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Evolutionary Contingency, Stability, and Biological Laws

Jani Raerinne, Ph.D.

Draft. Journal for General Philosophy of Science (forthcoming)

Department of Philosophy, History, Culture, and Art Studies

University of Helsinki

Address: P.O. Box 24 (Unioninkatu 40 A)

00014 University of Helsinki, Finland

E-mail: jani.raerinne@helsinki.fi

Abstract The contingency of biological regularities – and its implications for the

existence of biological laws – has long puzzled biologists and philosophers. The best

argument for the contingency of biological regularities is John Beatty’s evolutionary

contingency thesis, which will be re-analyzed here. First, I argue that in Beatty’s thesis

there are two versions of strong contingency used as arguments against biological laws

that have gone unnoticed by his commentators. Second, Beatty’s two different versions

of strong contingency are analyzed in terms of two different stabilities of regularities.

Third, I argue that Beatty and his commentators have focused on the more ineffective

trajectory stability version of the argument, whereas the constancy stability version

provides a more substantial and applicable argument against the existence of biological

laws. Fourth, I develop a counterexample to Beatty’s thesis. Finally, I discuss the

possibility of evolution producing repeatable and general non-lawlike regularities and

patterns by utilizing the notion of generative entrenchment and by criticizing the thesis

of multiple realizability of biological properties.

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Key words: evolutionary contingency thesis; generative entrenchment; laws;

multiple realization

1 Introduction

Many arguments have been made in support of the claim that there are no biological

laws. One argument in particular has been taken seriously by many, namely, the

argument that charges biological regularities with being contingent rather than necessary

in the sense that laws are supposed to be necessary. The best explication of this

argument is the evolutionary contingency thesis put forth by John Beatty (1995).

To Beatty (1995), there are two kinds of contingency to which his thesis refers.

By weak contingency Beatty means that biological regularities are riddled with

exceptions. By strong contingency Beatty means that biological regularities lack the

necessity associated with laws. Taken together, weak and strong contingency pose a

threat, both to biological regularities’ lawlikeness and to their truth. Biological

regularities’ strong contingency suggests that they are not lawlike, but rather

accidentally true regularities, whereas biological regularities’ weak contingency suggests

that they are not even accidentally true. Instead, they are false regularities, owing to the

regularities’ open-ended sets of exceptions that defy systematic and simple treatment.

My aim is to re-analyze Beatty’s thesis. In section 2, I review previous responses

to his thesis in terms of stability and scope. In section 3, I argue that in fact there are two

versions of strong contingency in Beatty’s thesis raised as arguments against biological

regularities’ lawlikeness and that the two are analyzable by different forms of stability of

regularities, the trajectory stability version and the constancy stability version. In section

4, I argue that Beatty and his commentators have concentrated on the more ineffective

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version of the strong contingency thesis, the trajectory stability version, whereas the

constancy stability version provides a more substantial and applicable argument against

the existence of biological laws. In section 5, I develop a counterexample to Beatty’s

thesis. The final section concludes by describing how my analysis of Beatty’s thesis

differs from previous responses and analyses.

The biological laws debate is an interesting philosophical topic in itself.

Moreover, many authors think of laws as having special or proprietary roles in the

sciences, or they think of lawlikeness as a central concept in analyzing other interesting

concepts, such as induction, confirmation, and theory structure. If Beatty’s thesis is

successful as an argument against biological laws, then the question arises of whether it

applies to other sciences and has similar consequences there. This shows that Beatty’s

thesis is of interest to philosophers other than those of biology alone. Finally, the

evolutionary contingency thesis is connected to other interesting topics, such as whether

evolution is capable of bringing about general and repeatable patterns or whether it

results in idiosyncratic and unique outcomes.

2 Previous Reponses to Beatty: Scope and Stability

As Beatty’s critics have argued, rather than being an explanans of why biological

regularities lack lawlikeness, evolutionary contingency is an explanandum that needs to

be analyzed. Mitchell’s (1997, 2000) idea is that strong contingency should be analyzed

in terms of stability of regularities. Waters (1998) suggested that weak contingency

should be analyzed in terms of scopes or “distributions” of regularities. Beatty’s

evolutionary contingency thesis can thus be analyzed as follows:

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1) Given their weak contingency, biological regularities are riddled with

exceptions, that is, their scope is far from universal.

