their research demonstrate natural selection?

'Y Figure 16-15 This graph shows the survival rate of one species of ground-feeding finches, the medium ground finch, Geospiza fortis. Using Tables and Graphs What trend does this graph show?

Bird Survival Based on Beak Size

o -i...-ss:i=---1-----t~-1--1----1

6 7 8 9 10 11 12 13 Beak Size (mm)


To find out more about ongoing research on the

1-------~ Galapagos, view the segment "The Galapagos Islands: A Glimpse Into the Past," on Videotape Two.

Analyzing Data

Speciation in Darwin's Finches The Grants' work demonstrates that finch beak size can be changed by natural selection. If we combine this information with other evolutionary concepts you have learned in this chapter, we can show how natural selection can lead to speciation. We can devise a hypothetical scenario for the evolution of all Galapagos finches from a single group of founding birds. ~ Speciation in the Gulupugos finches occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, reproductive isolation, and ecological competition.

Founders Arrive Many years ago, a few finches from the South American mainland-species A-flew or were blown to one of the Galapagos Islands, as shown in Figure 16-16. Finches are small birds that do not usually fly far over open water. These birds may have gotten lost, or they may have been blown off course by a storm. Once they arrived on one of the islands, they managed to survive and reproduce.

Geographic Isolation Later on, some birds from species A crossed to another island in the Galapagos group. Because these birds do not usually fly over open water, they rarely move from island to island. Thus, finch populations on the two islands were essentially isolated from each other and no longer shared a common gene pool.

CHECKPOINT How did finches arrive in the Galapagos Islands?

How Are These Fish Related? A research team studied two lakes in an area that sometimes experiences flooding. Each lake contained two types of similar fish: a dull brown form and an iridescent gold form. The team wondered how all the fish were related, and they considered the two hypotheses diagrammed on the right.

1. Interpreting Graphics Study the two diagrams. What does hypothesis A indicate about the ancestry of the fish in Lake 1 and Lake 2? What does hypothesis B indicate?

2. Comparing and Contrasting According to the two hypotheses, what is the key difference in the way the brown and gold fish populations might have formed?

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A= Possible ancestor

B = Contemporary brown form

G = Contemporary gold form

-Shows possible line of descent

3. Drawing Conclusions A DNA analysis showed that the brown and gold fish from Lake 1 are the most closely related. Which hypothesis does this evidence support?

4. Asking Questions To help determine whether the brown and gold fish are members of separate species, what question might scientists ask?

.... Figure 16-16 Speciation in the Galapagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Changes in the Gene Pool Over time, populations on each island became adapted to their local environments. The plants growing on the first island may have produced small thin-shelled seeds, whereas the plants on the second island may have produced larger thick-shelled seeds. On the second island, directional selection would favor individuals with larger, heavier beaks. These birds could crack open and eat the large seeds more easily. Thus, birds with large beaks would be better able to survive on the second island. Over time, natural selection would have caused that population to evolve larger beaks, forming a separate population, B.

Reproductive Isolation Now, imagine that a few birds from the second island cross back to the first island. Will the population-A birds breed with the population-B birds? Probably not. These finches choose their mates carefully. As part of their courtship behavior, they inspect a potential partner's beak very closely. Finches prefer to mate with birds that have the samesized beak as they do. In other words, big-beaked birds prefer to mate with other big-beaked birds, and smaller-beaked birds prefer to mate with other smaller-beaked birds. Because the birds on the two islands have different-sized beaks, it is likely that they would not choose to mate with each other. Thus, differences in beak size, combined with mating behavior, could lead to reproductive isolation. The gene pools of the two bird populations remain isolated from each other-even when individuals live together in the same place. The two populations have now become separate species.

Ecological Competition As these two new species live together in the same environment (the first island), they compete with each other for available seeds. During the dry season, individuals that are most different from each other have the highest fitness. The more specialized birds have less competition for certain kinds of seeds and other foods, and the competition among individual finches is also reduced. Over time, species evolve in a way that increases the differences between them. The species-B birds on the first island may evolve into a new species, C.

Continued Evolution This process of isolation on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire Galapagos island chain. Over many generations, it produced the 13 different finch species found there today. Use the steps in this illustration to explain how other Darwin finches, such as the vegetarian tree finch that feeds on fruit, might have evolved.

Galapagos Islands

Founders Arrive

SOUTH AMERICA

A few finches travel from South America to one of the islands. There, they survive and reproduce.

Geographic Isolation Some birds from species A cross to a second island. The two populations no longer share a gene pool.

Changes in the Gene Pool Seed sizes on the second island favor birds with larger beaks. The population on the second island evolves into a population, B, with larger beaks. Eventually, populations A and B evolve into separate species.


.& Figure 16-17 Paleontologists study fossils to find clues about previous life forms.

1. · Key Concept How is reproductive isolation related to the formation of new species?

2. Key Concept What type of isolating mechanism was important in the formation of Galapagos finch species?

3. Explain how behavior can play a role in the evolution of species.

410 Chapter 16

Studying Evolution Since Darwin It is useful to review and critique the strengths and weaknesses of evolutionary theory. Darwin made bold assumptions about heritable variation, the age of Earth, and relationships among organisms. New data from genetics, physics, and biochemistry could have proved him wrong on many counts. They didn't. Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past.

Limitations of Research The Grants' research clearly shows the effects of directional selection in nature. The Grants' data also show how competition and climate change affect natural selection. The work does have limitations. For example, while the Grants observed changes in the size of the finches' beaks, they did not observe the formation'of a new sp cies. Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved.

Unanswered Questions The studies of the Grants fit into an enormous body of scientific work supporting the theory of evolution. Millions of fossils show that life has existed on Earth for more than 3 billion years and that organisms have changed dramatically over this time. These fossils form just a part of the evidence supporting the conclusion that life has evolved. Remember that a scientific theory is defined as a well-tested explanation that accounts for a broad range of observations. Evolutionary theory fits this definition. To be sure, many new discoveries have led to new hypotheses that refine and expand Darwin's original ideas. No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain.

Why is understanding evolution important? Because evolution continues today, driving changes in the living world such as drug resistance in bacteria and viruses, and pesticide resistance in insects. Evolutionary theory helps us understand and respond to these changes in ways that improve human life.

4. What recent research findings support Darwin's theory of evolution?

5. Critical Thinking Inferring Suppose that a drought on an island eliminates all but plants that produce large, tough seeds. All the finches on the island have very small beaks. How might this environmental change impact the survival of this finch population?

Summarizing Write a paragraph that summarizes the Grants' research with Galapagos finches. Your summary should include the main points of the research. Hint: The first sentence in your summary might state the Grants' hypothesis.

Exploration

Investigating Genetic Diversity in Bacteria Genetic diversity can make a population of organisms more adaptable. Some bacteria are able to survive in the presence of antibiotics that kill other bacteria. In this investigation, you will test a population of bacteria for the presence of antibiotic-resistant bacteria.

Problem How common are antibiotic-resistant bacteria?

Materials • liquid bacterial culture •forceps • sterile swabs • transparent tape • sterile agar plate • 70% alcohol • glass-marking pencil • metric ruler • antibiotic paper disks

Skills Observing, Analyzing Data

Procedure ~ El~ CJ 0 Wash your hands thoroughly with soap and

warm water. Without opening the agar plate, use a glass-marking pencil to draw two lines at right angles on the bottom of the plate. This will divide the plate into four equal areas, or quadrants. Label the quadrants 1 to 4, as shown. Write your initials on the plate.

Wearing plastic gloves, dip a sterile swab in the bacterial culture. Remove the cover of the agar plate and rub the swab gently over the entire surface of the agar. Immediately replace the cover. Follow your teacher's directions for disposing of the swab.

Remove the cover of the agar plate again. Use clean forceps to place an antibiotic disk on the agar in the center of each quadrant. Replace the cover; then tape the plate closed.

Place the plate upside down in the area designated by your teacher.

Return the forceps to your teacher for disinfection. Wipe your work surface with 70% alcohol and a paper towel. Wash your hands well with soap and warm water before leaving the lab.

After 24 hours, observe the growth of bacteria around each antibiotic disk. Record your observations. CAUTION: Do not open the plate.

Use the metric ruler to measure the diameter of the zone of reduced bacterial growth, called the zone of inhibition, around each antibiotic disk. Record the diameter of each zone of inhibition.

C) Carefully observe the zones of inhibition. Do you see any evidence of bacterial growth there? Record your observations. Give the used plate to your teacher for safe disposal. Wipe your work surface with 70% alcohol and a paper towel. Wash your hands well with soap and warm water before leaving the lab.

Analyze and Conclude 1. Observing How did the antibiotic disks affect

the growth of the bacteria?

2. Classifying What type of selection (directional, stabilizing, or disruptive) occurred in this experiment? Explain your answer.

3. Drawing Conclusions Did your data support the idea that antibiotic-resistant bacteria are common? Explain your answer.

4. Evaluating How do you know your data and conclusion are valid? (Hint: Compare your data and conclusion with those of other students.)

Go Further

Applying Concepts Investigate genetic diversity further by using your school library and the Internet to research the use of wild relatives of food crops to increase genetic variation in plants.


16-1 Genes and Variation Key Concepts

• In genetic terms, evolution is any change in the relative frequency of alleles in a population.

• Biologists have discovered that there are two main sources of genetic variation: mutations and the genetic shuffling that results from sexual reproduction.

• The number of phenotypes produced for a given trait depends on how many genes control the trait.

Vocabulary gene pool, p. 394 relative frequency, p. 394 single-gene trait, p. 395 polygenic trait, p. 396

16-2 Evolution as Genetic Change Key Concepts

• Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.

• Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.

• In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

• Five conditions are required to maintain genetic equilibrium from generation to generation: There must be random mating; the population must be very large; and there can be no movement into or out of the population, no mutations, and no natural selection.