2) Given their strong contingency, biological regularities lack the stability that

guarantees that they would hold in many or all possible background conditions.

Let me call a regularity stable if it holds in many possible background conditions. Stable

regularities are relatively context insensitive, reliable, or projectable in that they

continue to hold in many possible background conditions.

The connection between strong contingency and stability is that these are concepts

of degree, which are inversely related to one another: the more strongly contingent a

regularity is, the less stability it has and vice versa. Stability considerations of

regularities are evaluated on the basis of how many background conditions there are and

what kinds of background conditions the regularities are dependent or contingent upon.

The more stable a regularity is, the less dependent it is in holding in these background

conditions. Of course, stability depends not only on the number of background

conditions, but also on their nature. Likewise, different sciences and disciplines differ on

the background conditions considered important in evaluating the stability of regularities

(cf. Lange 2005).

The application domain of a regularity in the past or present is described by its

scope. This domain includes those (dis)similar systems to which a regularity applies or

has applied. Biological regularities typically generalize about different taxa, features,

entities, and so forth in different times and/or places. As examples of scope, consider the

following: some of the members of the lineage of horses exhibited the pattern of Cope’s

rule in the Miocene and currently, rain forests are almost exclusively located at low

latitudes. Many biological regularities have narrow or limited scopes (Hairston 1989,

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Alroy 1998, Hecnar 1999), whereas traditionally it has been thought that laws have

unlimited or universal scopes.

Scope furnishes us with information about the application domain of a regularity

to (dis)similar systems rather than about the holding of a regularity in different

background conditions. The scope of Mendel’s rules is (only) all or nearly all sexually-

reproducing taxa. The conditions, like the evolution of mitosis and meiosis, on which

Mendel’s rules depend in their holding are the background conditions in the above sense

of stability. A regularity that has a narrow scope could have a high degree of stability

within this scope. The converse could also be true: a regularity that has a broad scope

could have a low degree of stability within its scope.

The above analyses help to clarify the implications of the evolutionary

contingency thesis. Waters shows that weak contingency is a serious threat to biological

regularities’ lawlikeness if one holds to an account of laws in which biological

regularities have unlimited or universal scopes. However, only a minority of

philosophers and biologists today think of biological laws in these traditional terms.

Likewise, there are accounts of laws that tolerate exceptions, such as the pragmatic,

paradigmatic, and inference ticket accounts (Lange 1993a, 1993b; Mitchell 1997, 2000).

Consequently, it can be claimed that weak contingency alone is not a serious difficulty

for the lawlike status of biological regularities.

Mitchell’s analysis shows that strong contingency is potentially a more damaging

threat to the existence of biological laws, because if biological regularities lack maximal

or high degrees of stability, then they lack the modal force of laws that is connected to

the laws’ ability to support counterfactuals. Regularities lacking high degrees of stability

would not hold had the background conditions for the regularities changed in space or

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time. Laws of nature are thought to hold in widely different conditions and even under

all the different background conditions.

However, what the previous commentators, including Mitchell, have missed is

that there are two versions of strong contingency in Beatty’s thesis which are used as

arguments against the existence of biological laws.

3 Two Senses of Strong Contingency

The following is the passage usually quoted from Beatty (1995, 46-47; see also Beatty

1995, 45-51, 57 for similar ideas) as a characterization of strong contingency:

All generalizations about the living world:

(a) are just mathematical, physical, or chemical generalizations (or deductive

consequences of mathematical, physical, or chemical generalizations plus initial

conditions),

or

(b) are distinctively biological, in which case they describe contingent outcomes

of evolution.

The first part of this claim is meant to acknowledge that there are generalizations

about the living world whose truth values are not a matter of evolutionary history.

Evolution has not and will not result in any forms of life that are not subject to the

laws of probability, or to Newton’s laws of motion. Nor will evolution result in

any carbon based forms of life that are not subject to the principles of organic

chemistry. But while these sorts of principles are true of the living world, we do

not call them “biological” principles. [Emphasis added.]

The meaning of this passage can be paraphrased as follows: evolution can lead to

different outcomes from the same or similar starting points given the same or similar

selection pressures. Thus, even from the same selection pressures, similar or identical

adaptations need not, and probably do not, follow, even given similar organisms in

similar environments. Regularities concerning biological phenomena are contingent,

accidental, or unique outcomes in that evolution is easily switched to another track or

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disturbed by small changes in its initial conditions. I will call this version of the

evolutionary strong contingency thesis ESCT1.

Beatty (1995, 57-59) gives reasons why biological regularities are strongly

contingent in the sense of ESCT1. First, selection often selects from a set of trait variants

that are similar in fitness, but differ in realization, that is, the multiple realizability thesis

is true of biological properties. Second, there are other evolutionary forces besides

selection, such as genetic drift and mutations, that affect evolution and its consequences.

Secondly and differently, by strong contingency Beatty (1995, 46, 47, 51-53, 63)

also means that even if biological regularities were true, there might be and quite likely

will be other background conditions in the universe or on this planet where these fail to

hold. Thus, biological regularities are at most representable as accidentally true

generalizations that hold because of certain background conditions. I will call this

version of the evolutionary strong contingency thesis ESCT2.

The two different versions of evolutionary strong contingency can be analyzed in

terms of two different stability concepts. With ESCT1, Beatty was concerned with

biological regularities’ sensitivity to changes in initial background conditions or what is

more commonly known as “trajectory stability,” whereas with ESCT2 he was concerned

with biological regularities’ general endurance during changes in the background

conditions or “constancy.”

What the idea of trajectory instability means when used to analyze the strong

contingency of biological regularities ESCT1 is that biological regularities are accidental

outcomes or products of (evolutionary) history. Biological regularities lack the

inevitability associated with laws, because instead of the current and prevailing

regularities, different regularities could have evolved on this planet had the historical or

initial conditions been different. Thus, biological regularities lack the necessity and the

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trajectory stability in the sense that their evolution could easily have been switched to

another track or disturbed by past historical or initial conditions.

The regularity described by Bergmann’s rule could be argued to have a low

degree of trajectory stability in the sense that its validity is dependent on certain initial or

historical conditions (but see the next section).1 Bergmann’s rule is a geographical or

latitudinal gradient in body size, according to which the members of a species of

endothermic animals are larger in their body size in colder regions or at higher latitudes

than members of the same species in warmer regions or at lower latitudes.

Endothermicity arose as an adaptation to fluctuating environmental or climatic

conditions. Now, if the earth’s past climates or environmental conditions had been

constant rather than fluctuating, then it is possible that endothermicity would not have

evolved. If endothermicity had not evolved, then the regularity described by Bergmann’s

rule would not hold, since the regularity applies to endothermic rather than to

ectothermic animals. Past environmental or climate conditions can be seen as an initial

condition that could have disturbed the validity of regularity described by Bergmann’s

rule had these conditions been different.

The same holds for the regularities described by Mendel’s rules and the Hardy-

Weinberg rule. Had there not been mitosis or had there been some equivalently fit or

fitter alternative, then meiosis would not have evolved, because meiosis evolved from

mitosis; thus, Mendel’s rules would not have evolved on this planet, because the holding

of Mendel’s rules is dependent on the operation of meiosis and mitosis. Moreover, had

Mendelian rules not evolved or had the conditions changed so that they no longer hold,

1 Bergmann’s rule is presented by Beatty (1995, 58-59) as an example of an evolutionary contingent

regularity. If the lack of trajectory stability of the rule can be challenged, then this casts doubt both on

Beatty’s reasoning and on his examples of evolutionary contingent regularities.

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then the Hardy-Weinberg rule might not hold, because it seems to be a consequence of

Mendelian rules. The regularities just described are contingent and accidental products

of history, and their evolution could have been switched to another track or disturbed

had some of their initial or historical background conditions been different. In other

words, they display trajectory instability.

What the idea of constancy instability means when used to analyze the strong

contingency of biological regularities ESCT2 is that biological regularities depend on

different background conditions in order to hold. If some of these background conditions

were to change, then the biological regularities would fail to hold. For instance, there are

non-Mendelian mechanisms of inheritance on our planet (Crow 1979). If the

environment changes so that these become as fit as or fitter than Mendelian mechanisms,

then they might become as omnipresent as Mendelian mechanisms are at present. Thus,

Mendel’s rules seem to have a low or moderate degree of constancy stability.