Vocabulary directional selection, p. 398 stabilizing selection, p. 399 disruptive selection, p. 399 genetic drift, p. 400 founder effect, p. 400 Hardy-Weinberg principle, p. 401 genetic equilibrium, p. 401

412 Chapter 16

16-3 The Process of Speclation Key Concepts

• As new species evolve, populations become reproductively isolated from each other.

• Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, reproductive isolation, and ecological competition .

Vocabulary speciation, p. 404 reproductive isolation, p. 404 behavioral isolation, p. 404 geographic isolation, p. 405 temporal isolation, p. 405

Thinking Visually Using the information in this chapter, complete the following concept map about evolution of populations:

Natural

acts on I

can change by I

i

i 0

can be affected by I


1. The combined genetic information of all members of a particular population forms a a. gene pool. c. phenotype. b. niche. d. population.

2. The success of an organism in surviving and reproducing is a measure of its a. fitness. c. speciation. b. polygenic traits. d. gene pool.

3. Traits that are controlled by more than one gene, such as human height, are known as a. single-gene traits. c. recessive traits. b. polygenic traits. d. dominant traits.

4. The type of selection in which individuals of average size have greater fitness than small or large individuals is called a. disruptive selection. b. stabilizing selection. c. directional selection. d. genetic drift.

5. The type of selection in which individuals at one end of a curve have the highest fitness is called a. stabilizing selection. b. disruptive selection. c. directional selection. d. the founder effect.

6. If coat color in a rabbit population is a polygenic trait, which process might have produced the graph below?

o~ ca ... :::J Cl> 'tJ .c ·-E .2!: :::J 'tJ z .E

Darkness of Coat ____.

a. stabilizing selection b. disruptive selection c. directional selection d. genetic equilibrium

7. A random change in a small population's allele frequency is known as a. a gene pool. b. genetic drift. c. variation. d. fitness.


8. A change in allele frequency that results from the migration of a small subgroup of a population is called a. natural selection. b. the Hardy-Weinberg principle. c. the founder effect. d. genetic equilibrium.

9. A group of individuals of the same species that interbreed make up a a. species. c. population. b. gene pool. d. genetic drift.

10. The evolution of Darwin's finches is an example of a. equilibrium. c. stabilizing selection. b. speciation. d. artificial selection.

Understanding Concepts 11. Explain what the term relative frequency means.

Include an example in your answer.

12. Explain why sexual reproduction is a source of genetic variation.

13. Explain what determines the number of phenotypes for a given trait.

14. What is meant by the term single-gene trait?

15. Why are certain polygenic traits represented by a bell curve?

16. Define evolution in genetic terms.

17. How are speciation and reproductive isolation related?

18. How do stabilizing selection and disruptive selection differ?

19. What is genetic drift? In what kinds of situations is it likely to occur?

20. What is genetic equilibrium? What conditions are required to maintain genetic equilibrium?

21. Explain how isolation of groups can be involved in speciation.

22. What two testable assumptions were the basis for Darwin's hypothesis about the evolution of the Galapagos finches?

23. What evidence did the work of Rosemary and Peter Grant provide that strengthened Darwin's hypothesis about finch evolution in the Galapagos Islands?

24. Explain how the Galapagos finches may have evolved.


Critical Thinking The graph below shows data on the lengths of the beaks of three species of Darwin's finches. The percentage of individuals in each category of beak length is given. Use this information to answer questions 25-28.

Beak Length in Three Bird Species

~ Species A Species B Species C ~ 50.-------------------. ~ 40 ~ 30 ~ 204-~--.--~ 0 10 -··- --- -1/1 'E 0 -·1"--r--.--...........,."--T---.--.--_,..,,._,........._~~~~---4

iii 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Beak Length (mm)

25. Interpreting Graphics What is the shortest beak length observed in species A? About what percentage of the birds of species A have this beak length?

26. Interpreting Graphics What are the longest beak lengths of each of the three species?

27. Interpreting Graphics What is the range of beak lengths for the birds of species C?

28. Inferring Based on these data, what can you infer about the sizes of the seeds eaten by each of these species of birds?

29. Applying Concepts Suppose a rock slide isolates a very small number of animals from the rest of their population. How might this reproductive isolation impact the long-term survival of the new, smaller population? (Hint: Think of the role that genetic variation might play, both positively and negatively.)

30. Evaluating Darwin hypothesized that natural selection shaped the beaks of different finch populations on the Galapagos Islands. Describe how the Grants tested this hypothesis. Did their data support or refute Darwin's hypothesis? Explain.

31. Inferring How might a limited resource, such as food, affect the survival of an individual organism? How might a severe limitation affect the long-term survival of a species?

414 Chapter 16

32. Formulating Hypotheses A botanist identifies two distinct species of violets growing in a field. Also in the field are several other types of violets that, although somewhat similar to the two known species, appear to be new species. Develop a hypothesis explaining how the new species may have originated.

Viola Viola Other violets pedatifida sagittata

33. Connecting Concepts Sometimes biologists say, "Evolution is ecology over time." Use what you learned in Unit 2 to explain that statement.

v"'"'\Nriting in Science When you write a summary, you use your own words to express the main ideas of something you have read or heard. Write a summary of the ways in which natural selection operates on polygenic traits. (Hint: Use the graphs in Figures 16-6, 16-7, and 16-8 to help identify the main ideas.)

Performance-Based Assessment In Your Community Use field guides or scientific literature to identify a species of tree, flowering plant, or insect in your neighborhood. Then investigate several examples of that species, noting the variations that you observe. Document the variations, using descriptive notes along with photographs or drawings. Describe how the variations may have contributed to the evolution of the species.

cGo nline .__ -----¥PHSchool.com

For: An interactive self-test Visit: PHSchool.com Web Code: cba-5160

Test-Taking Tip If you have trouble answering

a question, make a mark beside it and go on. (Do not write in this book.) You may find information in later questions that will allow you to eliminate some answer choices in your unanswered question.


1. Which of the following conditions is likely to result in speciation? (A) random mating (B) small population size (C) no migrations into or out of the population (D) absence of natural selection (E) lack of mutations

2. Which of the following is a source of genetic variation?

I. Mutations II. Polygenic traits

III. Genetic shuffling that results from sexual reproduction

(A) I only (B) II only (C) I and III only

(D) II and III only (E) I, II, and III

3. In a population of lizards, the smallest and largest lizards are more easily preyed upon than middlesized lizards. What kind of natural selection is most likely to occur in this situation? (A) genetic drift (B) sexual selection (C) stabilizing selection (D) directional selection (E) disruptive selection

4. When two species reproduce at different times, the situation is called (A) temporal isolation. (B) speciation. (C) genetic drift. (D) temporal selection. (E) geographic isolation.

5. A situation in which a population's allele frequencies remain relatively constant is called (A) genetic equilibrium. (B) polygenic traits. (C) a gene pool. (D) fitness. (E) genetic variation.


Questions 6-8 Each of the lettered choices below refers to the following numbered statements. Select the best lettered choice. A choice may be used once, more than once, or not at all.

(A) Fitness (B) Single-gene trait (C) Polygenic trait (D) Hardy-Weinberg principle (E) Gene pool

6. The combined genetic information of all members of a particular population

7. Survival and reproduction of individuals best suited to their environment

8. Characteristic of the traits that Mendel tracked in pea plants

Questions 9- 7 0

The graphs show the changes in crab color at one beach.

Graph A (1950) Graph B (1990)

- -Light Medium Dark Light Medium Dark tan tan tan tan tan tan

Crab Body Color Crab Body Color

9. What process occurred over the 40-year period? (A) artificial selection (B) sexual selection (C) stabilizing selection (D) disruptive selection (E) directional selection

10. Which of the following is most likely to have caused the change in distribution? (A) A new predator prefers dark-tan crabs. (B) A new predator prefers light-tan crabs. (C) A new beach color makes medium-tan crabs

the least visible to predators. (D) A new beach color makes medium-tan crabs

the most visible to pr • Which animals in a population are considered the

most genetically valuable?


Inquiry Activity

How can you date a rock 1

Procedure 1. Examine a piece of shale with a hand lens. This rock

formed from the sediment deposited at the bottom of an ancient lake. As the shale formed, one dark layer and one light layer were deposited each year.

2. Place a transparent metric ruler next to the shale sample. Count and record the number of dark layers in a 5-mm section of the shale.

About 25 million years ago, this scorpion was caught in sticky tree resin, which later hardened into amber. Fossils like this one provide evidence that enables scientists to build up a picture of Earth's history.

3. Divide your result in step 2 by 5 to determine the average number of layers per millimeter.

Think About It

1. Inferring How many years did it take for your specimen to form?

2. Calculating Suppose your specimen came from a deposit of shale that is 600 meters thick. How long did it take for the complete deposit to form?

17- 1 The Fossil Record

T he history oflife onEarth is filled with mystery, life-anddeath struggles, and bizarre plants and animals as amazing

as any mythological creatures. Studying life's history is one of the most fascinating and challenging parts of biology, and researchers go about it in several ways. One technique is to read the pieces of the story that are "written" in ancient rocks, in the petrified sap of ancient trees, in peat bogs and tar pits, and in polar glaciers. You may recall that these traces and preserved remains of ancient life are called fossils.

Fossils and Ancient Life Paleontologists (pay-lee-un-TAHL-uh-jists) are scientists who study fossils. They collect fossils such as the one shown in Figure 17-1. From these fossils, they infer what past life forms were like-the structure of the organisms, what they ate, what ate them, and the environment in which they lived. Paleontologists also classify fossil organisms. They group similar organisms together and arrange them in the order in which they livedfrom oldest to most recent. Together, all this information about past life is called the fossil record. · The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.