Regularities that hold owing to certain background conditions lack the necessity

associated with laws and are at most accidentally true. Laws of nature are thought to

hold in all or at least in most of the possible background conditions rather than holding

only because of certain accidental background conditions.

4 ESCT1 and ESCT2 and the Existence of Biological Laws

The trajectory stability version, ESCT1, is the most common interpretation of Beatty’s

strong contingency (Carrier 1995, 84, 89-90; Schaffner 1995, 102-103; Mitchell 1997,

S470-S472, 2000, 250-251; Powell 2009, 394). In fact, it seems to be the only

interpretation of strong contingency offered by previous authors. I will deal with it first

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by arguing that the trajectory stability version provides a weaker argument against the

existence of biological laws than does the constancy stability version, ESCT2.

Considerations of trajectory stability are important when we are explaining,

predicting, or manipulating. We have to be careful and sensitive in setting up and

studying the initial conditions of a process or a system with a low degree of trajectory

stability. However, the connection between trajectory stability and lawlikeness is a moot

point.

We should not confuse the process or mechanism with its results or outcomes.

The process or mechanism, such as natural selection, could be lawlike, although varying

in results if its initial conditions were varied (see Carrier 1995 for a similar view).

Although natural selection might produce alien-looking life forms on a planet that has

antecedent conditions different from ours, this difference in results does not provide

reasons to doubt the putative lawlike status of natural selection itself. Sensitivity or

variance of results or outcomes to initial conditions does not imply much about the

lawlike nature of the process or mechanism that produced the results. In other words,

one should not confuse contingent evolutionary history as a result with the putative

lawlike mechanisms or processes responsible for this history (Hempel 1965, 370, Ruse

1973, 211-212).

Beatty’s thesis was directed against the existence of “distinctively” biological

laws. A low degree of trajectory stability is not at all distinctive of biological

phenomena, processes, mechanisms, or systems. The same goes for the regularities that

describe the behavior or dynamics displayed by phenomena, mechanisms, processes, or

systems. There are non-biological systems in physics, economy, engineering, and

meteorology that exhibit low degrees of trajectory stability, such as chaotic systems in

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general. Thus, ESCT1 might fail to meet its objective if it does not target anything

distinctively special or unique about the status of biological regularities.

One reason why Beatty believes that biological regularities, in contrast to physical

ones, are strongly contingent in the sense of displaying trajectory instability is his idea

that evolution can lead to different outcomes from the same or similar starting points,

given the same or similar selection pressures, since natural selection often selects from a

set of trait variants that are similar in fitness, but different in realization. Thus, even from

the same selection pressures, similar or identical adaptations need not follow, even given

similar organisms in similar environments. However, this argument is based on a

concept of multiple realizability that rests on unreliable intuitions concerning what

counts as a different realization of the same property.

The traditional explanation for Bergmann’s rule is that the larger body size of an

endothermic animal is an adaptation to cold climates, which results from the fact that the

ratio of “surface area to volume” is smaller in animals with a larger body size than in

animals with a smaller body size. The reason is that, when body size becomes larger, the

surface area of the body increases as the square of the mass, whereas its volume

increases as the cubic of the mass. This allows larger-sized endothermic animals to

conserve their metabolized heat more effectively, because the heat is dissipated through

the surface area in cold environments. Accordingly, if the temperature of an environment

(during the cold season) is a critical factor for an organism’s survival, then selection

should generally and even universally favor and select for different body sizes — and

this is seen in the pattern of Bergmann’s rule, in which the members of a species of

endothermic animals in cold climates or at higher latitudes have larger body sizes than

their relatives in warmer climates or at lower latitudes.

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There are other means for reducing the dissipation of heat in cold climates.

Thicker and heavier fur or plumage and modifications of animals’ blood circulation

might have effects similar to heat dissipation as the increase in body size (Scholander

1955, Irving 1959). These enhance an animal’s insulation without affecting its surface

area to volume ratio or body size. This suggests that there are functionally equivalent,

but physically different means or realizations for achieving the same level of fitness in

cold climates. Hence, the “dissipation of heat” appears to be a multiply realizable

property. Had we “replayed life’s tape,” the latter traits might have been selected as

adaptations to cold climates instead of differences in body size. Thus, it is not necessary

for Bergmann’s rule to be valid as a result of evolutionary history, because there are

alternative trait variants or multiple realizations of the dissipation of heat that could have

led to the same result or effect and which do not affect the ratio of “surface area to

volume.” Thus, given the multiple realizability thesis of biological properties, the

regularity described by Bergmann’s rule seems to lack trajectory stability.

However, the claim that there are or could be different means for reducing heat

dissipation in cold climates is not valid. It is true that a larger body size results in smaller

“surface area to volume ratio” and that modifications of animals’ fur or plumage and

changes in blood circulation result in better insulation. But interestingly, there are not

many other realizers of the dissipation of heat. Thus, the realizers of the “dissipation of

heat” are not diverse and heterogeneous as far as different mechanisms for reducing heat

dissipation in cold climates are concerned. Furthermore, the idea of larger body size

reducing heat dissipation in cold climates is not accurate, since differences in body size

do not have an effect on an animal’s heat dissipation per se. Rather, body size functions

only via the reduction of the “surface area to mass ratio,” which has the effect of a

relatively smaller amount of metabolized heat being dissipated through the surface area

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of larger-sized animals than is dissipated in similar animals with smaller body sizes. The

same applies to such behavioral adaptations as dwelling underground and basking in the

sun, both of which may affect an animal’s fitness in cold climates, but which

nevertheless do not affect its heat dissipation per se. In fact, it could be argued that there

is only one or a few general mechanisms that affect heat dissipation per se in animals,

namely, such changes as modifications of animals’ fur or plumage and changes in blood

circulation that result in changes in the animals’ insulation.

It is thus debatable whether the “dissipation of heat” is a multiply realizable

biological property. Although larger body size may have positive effects on the fitness

of organisms in cold climates, larger body size is not a realization of lesser “dissipation

of heat,” because it does not affect dissipation of heat as do modifications of animals’ fur

or plumage and changes in blood circulation, which result in better insulation. The

former and the latter proceed via different mechanisms to different properties, effects, or

functions; thus, they are not “trait variants that are similar in fitness but differ in

realization.”

Conversely, the presupposition that biological properties are always or often

diverse and heterogeneous in realization can be challenged (Bechtel & Mundale 1999,

Batterman 2000, Shapiro 2000, Raerinne & Eronen 2012). The common theme in these

papers is that when the cases of multiple realizations are scrutinized, it appears that the

realizations of higher-level properties are not as heterogeneous as many have supposed.

Differences that exist between realizations turn out to be unimportant, while explanatory

or mechanistically important things are shared by realizations. A case of this kind was

already discussed above: modifications of animals’ fur or plumage and changes in blood

circulation have an effect on organisms’ heat dissipation. However, these different

realizations of “dissipation of heat” proceed via the same mechanism, namely, by

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improving the insulation of organisms, which suggests that the realizations share

functionally and mechanistically important things. Thus, they are not different

realizations of “dissipation of heat,” but mechanistically and causally similar, from

which it follows that they are not trait variants similar in fitness, but differing in

realization.

The above suggests that one of Beatty’s main reasons for the presence of

trajectory instability in biology can be questioned, namely, the idea that selection selects

from a set of trait variants that are similar in fitness, but differ in realization. If the traits

with similar effects are not so different in terms of the mechanisms that produce the

effects and/or if the set of different realizations is not diverse and heterogeneous after

all, then it follows – contra Beatty – that from the same selection pressures the same or

similar adaptations follow, especially given similar organisms in similar environments.

In other words, the evolution of Bergmann’s rule is not perhaps so easily switched to

another track, but the rule could have some inevitability and necessity after all in the

sense of not displaying trajectory instability.

Moreover, there are evolutionary forces or factors, such as generative

entrenchment (Schank & Wimsatt 1987, Wimsatt & Schank 1988, Wimsatt 1999, 2001),

that foster trajectory stability, despite the fact that a regularity, trait, characteristic, or

mechanism originally had a low degree of trajectory stability and/or constancy.