The fossil record reveals a remarkable fact: Fossils occur in a particular order. Certain fossils appear only in older rocks, and other fossils appear only in more recent rocks. In other words, the fossil record shows that life on Earth has changed over time. In fact, more than 99 percent of all species that have ever lived on Earth have become extinct, which means the species died out. Meanwhile, over billions of years, ancient unicellular organisms have given rise to the modern bacteria, protists, fungi, plants, and animals that you will study in later units .

.... Figure 17-1 This remarkably complete dinosaur fossil reveals many characteristics of the original animal. Observing What anatomical similarities can you observe between this fossil and any organism alive today?

Guide for Reading

Key Concepts • What is the fossil record? • What information do relative

dating and radioactive dating provide about fossils?

• What are the main divisions of the geologic time scale?

Vocabulary paleontologist •fossil record extinct • relative dating index fossil • half-life radioactive dating geologic time scale • era period

Reading Strategy: Finding Main Ideas Before you read, write down this idea: Scientists use the fossil record to learn about the history of life on Earth. As you read, make a list of the kinds of evidence that support this main idea.

The History of Life 417

D Water carries small rock particles to lakes and seas.

Dead organisms are buried by layers of sediment, which forms new rock.

The preserved remains may later be discovered and studied.

.A Figure 17-2 The fossil record provides evidence about the history of life on Earth. Most fossils are formed in sedimentary rock.

Go nline active ar

For: Fossil Formation activity Visit: PHSchool.com Web Code: cbp-5171

418 Chapter 17

How Fossils Form A fossil can be as large and complete as an entire, perfectly preserved animal, or as small and incomplete as a tiny fragment of a jawbone or leaf There are fossil eggs, fossil footprints, and even fossilized animal droppings. For a fossil to form, either the remains of the organism or some trace of its presence must be preserved. The formation of any fossil depends on a precise combination of conditions. Because of this, the fossil record provides incomplete information about the history of life. For every organism that leaves a fossil, many more die without leaving a trace.

Most fossils form in sedimentary rock, as shown in Figure 17-2. Sedimentary rock is formed when exposure to rain, heat, wind, and cold breaks down existing rock into small particles of sand, silt, and clay. These particles are carried by streams and rivers into lakes or seas, where they eventually settle to the bottom. As layers of sediment build up over time, dead organisms may also sink to the bottom and become buried. If conditions are right, the remains may be kept intact and free from decay. The weight of layers of sediment gradually compresses the lower layers and, along with chemical activity, turns them into rock.

The quality of fossil preservation varies. In some cases, the small particles of rock surrounding the remains of an organism preserve an imprint of its soft parts. In other cases, the hard parts are preserved when wood, shells, or bones are saturated or replaced with long-lasting mineral compounds. Occasionally, organisms are buried quickly in finegrained clay or volcanic ash before they begin to decay, so they are perfectly preserved.

CHECXPOINT Why is the fossil record described as an incomplete record of life's history?

Interpreting Fossil Evidence The natural forces that form sedimentary rock can also reveal fossils that have been hidden in layers of rock for millions of years. Forces inside Earth lift rocks up into mountain ranges, where wind, rain, and running water erode the rock. Bit by bit, water and wind wear away the upper, younger layers, exposing the older fossil-bearing layers beneath.

When a fossil is exposed, a fortunate (and observant) paleontologist may happen along at just the right time and remove the fossil for study.

Paleontologists occasionally unearth the remains of an entire organism. More often, though, they must reconstruct an extinct species from a few fossil bits-remains of bone, a shell, leaves, or pollen. When paleontologists study a fossil, they look for anatomical similarities-and differences-between the fossil and living organisms. Also, a fossil's age is extremely important. Paleontologists determine the age of fossils using two techniques: relative dating and radioactive dating.

Relative Dating About two centuries ago, geologists noted that rock layers containing certain fossils consistently appeared in the same vertical order no matter where they were found. Also, a particular species of trilobite-a common fossil and an extinct relative of horseshoe crabs-might be found in one rock layer but be absent from layers above or below it. How might such a pattern be useful?

In relative dating, the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock, as shown in Figure 17-3. Recall that sedimentary rock is formed from the gradual deposition of layers of sand, rock, and other types of sediment. The rock layers form in order by agethe oldest layers on the bottom, with more recent layers on top, closer to Earth's surface.

Scientists also use index fossils to compare the relative ages of fossils. To be used as an index fossil, a species must be easily recognized and must have existed for a short period but have had a wide geographic range. As a result, it will be found in only a few layers of rock, but these specific layers will be found in different geographic locations. Relative dating allows paleontologists to estimate a fossil's age compared with that of other fossils. However, it provides no information about its absolute age, or age in years.

Word Origins .., ________ .... ______ iiiiil

The word part paleo- means "ancient" or "early," and -zoic means "life." The word part mesomeans "middle." The word part ceno- means "recent." Use this information to explain the meanings of paleozoic, mesozoic, and cenozoic.

T Figure 17-3 ~In relative dating, a paleontologist estimates a fossil's age in comparison with that of other fossils. Each of these fossils is an index fossil. It enables scientists to date the rock layer in which it is found. Scientists can also use index fossils to date rocks from different locations.

Quick Lab

What is a half-life? Materials 100 1-cm squares of paper, plastic or paper cup

Procedure ftn 1. Construct a data table with

2 columns and 5 blank rows. Label the columns "Spill Number" and "Number of Squares Returned."

2. Place an X on each square of paper, and put all the squares in the cup.

3. Mix up the squares in the cup. Then, spill them out and separate all squares that overlap.

4. Remove the squares that have an X showing. Record the number of squares remaining and return them to the cup.

5. Repeat steps 3 and 4 until there are 5 or fewer squares remaining. Make a graph of your results with the number of spills on the x-axis and the number of squares remaining on the y-axis.

Analyze and Conclude 1. Analyzing Data How many

spills were required to remove half of the squares? To remove three fourths?

2. Calculating If each spill represents one year, what is the half-life of the squares?

To find out about radioactive dating, view the segment "Mummies: Ties

to the Past," on Videotape Two.

420 Chapter 17

Radioactive Dating Scientists use radioactive decay to assign absolute ages to rocks. Some elements found in rocks are radioactive. Radioactive elements decay, or break down, into nonradioactive elements at a steady rate, which is measured in a unit called a half-life. A half-life is the length of time required for half of the radioactive atoms in a sample to decay. As shown in Figure 17-4, after one half-life, halfofthe original radioactive atoms in a sample have decayed. Of those remaining atoms, half again are decayed after another half-life.

Radioactive dating is the use of half-lives to determine the age of a sample. · In radioactive dating, scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains. Different radioactive elements have different half-lives and therefore provide natural clocks that "tick" at different rates. Carbon-14, for example, has a half-life of about 5730 years. Carbon-14 is taken up by living things while they are alive. After an organism dies, the carbon-14 in its body begins to decay to form nitrogen-14, which escapes into the air. Carbon-12, the most common isotope of carbon, is not radioactive and does not decay. By comparing the amounts of carbon-14 and carbon-12 in a fossil, researchers can determine when the organism lived. The more carbon-12 there is in a sample compared to carbon-14, the older the sample is.

Because carbon-14 has a relatively short half-life, it is useful only for dating fossils younger than about 60,000 years. To date older rocks, researchers use elements with longer half-lives. Potassium-40, for example, decays to the inert gas argon-40 and has a half-life of 1.26 billion years.

CHECKPOINT What is a half-life?

Radioactive Decay of Potassium-40

0 2 3 4 5 Time (billions of years)

.& Figure 17-4 ~ Radioactive dating involves measuring the amounts of radioactive isotopes in a sample to determine its actual age. Such measurements enable scientists to determine the absolute age of rocks and the fossils they contain.

Geologic Time Scale Paleontologists use divisions of the geologic time scale to represent evolutionary time. Figure 17-5 shows the most recent version of the geologic time scale. Scientists first developed the geologic time scale by studying rock layers and index fossils worldwide. With this information, they placed Earth's rocks in order according to relative age. As geologists studied the fossil record, they found major changes in the fossil animals and plants at specific layers in the rock. These times were used to mark where one segment of geologic time ends and the next beginslong before anyone knew how long these various segments actually were.

Years later, radioactive dating techniques were used to assign specific ages to the various rock layers. Not surprisingly, the divisions of the geologic time scale did not turn out to be of standard lengths, such as 100 million years. Instead, geologic divisions vary in duration by many millions of years. Scientists use several levels of divisions for the geologic time scale. Geologic time begins with Precambrian (pree-KAM-bree-un) Time. Although few multicellular fossils exist from this time, the Precambrian actually covers about 88 percent of Earth's history as shown in Figure 17-6 on the next page. ~After Precambrian Time, the basic divisions of the geologic time scale are eras and periods.

Eras Geologists divide the time between the Precambrian and the present into three eras. They are the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. The Paleozoic (pay-lee-oh~ZOH-ik) began about 544 million years ago and lasted for almost 300 million years. Many vertebrates and invertebrates-animals with and without backbones-lived during the Paleozoic.

The Mesozoic (mez-uh-ZOH-ik) began about 245 million years ago and lasted about 180 million years. Some people call the Mesozoic the Age of Dinosaurs, yet dinosaurs were only one of many kinds of organisms that lived during this era. Mammals began to evolve during the Mesozoic.

Earth's most recent era is the Cenozoic (sen-uh-ZOH-ik). It began about 65 million years ago and continues to the present. The Cenozoic is sometimes called the Age of Mammals because mammals became common during this time.

Geologic Time Scale

Era Period Time (millions of years ago)

Quaternary 1.8 - present

Tertiary 65 - 1.8

€t.etaceou 145 - 65

Jurassic 208 - 145

Triassic 245 - 208

Carbonlferovs 360 - 290

Devonian 410 - 360

Silu~ian 440 - 410

Ordovician 505-440

Cambrian 544- 505

Verid iar.t 650 - 544

• Figure 17-5 ~The basic units of the geologic time scale after Precambrian Time are eras and periods. Each era is divided into periods.