Although on this planet it is a true generalization that hereditary information is

carried by nucleic acids, this seems to be “an accident” – a conditional fact of our

planet’s and life’s history. Had the historical conditions or other initial background

conditions been different, then other materials could have evolved to do the same thing.2

2 For the sake of argument, I presume that multiple realizability holds in the case of heredity information.

Note that the argument presented here for the stability of generatively entrenched traits is independent of

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Thus, the ubiquity of the genetic code is an accident whose evolution could have been

disturbed or switched to another track. Yet in the course of the history of life on earth,

this code has become so generatively entrenched as to be nearly impossible to change

because so many other things depend on it, i.e., it now represents a functional necessity

or a “frozen accident.” A similar argument can be made in the case of the generative

entrenchment of endothermicity and the holding of Bergmann’s rule. Much of what was

just said above is probably also true of many other basic biological mechanisms, the

presence and functioning of many ancestral or primitive characters or traits,

developmental constraints, and so on, such as Mendel’s rules or mitosis and meiosis,

diplobiontic life cycle, cell respiration, the Krebs cycle, and the use of ATP in metabolic

processes, eukaryoticity, homeobox genes, the mechanism of photosynthesis, bilateral

symmetry, dorsal-ventral polarity, initiator and terminator codons, DNA ligase and DNA

methylation, morphogenesis and organonesis in general, and, for instance, the specific

mechanisms of apoptosis in ontogeny.

In addition to multiple realizability of biological properties, Beatty mentions

mutation and drift as the second source for biological regularities’ strong contingency in

the sense of ESCT1. Generatively entrenched traits are functional necessities for

organisms’ development, survival, and/or reproduction, which resist evolutionary

change, even if we suppose that evolution proceeds via other “forces” than selection,

such as mutation and drift. For a trait to become generatively entrenched, it does not

matter how or under what evolutionary forces it has evolved or will evolve; what matters

the truth of the multiple realizability thesis in the sense that, even if the thesis is presumed to be true,

generatively entrenched traits are still capable of displaying trajectory stability. At the same time, if the

multiple realizability thesis can be questioned, then generatively entrenched traits become even more

stable in the sense of displaying high degrees of trajectory stability, because the functional necessities or

generatively entrenched traits become realization “necessities” as well.

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is how many other traits come to depend on the functioning or development of the trait

in question. A change in a deeply generatively entrenched trait has serious, deleterious

effects on other traits in the development or functioning of an organism. This is why

deeply generatively entrenched traits are preserved and resist evolutionary change,

regardless of whether the trait was a consequence of drift, mutation, or selection.

Generative entrenchment thus provides a counterexample to the second of Beatty’s

reasons for why trajectory instability is present in biology or evolution.

The points just outlined question the validity or applicability of the trajectory

stability version ESCT1 of strong contingency as an argument against biological laws.3

We obtain a more general, substantial, or applicable version of the thesis if we focus on

the lack of stability of biological regularities in the sense of constancy ESCT2 rather

than focusing on trajectory stability ESCT1. Regularities with low and moderate degrees

of constancy stability hold in special or limited background conditions, whereas laws are

3 Beatty developed his evolutionary contingency thesis as an elaboration of Gould’s ideas of the

contingency of macroevolution (for an excellent review of Gould’s ideas, see Powell 2012). Beatty

(2006) develops Gould’s evolutionary contingency further, distinguishing between the unpredictability

notion and the causal dependency notion of evolutionary contingency. According to the unpredictability

notion, evolutionary contingency means that unpredictable outcomes arise from the same or

indistinguishable prior states. According to the causal dependency notion, evolutionary contingency

means that a particular evolutionary outcome depends on which particular states preceded it. Both notions

capture some elements of my trajectory stability version of strong contingency, yet neither is identical to

it. I will not discuss Beatty’s two notions here; Powell (2009, 2012) and Turner (2010) already provide

criticisms of them as meaningful interpretations of evolutionary contingency in the context of

macroevolution. It suffices to point out that Powell’s (2009, 2012) idea of radical contingency, a notion

that is meant both to unify the two Beattyan contingency concepts above and to give them a fair

Gouldian reading, comes very close to my meaning of trajectory stability.