Go For: Links on the

fossil record Visit: www.Scilinks.org Web Code: cbn-5171


• Cenozoic Era

• Mesozoic Era

Radiation of mammals

First humans

• Paleozoic Era First land --L--

plants • Precambrian Time

First prokaryotes

First multicellular -,::------lii:w organisms

First eukaryotes

~:ii;Ll:il~~~---Accumulation of atmospheric oxygen

.A Figure 17-6 Earth's history is often compared to a familiar measurement, such as the twelve hours between noon and midnight. In such a comparison, notice that Precambrian Time lasts from noon until after 10:30 PM. Interpreting Graphics Using this model, about what time did life appear? The first plants? The first humans?

1. ~Key Concept What can be learned from the fossil record?

2. ~Key Concept Which type of dating provides an absolute age for a given fossil? Describe how this is done.

3. · Key Concept How are eras and periods related?

4. How do fossils form?

422 Chapter 17

Periods Eras are subdivided into periods, which range in length from tens of millions of years to less than two million years. The Mesozoic Era, for example, includes three periods: the Triassic Period, the Jurassic Period, and the Cretaceous Period. Many periods are named for places around the world where geologists first described the rocks and fossils of that period. The name Cambrian, for example, refers to Cambria, the old Roman name for Wales. Jurassic refers to the Jura Mountains in France. The Carboniferous ("carbon-bearing'') Period, on the other hand, is named for the large coal deposits that formed during that period.

5. What geologic era is known as the Age of Mammals? When did this era begin?

6. Critical Thinking Drawing Conclusions Many more fossils have been found since Darwin's day, allowing several gaps in the fossil record to be filled. How might this information make relative dating more accurate?

Constructing a Time Line Create a time line that shows the four main divisions in the geologic time scale and the key events that occurred during those divisions. Then, as you read Section 17-3, add more events to your time line ..

17-2 Earth's Early History

I f ]jfe comes only from life, then how did life on Eaxth first begin? This section presents the current scientific view of

events on the early Earth. These hypotheses, however, are based on a relatively small amount of evidence. The gaps and uncertainties make it likely that scientific ideas about the origin of life will change.

Formation of Earth Geologic evidence shows that Earth, which is about 4.6 billion years old, was not "born" in a single event. Instead, pieces of cosmic debris were probably attracted to one another over the course of about 100 million years. While the planet was young, it was struck by one or more objects, possibly as large as the planet Mars. This collision produced enough heat to melt the entire globe.

Once Earth melted, its elements rearranged themselves according to density. The most dense elements formed the planet's core. There, radioactive decay generated enough heat to convert Earth's interior into molten rock. Moderately dense elements floated to the surface, much as fat floats to the top of hot chicken soup. These elements ultimately cooled to form a solid crust. The least dense elements-including hydrogen and nitrogen-formed the first atmosphere.

This infant planet was very different from today's Earth. Figure 17-7 shows how it might have looked. The sky was probably not blue but pinkish-or ange. Earth's early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water. Had you been there, a few deep breaths would have killed you!

, ~ i r'- '

Guide for Reading

Key Concepts • Wha t substances made up

Earth's early atmosphere? • What did Miller and Urey's

experiments show? • What occurred when oxygen

was added to Earth's atmosphere?

• What hypothesis explains the origin of eukaryotic cells?

Vocabulary proteinoid microsphere microfossil endosymbiotic theory

Reading Strategy: Making Comparisons Before you read, write three sentences about Earth as it is today. As you read, write three sentences that describe how Earth was very different in the past.

'. . ' ll• . •

'( . ! . . ~. • • lo. < "ff Figure 17-7 The early Earth was much hotter than it

is now, and there was little or no oxygen in the atmosphere. ~ Earth's early atmosphere was probably made up of hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.

~

---

'/ ,,<:J ( l Condensation -l

chamber +-I Cold

water cools chamber, causing droplets to form.

~Water vapor

Liquid containing amino acids and other organic compounds

.& Figure 17-8 Miller and Urey produced amino acids, which are needed to make proteins, by passing sparks through a mixture of hydrogen, methane, ammonia, and water. ~This and other experiments suggested how simple compounds found on the early Earth could have combined to form the organic compounds needed for life.

424 Chapter 17

Geologists infer that about 4 billion years ago, Earth cooled enough to allow the first solid rocks to form on its surface. For millions of years afterward, violent volcanic activity shook Earth's crust. Comets and asteroids bombarded its surface. Oceans did not exist because the surface was extremely hot.

About 3.8 billion years ago, Earth's surface cooled enough for water to remain a liquid. Thunderstorms drenched the planet, and oceans covered much of the surface. Those primitive oceans were brown because they contained lots of dissolved iron. The earliest sedimentary rocks, which were deposited in water, have been dated to this period. This was the Earth on which life appeared.

CHECKPQJl.Yk Why did the early Earth not have oceans?

The First Organic Molecules For several reasons, atoms do not assemble themselves into complex organic molecules or living cells on Earth today. For one thing, the oxygen in the atmosphere is very reactive and would destroy many kinds of organic molecules not protected within cells. In addition, as soon as organic molecules appeared, something-bacteria or some other life form-would probably eat them! But the early Earth was a very different place. Could organic molecules have evolved under those conditions?

In the 1950s, American chemists Stanley Miller and Harold Urey tried to answer that question by simulating conditions on the early Earth in a laboratory setting. They filled a flask with hydrogen, methane, ammonia, and water to represent the atmosphere. They made certain that no microorganisms could contaminate the results. Then, as shown in Figure 17-8, they passed electric sparks through the mixture to simulate lightning.

The results were spectacular. Over a few days, several amino acids-the building blocks of proteins-began to accumulate. ~Miller and Urey's experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth. Scientists now know that Miller and Urey's original simulations of Earth's early atmosphere were not accurate. However, similar experiments based on more current knowledge of Earth's early atmosphere have also produced organic compounds. In fact, one of Miller's experiments in 1995 produced cytosine and uracil, two of the bases found in RNA.

The Puzzle of Life's Origins A stew of organic molecules is a long way from a living cell, and the leap from nonlife to life is the greatest gap in scientific hypotheses of Earth's early history. Geological evidence suggests that about 200 to 300 million years after Earth cooled enough to carry liquid water, cells similar to modern bacteria were common. How might these cells have originated?

Formation of Microspheres Under certain conditions, large organic molecules can form tiny bubbles called proteinoid microspheres, as shown in Figure 17-9. Microspheres are not cells, but they have some characteristics of living systems. Like cells, they have selectively permeable membranes through which water molecules can pass. Microspheres also have a simple means of storing and releasing energy. Several hypotheses suggest that structures similar to proteinoid microspheres might have acquired more and more characteristics of living cells.

Evolution of RNA and DNA Another unanswered question in the evolution of cells is the origin of DNA and RNA Remember that all cells are controlled by information stored in DNA, which is transcribed into RNA and then translated into proteins. How could this complex biochemical machinery have evolved?

Science cannot yet solve this puzzle, although molecular biologists have made surprising discoveries in this area. Under the right conditions, some RNA sequences can help DNA replicate. Other RNA sequences process messenger RNA after transcription. Still others catalyze chemical reactions. Some RNA molecules can even grow and duplicate themselvessuggesting that RNA might have existed before DNA A series of experiments that simulated conditions of the early Earth have suggested that small sequences of RNA could have formed and replicated on their own. From this relatively simple RNA-based form oflife, several steps could have led to the system ofDNAdirected protein synthesis that exists now. This hypothesis is shown in Figure 17-10. Future experiments are aimed at refining and retesting this hypothesis.

_... Simple organic _.... ------...- molecules ---,..-

(magnification: about 1 O,OOOX)

A Figure 17-9 Large organic molecules can sometimes form tiny proteinoid microspheres like the ones shown here. Comparing and Contrasting How are proteinoid microspheres similar to cells? How are they different?

'Y Figure 17-10 One hypothesis about the origin of life, illustrated here, suggests that RNA could have evolved before DNA. Scientists have not yet demonstrated the later stages of this process in a laboratory setting. Interpreting Graphics How might RNA have stored genetic information?

• /

Proteins build cell structures and catalyze chemical reactions

? ... • RNA helps in

protein synthesis

Abiotic "stew" of inorganic matter

RNA nucleotides • RNA able to replicate itself, synthesize proteins, and function in information storage DNA functions in

information storage and retrieval


.... Figure 17-11 ~Ancient photosynthetic organisms produced a rise in oxygen in Earth's atmosphere. These rocklike formations, called stromatolites, were made by cyanobacteria, which were probably among the earliest organisms to evolve on Earth.

426 Chapter 17

Free Oxygen Microscopic fossils, or microfossils, of single-celled prokaryotic organisms that resemble modern bacteria have been found in rocks more than 3.5 billion years old, as shown in Figure 17-11. Those first life forms must have evolved in the absence of oxygen, because Earth's first atmosphere contained very little of that highly reactive gas.

Over time, as indicated by fossil evidence, photosynthetic bacteria became common in the shallow seas of the Precambrian. By 2.2 billion years ago at the latest, these organisms were steadily churning out oxygen, an end product of photosynthesis. One of the first things oxygen did was to combine with iron in the oceans. In other words, it caused the oceans to rust! When iron oxide was formed, it fell from the sea water to the ocean floor. There, it formed great bands of iron that are the source of most of the iron ore mined today. Without iron, the oceans changed color from brown to blue-green.

Next, oxygen gas started accumulating in the atmosphere. As atmospheric oxygen concentrations rose, concentrations of methane and hydrogen sulfide began to decrease, the ozone layer began to form, and the skies turned their present shade of blue. Over the course of several hundred million years, oxygen concentrations rose until they reached today's levels.