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traditionally understood to be very stable or maximally stable regularities that hold in

most, if not in all, possible background conditions.

Low degrees of constancy stability deprive regularities of their necessity and

nomic force. With low degrees of constancy stability goes the support of counterfactuals,

the distinctive property of laws, according to which laws govern not just what actually

happens, but what would have happened under various background conditions that did

not actually happen. Nor are unstable regularities projectable or extrapolable in the sense

that laws are sometimes thought to be.

One reason why this latter form of stability deprives regularities of their lawlike

status is that with degrees of constancy other than maximal or very high degrees come

restrictions on where and under what background conditions the regularity holds, i.e.,

such regularities are accidentally true rather than lawlike. Another reason is that

biological regularities in general seem to have low or moderate degrees of constancy. By

contrast, a maximal or very high degree of constancy is usually attributed to physical

regularities.

Consequently, Beatty’s strong contingency, when interpreted along the lines of

ESCT2, represents a problem for biological regularities’ lawlikeness, because laws are

commonly understood to be maximally or highly stable regularities in the sense of

constancy. And if biological regularities display low or moderate degrees of this form of

stability, then they are accidental regularities.

Finally, when ESCT1 has force as an argument against biological laws, it rides

piggyback on ESCT2. A low degree of trajectory stability of a regularity is a

consequence of its low degree of constancy. Similarly, a high degree of constancy

results in a high degree of trajectory stability. Regularities that hold in many or all of the

different background conditions, i.e., the regularities that show maximal or high degrees

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of constancy stability, are the ones that quite likely would have held had the initial or

past historical conditions been different, i.e., the regularities also have high degrees of

trajectory stability. Thus, if evolution or evolutionary history is law-governed in the

sense of displaying high degrees of constancy, then we should expect it also to show a

high degree of trajectory stability.

However, regularities displaying high degrees of trajectory stability are not

necessarily regularities that display high degrees of constancy, that is, the two forms of

stability de-couple (see Figure 1). Generative entrenchment and lack of multiple

realizability of biological properties can foster trajectory stability despite the lack of a

high degree of constancy. Here we have a mechanism begetting stability to evolution

without lawlike regularities or mechanisms in the background.4 Thus, a lack of high

degree of trajectory stability is indicative of non-lawlikeness of biological regularities

(this is what Beatty got right with his ESCT1), because a low degree of trajectory

stability follows from a low degree of constancy. But a high degree of trajectory stability

is not necessarily indicative of the lawlikeness of biological regularities by contrast to

what Beatty and his commentators believe, because this does not imply a high degree of

constancy of biological regularities, given that functional necessities and realization

necessities can resist evolutionary change.

4 Generative entrenchment, such as a developmental constraint, is interpreted even by the proponents of

the evolutionary contingency thesis as a mechanism begetting stability to biological systems (Powell

2012, 357-358; see also Wimsatt 1999, 139-152).

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5 A Counterexample to the Evolutionary Contingency Thesis

According to the competitive exclusion principle (CEP), species with similar niches, not

to mention those with identical niches, cannot stably coexist in the same habitat for long

periods of time: n number of sympatric competing species cannot coexist in equilibrium

indefinitely on fewer than n number of common limiting resources. The magnitude or

intensity of competition between ecologically similar species is thought to be

proportional to the degree of overlap in their niches or use of resources. Species that

stably coexist do so because there are “important differences” in their niches or in their

use of resources.

(-)

Deg

ree

of

stab

ilit

y (

+)

Fig 1. Connections between constancy stability (solid arrows), trajectory stability (broken

arrows), generative entrenchment (GE), and multiple realizability (MR).

The presence of GE

and/or the lack of

MR

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I will argue that CEP is not riddled with genuine exceptions, only apparent ones.