Biologists hypothesize that the increase in this highly reactive gas created the first global "pollution" crisis. To the first cells, oxygen was a deadly poison! · The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration. Organisms that had evolved in an oxygen-free atmosphere were forced into a few airless habitats, where their anaerobic descendants remain today. Some organisms, however, evolved ways of using oxygen for respiration and protecting themselves from oxygen's powerful reactive abilities. The stage was set for the evolution of modern life.

CHECKPOll>IT What process added oxygen to Earth's atmosphere?

Origin of Eukaryotic Cells Several important events in the history of life have been revealed through molecular studies of cells and their organelles. One of these events is the origin of eukaryotic cells, which are cells that have nuclei . About 2 billion years ago, prokaryotic cells-cells without nuclei-began evolving internal cell membranes. The result was the ancestor of all eukaryotic cells.

The Endosymbiotic Theory Then, something radical seems to have happened. Other prokaryotic organisms entered this ancestral eukaryote. These organisms did not infect their host, as parasites would have done, and the host did not digest them, as it would have digested prey. Instead, the smaller prokaryotes began living inside the larger cell, as shown in Figure 17-12. Over time, a symbiotic, or interdependent, relationship evolved. According to the endosymbiotic theory, eukaryotic cells formed from a symbiosis among several different proka.ryotic organisms. One group of prokaryotes had the ability to use oxygen to generate energy-rich molecules of ATP. These evolved into the mitochondria that are now in the cells of all multicellular organisms. Other prokaryotes that carried out photosynthesis evolved into the chloroplast s of plants and algae.

The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.

This hypothesis was proposed more than a century ago, when microscopists saw that the membranes of mitochondria and chloroplasts resembled the plasma membranes of free-living prokaryotes. Yet, the endosymbiotic theory did not receive much support until the 1960s, when it was championed by Lynn Margulis of Boston University.

Aerobic -- Ancient Prokaryotes bacteria

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T Figure 17-12 ~The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. Ancient prokaryotes may have entered primitive eukaryotic cells and remained there as organelles.

Chloroplast

Plants and plantlike protists

Animals, fungi, and non-plantlike protists


A Figure 17-13 This ancient jellyfish, an early multicellular animal from Precambrian Time, did not have bones or other hard parts, but it left behind a fossil that allowed biologists to infer its overall shape. Observing What evidence shows that this organism had body parts arranged around a central point?

1. Key Concept What substances probably made up Earth's early atmosphere?

2. Key Concept What molecules were the end products in Miller and Urey's experiments?

3. · Key Concept How did the addition of oxygen to Earth's atmosphere affect life of that time? 1

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The Evidence Lynn Margulis and her supporters built their argument on several pieces of evidence: First, mitochondria and chloroplasts contain DNA similar to bacterial DNA. Second, mitochondria and chloroplasts have ribosomes whose size and structure closely resemble those of bacteria. Third, like bacteria, mitochondria and chloroplasts reproduce by binary fission when the cells containing them divide by mitosis. Thus, mitochondria and chloroplasts have many of the features of free-living bacteria. These similarities provide strong evidence of a common ancestry between free-living bacteria and the organelles of living eukaryotic cells.

Sexual Reproduction and Multicellularity Some time after eukaryotic cells arose, those cells began to reproduce sexually. This development enabled evolution to take place at far greater speeds than ever before. How did sexual reproduction speed up the evolutionary process?

Most prokaryotes reproduce asexually. Often, they simply duplicate their genetic material and divide into two new cells. Although this process is efficient, it yields daughter cells that are exact duplicates of the parent cell. This type of reproduction restricts genetic variation to mutations in DNA. Sexual reproduction, on the other hand, shuffles and reshuffles genes in each generation, much like a person shuffling a deck of cards. The offspring of sexually reproducing organisms, therefore, never resemble their parents exactly. By increasing the number of gene combinations, sexual reproduction increases the probability that favorable combinations will be produced. Favorable gene combinations greatly increase the chances of evolutionary change in a species due to natural selection.

A few hundred million years after the evolution of sexual reproduction, evolving life forms crossed another great threshold: the development of multicellular organisms from singlecelled organisms. These first multicellular organisms, such as the one shown in Figure 17-13, experienced a great increase in diversity. The evolution of life was well on its way.

4. Key Concept According to the endosymbiotic theory, how might chloroplasts and mitochondria have originated?

5. Critical Thinking Predicting You just read that life arose from non life billions of years ago. Could life arise from nonlife today? Explain.

Eukaryotic Cells The endosymbiotic theory accounts for the evolution of mitochondria and chloroplasts in eukaryotic cells. Review the description of eukaryotic cells in Chapter 7, and then describe the structure and function of mitochondria and chloroplasts.

17- 3 Evolution of Multicellular Life

A lthough the fossil record has missing pieces, paleontologists have assembled good evolutionary histories for many

groups of organisms. Furthermore, the fossil record indicates that major changes occurred in Earth's climate, geography, and life forms. In this section, you will get an overview of how multicellular life evolved from its earliest forms to its presentday diversity.

Precambrian Time Recall that almost 90 percent of Earth's history occurred during the Precambrian. During this time, simple anaerobic forms of life appeared and were followed by photosynthetic forms, which added oxygen to the atmosphere. Aerobic forms oflife evolved, and eukaryotes appeared. Some of those organisms gave rise to multicellular forms that continued to increase in complexity. Few fossils exist from this time because the animals were all soft-bodied. Life existed only in the sea.

Paleozoic Era ~ Rich fossil evidence shows that early in the Paleozoic Era, there was a diversity of marine life. Scientists once thought that those different forms oflife evolved rapidly at the beginning of the Paleozoic, but increasing evidence from Precambrian fossils and DNA studies suggests that life began to diversify much earlier. Regardless of when these forms evolved, fossil evidence shows that life was highly diverse by the first part of the Paleozoic Era, the Cambrian Period. An artist's portrayal of Cambrian life, which included many kinds of invertebrate animals, is shown in Figure 17-14.

Guide for Reading

-.--------------~ ~ Key Concept • What were the characteristic

forms of life in the Paleozoic, Mesozoic, and Cenozoic eras?

Vocabulary mass extinction

Reading Strategy: Using Graphic Organizers As you read, make a table of the three geologic eras described in the section. Include information about the typical organisms and main evolutionary events of each era.

T Figure 17-14 ~The fossil record shows evidence of many types of marine life early in the Paleozoic Era. These and other unfamiliar organisms dwelt in the sea during the Cambrian Period, a time when animals with hard parts evolved.

.& Figure 17-15 During the Ordovician Period, aquatic arthropods like this eurypterid evolved. Eurypterids had segmented bodies and lived in water. Some of them grew to a length of almost 1 3 meters. Eurypterids are now extinct. Comparing and Contrasting Which of today's animals do eurypterids resemble?

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Cambrian Period Paleontologists call the diversification of life during the early Cambrian Period the "Cambrian Explosion." For the first time, many organisms had hard parts, including shells and outer skeletons. During the Cambrian Period, the first known representatives of most animal phyla evolved. Invertebrates-such as jellyfishes, worms, and sponges-drifted through the water, crawled along the sandy bottom, or attached themselves to the ocean floors. Brachiopods, which were small animals with two shells, were especially common. They resembled-but were unrelated to-modern clams. Trilobites were also common. Trilobites were arthropods, which are invertebrates with segmented bodies, jointed limbs, and an external skeleton.

CH~CKPOINT,, What is the "Cambrian Explosion"?

Ordovician and Silurian Periods During the Ordovician (awr-duh-VISH-un) and Silurian (sih-LOOR-

ee-un) periods, the ancestors of the modern octopi and squid appeared, as did aquatic arthropods like the one in Figure 17-15. Some arthropods became the first animals to live on land. Among the first vertebrates (animals with backbones) to appear were jaw less fishes, which had suckerlike mouths. The first land

plants evolved from aquatic ancestors. These simple plants grew low to the ground in damp areas.

Devonian Period By the Devonian (dih-VOH-nee-un) Period, some plants, such as ferns, had adapted to drier areas, allowing them to invade more habitats. Insects, which are arthropods, appeared on land. In the seas, both invertebrates and vertebrates thrived. Even though the invertebrates were far more numerous, the Devonian is often called the Age of Fishes because many groups of fishes were present in the oceans. Most fishes of this time had jaws, bony skeletons, and scales on their bodies. Sharks appeared in the late Devonian.

During the Devonian, vertebrates began to invade the land. The first fishes to develop the ability to crawl awkwardly on leglike fins were still fully aquatic animals. Some of these early four-legged vertebrates evolved into the first amphibians. An amphibian (am-FIB-ee-un) is an animal that lives part of its life on land and part of its life in water.

Carboniferous and Permian Periods Throughout the rest of the Paleozoic Era, life expanded over Earth's continents. Other groups of vertebrates, such as reptiles, evolved from certain amphibians. Reptiles are animals that have scaly skin and lay eggs with tough, leathery shells. Winged insects evolved into many forms, including huge dragonflies and cockroaches. Giant ferns and other plants formed vast swampy forests, shown in Figure 17-16. The remains of those ancient plants formed thick deposits of sediment that changed into coal over millions of years, giving the Carboniferous its name.

At the end of the Paleozoic, many organisms died out. This was a mass extinction, in which many types ofliving things became extinct at the same time. The mass extinction at the end of the Paleozoic affected both plants and animals on land and in the seas. As much as 95 percent of the complex life in the oceans disappeared. For example, trilobites, which had existed since early in the Paleozoic, suddenly became extinct. Many amphibians also became extinct. Not all organisms disappeared, however. The mass extinction did not affect many fishes. Numerous reptiles also survived.

Mesozoic Era The Mesozoic Era lasted approximately 180 million years.

Events during the Mesozoic include the increasing dominance of dinosaurs. The Mesozoic is marked by the appearance of flowering plants.

Triassic Period Those organisms that survived the Permian mass extinction became the main forms oflife early in the Triassic (try-AS-ik) Period. Important organisms in this new ecosystem were fishes, insects, reptiles, and conebearing plants like the one in Figure 17-17. Reptiles were so successful during the Mesozoic Era that this time is often called the Age of Reptiles.

About 225 million years ago, the first dinosaurs appeared. One of the earliest dinosaurs, Coelophysis, was a meat-eater that had light, hollow bones and ran swiftly on its hind legs. Mammals also first appeared during the late Triassic Period, probably evolving from mammallike reptiles. Mammals of the Triassic were very small, about the size of a mouse or shrew.

_.. Figure 17-16 Ancient forests like this one from the Carboniferous Period were characterized by a huge variety of life forms. At the end of the Paleozoic Era, many types of animals and plants became extinct.

~ Figure 17-17 Among the seed plants of the Triassic Period were cone-bearing plants called cycads, which left this modern descendant. Applying Concepts What other organisms were important In the Triassic Period?

..&. Figure 17-18 ~ During the Mesozoic Era, dinosaurs were dominant. Dicraeosaurus (foreground) was a plant-eater that grew to about 20 meters in length.

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Jurassic Period During the Jurassic (joo-RAS-ik) Period, dinosaurs became the dominant animals on land. Dinosaurs "ruled" Earth for about 150 million years, but different types lived at different times. At 20 meters long, Dicraeosaurus, shown in Figure 17-18, was one of the larger dinosaurs of the Jurassic Period.

One of the first birds, called Archaeopteryx, appeared during this time. Many paleontologists now think that birds are close relatives of dinosaurs. Since the 1990s, scientists working in China have found evidence for this hypothesis in other fossils that have the skulls and teeth of dinosaurs but the body structure and feathers of birds.

Cretaceous Period Reptiles were still the dominant vertebrates throughout the Cretaceous (krih-TAY-shus) Period. Dinosaurs such as the meat-eating Tyrannosaurus rex dominated land ecosystems, while flying reptiles and birds soared in the sky. Flying reptiles, however, became extinct during the Cretaceous. In the seas, turtles, crocodiles, and extinct reptiles such as plesiosaurs swam among fishes and marine invertebrates.

The Cretaceous also brought new forms oflife, including leafy trees, shrubs, and small flowering plants like those you see today. Unlike the conifers, flowering plants produce seeds enclosed in a fruit, which protects the seed and aids in dispersing it to new locations.

At the close of the Cretaceous, another mass extinction occurred. More than half of all plant and animal groups were wiped out, including all of the dinosaurs.

CHECKPOINT When did flowering plants evolve?

.... Figure 17-19 During the Cenozoic Era, mammals evolved adaptations that allowed them to live on land, in water, and even in the air. Two of the traits that contributed to the success of mammals were a covering of hair that provided insulation against the cold and the protection of the young before and after birth.

Cenozoic Era During the Mesozoic, early mammals competed with dinosaurs for food and places to live. The extinction of dinosaurs at the end of the Mesozoic, however, created a different world. During the Cenozoic, mammals evolved adaptations that allowed them to live in various environments-on land, in water, and even in the air. One land mammal from the early Cenozoic is shown in Figure 17-19. Paleontologists often call the Cenozoic the Age of Mammals.

Tertiary Period During the Tertiary Period, Earth's climates were generally warm and mild. In the oceans, marine mammals such as whales and dolphins evolved. On land, flowering plants and insects flourished. Grasses evolved, providing a food source that encouraged the evolution of grazing mammals, the ancestors of today's cattle, deer, sheep, and other grass-eating mammals. Some mammals became very large, as did some birds.

Fossil Preparer job Description: work for private industries, museums, or universities to expose fossils covered by rock or soil or to construct missing fossil parts

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Skills: be knowledgeable about many areas of science, ability to use fine tools under a microscope, self-motivated, patient, ability to handle very fragile specimens for long periods

cGo nline '--------¥PHSchool.com

Highlights: work with fossils; collaborate with many types of people-from amateur fossil collectors to professional paleontologists

For: Career links Visit: PHSchool.com Web Code: cbb-5173


~ Figure 17-20 During the Quaternary Period, Earth's climate cooled, producing a series of ice ages. Among the characteristic animals of the time were these huge mammoths. Inferring How might the change to a colder climate have affected different types of organisms?

1. · Key Concept Where did life exist during the early Paleozoic Era?

2. Key Concept What evolutionary milestone involving animals occurred during the Devonian Period?

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Quaternary Period Mammals that had evolved during the Tertiary Period eventually faced a changing environment during the Quaternary Period. During this time, Earth's climate cooled, causing a series of ice ages. Repeatedly, thick continental glaciers advanced and retreated over parts of Europe and North America. So much of Earth's water was frozen in continental glaciers that the level of the oceans fell by more than 100 meters. Then, about 20,000 years ago, Earth's climate began to warm. Over the course of thousands of years, the continental glaciers melted. This caused sea levels to rise again.

In the oceans, algae, coral, mollusks, fishes, and mammals thrived. Insects and birds shared the skies. On land, mammalssuch as bats, cats, dogs, cattle, and the mammoths shown in Figure 17-20-became common. The fossil record suggests that the early ancestors of our species appeared about 4.5 million years ago but that they did not look entirely human. The first fossils assigned to our own species, Homo sapiens, may have appeared as early as 200,000 years ago in Africa. According to one hypothesis, members of our species began a series of migrations from Africa that ultimately colonized the world.

3. · Key Concept What are two key events from the Mesozoic Era?

4. Critical Thinking Inferring If you were a paleontologist investigating fossils from the Cenozoic Era, what fossils might you find?

Creative Writing Choose one of the periods described in this section. Then, write a story about life during that time. Include information about the life forms, weather, and other characteristics.

17-4 Patterns of Evolution

B iologist s often use the term macroevolution to refer to large-scale evolutionary patterns and processes that occur

over long periods of time. Six important topics in macroevolution are extinction, adaptive radiation, convergent evolution, coevolution, punctuated equilibrium, and changes in developmental genes.

Extinction More than 99 percent of all species that have ever lived are now extinct. Usually, extinctions happen for the reasons that Darwin proposed. Species compete for resources, and environments change. Some species adapt and survive. Others gradually become extinct in ways that are often caused by natural selection.

Several times in Earth's history, however, mass extinctions wiped out entire ecosystems. Food webs collapsed, and this disrupted energy flow through the biosphere. During these events, some biologists propose, many species became extinct because their environment was collapsing around them, rather than because they were unable to compete. Under these environmental pressures, extinction is not necessarily related to ordinary natural selection.

Until recently, most researchers looked for a single, major cause for each mass extinction. For example, one hypothesis suggests that at the end of the Cretaceous Period, the impact of a huge asteroid, as shown in Figure 17-21, wiped out the dinosaurs and many other organisms. Scientific evidence confirms that an asteroid did strike Earth at that time. The impact threw huge amounts of dust and water vapor into the atmosphere and probably caused global climate change. It is reasonable to assume that this kind of event played a role in the end of the dinosaurs.

Many paleontologists, however, think that most mass extinctions were caused by several factors. During several mass extinctions, many large volcanoes were erupting, continents were moving, and sea levels were changing. Researchers have not yet determined the precise causes of mass extinctions.

What effects have mass extinctions had on the history of life? Each disappearance of so many species left habitats open and provided ecological opportunities for those organisms that survived. The result was often a burst of evolution that produced many new species. The extinction of the dinosaurs, for example, cleared the way for the evolution of modern mammals and birds.

Guide for Reading ..... ____________ ...... Key Concept • What are six important

patterns of macroevolution?

Vocabulary macroevolution adaptive radiation convergent evolution coevolution punctuated equilibrium

Reading Strategy: Summarizing List the six patterns of macroevolution described in this section. As you read, write a statement describing each pattern.

T Figure 17-21 (> Mass extinctions are one pattern of macroevolution. A huge asteroid hitting Earth may have caused the extinction of the dinosaurs at the end of the Cretaceous Period. This illustration shows an artist's conception of that event.

Artiodactyls Cetaceans Perissodactyls Tubulidentates Hyracoids Sirenians Proboscideans

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A Figure 17-22 This diagram shows part of the adaptive radiation of mammals, emphasizing current hypotheses about how a group of ancestral mammals diversified over millions of years into several related living orders. Note that the dotted lines and question marks in this diagram indicate a combination of gaps in the fossil record and uncertainties about the timing of evolutionary branching. Interpreting Graphics According to this diagram, which mammal group is the most closely related to elephants?

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Adaptive Radiation Often, studies of fossils or of living organisms show that a single species or a small group of species has evolved, through natural selection and other processes, into diverse forms that live in different ways. This process is known as adaptive radiation. In the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species.

Adaptive radiations can also occur on a much larger scale. Dinosaurs, for example, were the products of a spectacular adaptive radiation among ancient reptiles. The first dinosaurs and the earliest mammals evolved at about the same time. Dinosaurs and other ancient reptiles, however, underwent an adaptive radiation first and "ruled" Earth for about 150 million years. During that time, mammals remained small and relatively scarce. But the disappearance of the dinosaurs cleared the way for the great adaptive radiation of mammals. This radiation, part of which is shown in Figure 17-22, produced the great diversity of mammals of the Cenozoic.

Convergent Evolution Adaptive radiations can have an interesting evolutionary "side effect." They can produce unrelated organisms that look remarkably similar to one another. How does that happen? Sometimes, groups of different organisms, such as mammals and dinosaurs, undergo adaptive radiation in different places or at different times but in ecologically similar environments. These organisms start out with different "raw material" for natural selection to work on, but they face similar environmental demands, such as moving through air, moving through water, or eating similar foods.

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Figure 17-23 Each of these animals has a streamlined body and various appendages that enable it to move rapidly through water. Yet, the shark (above) is a fish, the penguin (center) is a bird, and the dolphin (bottom) is a mammal. Applying Concepts How did these different animals come to resemble one another?

In these situations, natural selection may mold different body structures, such as arms and legs, into modified forms, such as wings or flippers. The wings or flippers function in the same way and look very similar. This process, by which unrelated organisms come to resemble one another, is called convergent evolution. Convergent evolution has occurred time and time again in both animals and plants.

Consider swimming animals, for example. An animal can move through the water rapidly with the least amount of energy if its body is streamlined and ifit has body parts that can be used like paddles. That is why convergent evolution involving :fishes, two different groups of aquatic mammals, and swimming birds has resulted in sharks, dolphins, seals, and penguins whose streamlined bodies and swimming appendages look a lot alike, as shown in Figure 17-23. Structures such as a dolphin's flukes and a fish's tail fin, which look and function similarly but are made up of parts that do not share a common evolutionary history, are called analogous structures. There are a surprising number of animals (including one of Darwin's :finches) that have evolved adaptations analogous to those of woodpeckers for feeding on insects living beneath the bark of trees and in rotted wood.

CHECKPOINT How do biologists explain the similar shapes of sharks and dolphins?

Coevolution Sometimes organisms that are closely connected to one another by ecological interactions evolve together. Many flowering plants, for example, can reproduce only if the shape, color, and odor of their flowers attract a specific type of pollinator. Not surprisingly, these kinds ofrelationships can change over time. An evolutionary change in one organism may also be followed by a corresponding change in another organism. The process by which two species evolve in response to changes in each other over time is called coevolution.

Penguins

Dolphin


Analyzing Data

Changing Number of Marine Families

.... Figure 17-24 This orchid has an unusually long spur containing a supply of nectar within its tip . The hawk moth has an equally long feeding tube that enables it to feed on the nectar. The flower spur and the feeding tube are an example of coevolution. Inferring How might natural selection bring about the evolution of this orchid and the moth?

The pattern of coevolution involving flowers and insects is so common that biologists in the field often discover additional examples. When Charles Darwin saw an orchid like the one in Figure 17-24, he closely examined the long structure called a spur. Inside the tip of that 40-centimeter spur is a supply of nectar, which serves as food for many insects. Darwin predicted the discovery of a pollinating insect with a 40-centimeter structure that could reach the orchid's nectar. About fifty years later, researchers discovered a moth that matched Darwin's prediction.

Consider another example, the relationships between plants and plant-eating insects. Insects have been feeding on flowering plants since both groups emerged during the Mesozoic. Over time, a number of plants have evolved poisonous compounds that prevent insects from feeding on them. In fact, some of the most powerful poisons known in nature are plant compounds that have evolved in response to insect attacks. But once plants began to produce poisons, natural selection in herbivorous insects began to favor any variants that could alter, inactivate, or eliminate those poisons. In a few cases, coevolutionary relationships can be traced back over millions of years.

CHECKPOINT What happens during coevolution?

Diversity of Marine Life Through Time

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Using fossil evidence, scientists make inferences about the kinds and number of organisms that lived at different times in the past. Further, they classify those organisms in ways that facilitate comparisons between past and present types. The graph on the right gives an estimate of the number of oceandwelling families over time. In biology, a family consists of several groups of related species.

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3. Inferring What kind of event(s) might explain the changes at the end of the Paleozoic and Mesozoic eras?

1. Using Tables and Graphs What overall trend does this graph show?

2. Calculating What was the change in the number of marine families at the end of the Paleozoic Era? At the end of the Mesozoic?

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4. Predicting What factors might cause this graph to change in the next 1000 years? The next 10,000,000 years? Explain.

Punctuated Equilibrium How quickly does evolution operate? Does it always occur at the same speed? These are questions on which some modern biologists would disagree with Darwin. Recall that Darwin was enormously impressed by the way Hutton and Lyell discussed the slow and steady nature of geologic change. Darwin, in turn, felt that biological change also needed to be slow and steady, an idea known as gradualism. In many cases, the fossil record confirms that populations of organisms did, indeed, change gradually over time.

But there is also evidence that this pattern does not always hold. Some species, such as horseshoe crabs, have changed little from the time they first appeared in the fossil record. In other words, much of the time these species are in a state of equilibrium, which means they do not change very much. Every now and then, however, something happens to upset the equilibrium. At several points in the fossil record, changes in animals and plants occurred over relatively short periods of time. Some biologists suggest that most new species are produced by periods of rapid change. (Remember that "short" and "rapid" are relative to the geologic time scale. Short periods of time for geologists can be hundreds of thousands-even millions-of years!)

Rapid evolution after long periods of equilibrium can occur for several reasons. It may occur when a small population becomes isolated from the main part of the population. This small population can then evolve more rapidly than the larger one because genetic changes can spread more quickly among fewer individuals. Or it may occur when a small group of organisms migrates to a new environment. That's what happened with the Galapagos finches, for example. Organisms evolve rapidly to fill available niches. In addition, mass extinctions can open many ecological niches and provide new opportunities to those organisms that survive. Thus, it is not surprising that some groups of organisms have evolved rapidly following mass extinctions.

Scientists use the term punctuated equilibrium to describe this pattern oflong, stable periods interrupted by brief periods of more rapid change. The concept of punctuated equilibrium, illustrated in Figure 17-25, has generated much debate and is still somewhat controversial among biologists today. It is clear, however, that evolution has often proceeded at different rates for different organisms at different times during the long history of life on Earth.

Model of Gradualism

Model of Punctuated Equilibrium

A Figure 17-25 Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent. Interpreting Graphics How do the diagrams illustrate these explanations?


Ancient Insect

Pairs of wings on many segments

Two Types of Modern Insects

One pair of wings

Two pairs of wings

_. Figure 17-26 Some ancient insects, such as the mayfly nymph (top), had winglike structures on many body segments. Modern insects have only four wings or two wings. ~Changes in the expression of developmental genes may explain how these differences evolved.

1. · Key Concept What is macroevolution? Describe two patterns of macroevolution.

2. What role have mass extinctions played in the history of life?

3. What is convergent evolution? Describe an example.

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Developmental Genes and Body Plans Biologists have long suspected that changes in the genes for growth and differentiation during embryological development could produce transformations in body shape and size. Until recently, however, researchers had only limited ability to affect gene activity in embryos. Therefore, they couldn't develop many of those hunches into testable scientific hypotheses. Molecular tools have changed all that. We can now perform experiments with gene expression by turning genes on or off and examining the results. These studies shed new light on how genetic change can produce major evolutionary transformations.

For example, as you saw in Chapter 12, "master control genes," called hox genes, guide development of major body structures in animals. Some determine which parts of an embryo become front and rear, or top and bottom. Others control the size and shape of arms, legs, or wings. Homologous control genes serve similar functions in animals as different as insects and humans-even though those animals haven't shared a common ancestor in at least 700 million years!

Small changes in the activity of control genes can affect many other genes to produce large changes in adult animals. If one gene, called "wingless," is turned on in an insect body segment, that segment grows no wings. This is interesting because some ancient insects, shown in Figure 17-26, had winglike structures on all body segments. Yet modern insects have wings on only one or two segments. Changes in the activation of this gene could have enabled many-winged ancestors of modern insects to evolve into four-winged and two-winged forms.

Small changes in the timing of cell differentiation and gene expression can make the difference between long legs and short ones, between long, slender fingers or short, stubby toes. In fact, recent studies suggest that differences in gene expression may cause many of the differences between chimpanzee brains and human brains. Small wonder that this new field is one of the hottest areas in all of evolutionary biology!

4. How might hox genes contribute to variation?

5. Crltlcal Thinking Comparing and Contrasting Compare and contrast the hypotheses of gradualism and punctuated equilibrium.

Making a Table Create a table that lists each of the six patterns of macroevolution, explains each pattern, and gives one example for each. Add a title to your table.

Exploration

Modeling Coevolution Flowering plants and the animals that pollinate their flowers include many examples of coevolving species. In this investigation, you will model how these plants and animals evolve in response to one another.

Problem How do flowering plants and their pollinators coevolve?

Materials • long forceps •spoon •dried peas

• 3 25-ml graduated cylinders • 3 100-ml beakers • watch or clock with second hand

Skills Using Models, Inferring

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Work in groups of three. Each group member represents a different bird species. To represent the birds' beaks, one group member will use forceps, the second group member will use a spoon, and the third will use two fingertips.

On a separate sheet of paper, make a copy of the data table shown. The beakers represent short, open flowers and the graduated cylinders represent long, narrow flowers. The dried peas represent the flowers' nectar, which is the birds' food . Fill the beakers and the graduated cylinders halfway with dried peas. CAUTION: Handle the beakers and graduated cylinders carefully. If one breaks, tell your teacher immediately.

For 1 minute, use the method you chose in step 1 to remove the peas . Remove them one at a time from your beaker. Do not move or tip the beaker as you do this.

Record the number of peas you removed in your data table.

To produce seeds, a flower must be pollinated by a member of its own species. Assume that 1 flower was pollinated for every 5 peas removed. Record the number of pollinations for each bird.

Data Table

Beak Individual Data Claaa Avera0e Type Peaa Polllnatlona Peaa Polllnatlona

Forceps

Spoon

Fingers

Repeat steps 3 through 5, using the graduated cylinders instead of the beakers.

Calculating Exchange data with your classmates and record the class averages for each bird species in your data table.

Analyze and Conclude 1. Analyzing Data Which bird species obtained

the most nectar from the beakers? From the graduated cylinders?

2. Analyzing Data From which type of flower was each bird most successful in obtaining food?

3. Inferring What is the benefit to a plant of short, open flowers?

4. Inferring What is the benefit to a bird of a long, narrow beak?

S. Drawing Conclusions Which type of bird is the best pollinator for long, narrow flowers?

6. Evaluating and Revising How does this model represent coevolution? How could you improve this model?

Go Further

Using Models Construct an alternative model of coevolution between flowering plants and birds that feed on their nectar. Then, compare your model to the one you used in this lab. Analyze the strengths and weaknesses of each model.


17- 1 The Fossil Record Key Concepts

• The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time.

• Relative dating allows paleontologists to estimate a fossil's age compared with that of other fossils.

• In radioactive dating, scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains.

• After Precambrian Time, the basic divisions of the geologic time scale are eras and periods.

Vocabulary paleontologist, p. 41 7 fossil record, p. 41 7 extinct, p. 41 7 relative dating, p. 419 index fossil, p. 419 half-life, p. 420 radioactive dating, p. 420 geologic time scale, p. 421 era, p. 421 period, p. 422

17- 2 Earth's Early History Key Concepts

• Earth's early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.

• Miller and Urey's experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.

• The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration.

• The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.

Vocabulary proteinoid microsphere, p. 425 microfossil, p. 426 endosymbiotic theory, p. 427

442 Chapter 17

17- 3 Evolution of Multicellular Life Key Concepts

• Rich fossil evidence shows that early in the Paleozoic Era, there was a diversity of marine life.

• During the Devonian, vertebrates began to invade the land.

• The mass extinction at the end of the Paleozoic affected both plants and animals on land and in the seas. As much as 95 percent of the complex life in the oceans disappeared.

• Events during the Mesozoic include the increasing dominance of dinosaurs. The Mesozoic is marked by the appearance of flowering plants.

• During the Cenozoic, mammals evolved adaptations that allowed them to live in various environments-on land, in water, and even in the air.

Vocabulary mass extinction, p. 431

17-4 Patterns of Evolution Key Concept

• Six important topics in macroevolution are extinctions, adaptive radiation, convergent evolution, coevolution, punctuated equilibrium, and changes in developmental genes.

Vocabulary macroevolution, p. 435 adaptive radiation, p. 436 convergent evolution, p. 437 coevolution, p. 437 punctuated equilibrium, p. 439

Thinking Visually Use information from the chapter to create a flowchart that illustrates how natural selection can lead to the extinction of a species.


1. Scientists who specialize in the study of fossils are called a. biologists. c. zoologists. b. paleontologists. d. anthropologists.

2. Sedimentary rocks form when layers of small particles are compressed a. in the atmosphere. c. in mountains. b. in a snow field. d. under water.

3. Radioactive dating of rock samples a. is a method of absolute dating. b. is a method of relative dating. c. forms a geologic column. d. forms a geologic time scale.

4. Half-life is the length of time required for half the atoms in a radioactive sample to a. decay. c. expand. b. double. d. be created.

5. Earth's first atmosphere contained little or no a. hydrogen cyanide. c. nitrogen. b. hydrogen sulfide. d. oxygen.

6. In Miller and Urey's experiments with the origin of life forms, electric sparks were passed through a mixture of gases to a. simulate temperature. b. simulate sunlight. c. sterilize the gases. d. simulate lightning.

7. Outlines of ancient cells that are preserved well enough to identify them as prokaryotes are a. microfossils. c. autotrophs. b. heterotrophs. d. phototrophic.

8. Which event occurred at the end of the Paleozoic Era? a. coevolution c. punctuated equilibrium b. mass extinction d. convergent evolution

9. The process that produces a similar appearance among unrelated groups of organisms is a. adaptive radiation. c. convergent evolution. b. coevolution. d. mass evolution.

10. As a group, the large-scale evolutionary changes that take place over long periods of time are called a. macroevolution. c. convergent evolution. b. coevolution. d. geologic time.


Understanding Concepts 11. How does relative dating enable paleontologists

to estimate a fossil's age?

12. Explain how radioactivity is used to date rocks.

13. What is the geologic time scale? How was it developed?

14. Discuss what scientists hypothesize about Earth's early atmosphere and the way oceans formed.

15. Use the diagram below to explain the significance of Miller and Urey's experiment.

Mixture of methane, ammonia,

and hydrogen

enters

I _ ----- Spark

I V''~ Electrodes \

Condenser

Mixture of

16. How are proteinoid microspheres like living cells?

17. How did the addition of oxygen to Earth's atmosphere affect the evolution of life?

18. Describe the endosymbiotic theory.

19. Describe life as it existed in Precambrian Time.

20. During what era did marine life become diverse?

21. What significant mammalian adaptations led to their success during the Cenozoic Era?

22. What events led to the diversification of mammals?

23. Explain the process of adaptive radiation. Give an example.

24. Explain the pattern known as punctuated equilibrium.

25. Use an example to explain the concept of coevolution.

26. How can hox genes provide evidence of evolution?


Critical Thinking 27. Using Models What part of Miller and Urey's

apparatus represents rain? What important part would rain play in chemical evolution?

28. Applying Concepts In what way might the cells that took in the ancestors of mitochondria and chloroplasts have benefited from the relationship?

29. Inferring Geologic changes often accompany mass extinctions of life forms. Why do you think this is true?

30. Problem Solving The half-life of carbon-14 is 5730 years. What is the age of a fossil containing 1 /16 the amount of carbon-14 of living organisms? Explain your reasoning.

31. Applying Concepts The graph shows an approximation of the amount of oxygen in the atmosphere since life began. What event occurred at the point indicated by the arrow?

Oxygen in Earth 's Atmosphere

.-. ~ -~ ~ 20 c. Ill 0 E < ·5 10 Ill

~ c Q)

g? 0

-

>< 0 3.5

' ...

'i) I

I I 3.0 2.5 2.0 1.5 1.0 0.5

Biiiion Years Present

32. Applying Concepts Evolutionary biologists say that there is good reason for gaps in the fossil record. Can you explain why some extinct animals and plants were never fossilized?

33. Comparing and Contrasting Compare mass extinction to the extinction of species through the more typical processes of natural selection. Be sure to say how the processes are similar as well as different.

444 Chapter 17

34. Asking Questions Suppose you are part of a scientific expedition searching for fossils. Someone brings you a fossilized bone that resembles one of the leg bones of a modern frog. What are some questions you would need to have answered in order to determine the age of the fossil? In what ways might the fossil provide evidence of change in species?

35. Applying Concepts What is the role of natural selection in adaptive radiation? How do these processes lead to diversity?

36. Connecting Concepts Recall what you learned about chemistry in Chapter 2. When Earth's atmosphere first began to form, it did not contain oxygen (Oz), and hydrogen (Hz) was the most abundant element in the solar system. However, there is very little Hz in the atmosphere today, and the element makes up less than 1 percent of Earth's mass. What might have happened to the Hz?

V"\lvriting in Science When you compare two items, you explain how they are similar and different. Write a paragraph comparing conditions on early Earth (about 3.8 billion years ago) with those on modern Earth. (Hints: When you write a comparison, it isn't sufficient to summarize the characteristics of both items separately. Instead, you need to say specifically how the items are like one another and how they differ from one another.)

Performance-Based Assessment

Prepare a Booklet Suppose you could observe the formation and early history of Earth. Write your experiences in the form of a booklet four to six pages long to be read by middle-school students. Include a table of contents, color illustrations, and an activity at the end to evaluate students' understanding of the concept. The activity could be a puzzle, a completion activity, or another similar activity.

cGo nline '-------PHSchool.com For: An interactive self-test Visit: PHSchool.com Web Code: cba-5170

Test-Taking Tip If you find particular questions

difficult, put a light mark beside them and keep working. As you answer later questions, you may find information that helps you find the answers you still need. (Do not write in this book.)


1. Which of the following is characteristic of an index fossil?

I. Distinctive species II. Lived in a wide geographic range

III. Lived for a long period of time (A) I only (D) II and III only (B) II only (E) I, II, and III (C) I and II only

2. In which geologic era do you live? (A) Cenozoic (D) Precambrian (B) Mesozoic (E) Paleozoic (C) Cambrian

3. The endosymbiotic theory includes all of the following EXCEPT (A) Photosynthetic prokaryotes evolved into

chloroplasts. (B) Aerobic prokaryotes evolved into

mitochondria. (C) Eukaryotic cells arose from the merging of

different prokaryotic organisms. (D) All organelles evolved from specialized

enfoldings of the plasma membrane. (E) Eukaryotic cells are the result of an inter

dependent relationship among different organisms.

4. Which of the following is evidence for the endosymbiotic theory?

I. Mitochondria and chloroplasts contain DNA similar to bacterial DNA.

II. Mitochondria and chloroplasts contain ribosomes that differ from bacterial ribosomes.

III . Mitochondria and chloroplasts reproduce by binary fission.

(A) I only (B) II only (C) I and III only

(D) II and III only (E) I, II, and III


5. Potassium-40 is useful for dating very old fossils because (A) it has a very long half-life. (B) it has a very short half-life. (C) most organisms contain more potassium than

carbon . (D) it does not undergo radioactive decay. (E) it is found only in certain rock layers.

6. The Cambrian Period is also called the (A) Age of Humans. (D) Age of Invertebrates. (B) Age of Fishes. (E) Age of Vertebrates. (C) Age of Dinosaurs.

Questions 7 and 8

The graph shows the radioactive decay of an isotope. Use the information in the graph to answer the questions that follow.

Amount of Isotope in a Sample

Q) c. 4 0 -0

.!!!. 3 0 -c ::J 0 E <(

0 0 2 3 4 5 6

Half-life

7. The half-life of thorium-230 is 75,000 years. How long will it take for 7 /8 of the original amount of thorium-230 in a sample to decay? (A) 75,000 years (D) 70,000 years (B) 225,000 years (E) 150,000 years (C) 25,000 years

8. The half-life of potassium-40 is about 1 300 million years. After four half-lives have passed, how much of the original sample will be left? (A) t\ (B) t\ x 1 300 million grams (C) t (D) t x 1 300 million grams (E) -!


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