This suggests that the principle might be universally true. I will also show how

ecologists recognize two kinds of coexistence mechanisms capable of producing

apparent exceptions to CEP. The two mechanisms specify the relevant background

conditions under which the principle does not necessarily hold. Moreover, I argue that

CEP is a strict law rather than a ceteris paribus law, because the law can be strictly

complemented or its proviso clause can be eliminated. Furthermore, I argue that CEP is

distinctively biological or autonomous as a law, because ecologists are capable of

accounting for the exceptions to CEP in terms of proprietary or distinctively ecological

terminology rather than using the mechanisms, causes, and so on of a lower-level

science to correct the principle. The competitive exclusion principle is thus presented as

a counterexample to Beatty’s thesis.5

There are many “exceptions” to CEP. For instance, Hutchinson (1961) described a

situation in which there are many sympatric phytoplankton species living in bodies of

water, despite the fact that the common limiting resources of species are both scarce and

fewer in number than the number of coexisting species. In such a situation, one would

expect that the competitive dominant species would drive other species into local

extinction by outcompeting competitively less superior species from their common

habitat.

When faced with a situation that seems to be an exception to CEP, for instance, a

guild of birds of prey that hunt for the same food, such as the voles of the genus

Microtus, ecologists make a closer investigation of the situation and the ecology of the

species. Often, competing species are found to be less similar in their niches or use of

5 The strict lawlike nature of CEP and its “same-level” explanations to exceptions are discussed by the

author in detail in two other articles (under review).

21

resources than it first appears (Heatwole & Davis 1965). Thus, what seemed to be a

genuine exception to the principle is found to be an apparent exception, since the

principle does not apply to cases where there are “important” differences between the

niches of competing species.

An apparent exception is a prima facie exception to a generalization, which is

found to be an instance to which the generalization does not apply. An instance in which

a generalization does not hold is not an instance that could show that the generalization

is false. Genuine exceptions are instances in which the generalization applies or holds,

and, as exceptions, they represent falsifying or disconfirming instances of the

generalization.

What makes an exception to a generalization apparent rather than genuine? Many

philosophers and scientists maintain that if an exception can be explained independently

and in a non-ad-hoc way, then the exception is apparent, not genuine (Fodor 1991,

Pietroski & Rey 1995, Carrier 1998).

There are background conditions under which CEP is not expected to hold,

producing apparent exceptions to the principle. For instance, environmental

heterogeneity can alter the competitive rankings between the species so that the species

do not reach their competitive equilibrium, namely, the local extinction of competitively

inferior species by competitively superior species from their common habitat. This was

the explanation that Hutchinson (1961) gave for his paradox of the plankton discussed

above. In high latitudes, for instance, there are temporal variations of common limiting

resources of many species and guilds: nesting sites, amount of sunlight, minerals,

nutrients, etc. Roughly speaking, if temporal variation of resources is faster – yet not too

fast – than the time it takes for competitive systems to reach their equilibriums, then the

temporal variation of resources can prevent competitive systems from reaching their

22

competitive equilibriums. Thus, one (or more) species is locally not outcompeted and

displaced by another, ecologically more efficient species.

Spatial environmental heterogeneity works differently. There might be poor-

quality habitat patches that provide “refugee camps” for a competitively inferior species

and from which the species recolonizes the high-quality patches from which it was

previously displaced by a competitively superior species. If vagility or the intrinsic

growth rate of the competitively inferior species is sufficiently high, and/or it has a

broader environmental tolerance than the competitively superior species, then this can

lead to the coexistence of both species, despite the fact that one is superior insofar as

competition and the use of resources are concerned. Therefore, different instances of

environmental heterogeneity have a common result: they create apparent exceptions to

CEP and thus make coexistence possible, or at least they considerably slow down the

rate at which competitive exclusion occurs.

In rocky intertidal marine habitats, the top predators (e.g., starfish and gastropod

species of the genera Thais and Pisaster) prey on different consumer species (the

different mussels, barnacles, and other species of many genera). In such habitats, there is

severe and intense competition among the consumer species for living space, which is

the major common limiting resource and a critical one. The consumer species compete

for living space, because, as sessile organisms, they are permanently attached to a solid

substrate at their mature stage. There is surprisingly high alpha or local diversity of the

consumer species in many such habitats. It is surprisingly high, because, on the basis of

CEP, one would expect only a few competing species to be sympatric and coexisting,

given the keen competition for living space.

Paine (1966) discovered experimentally that removing one or more of the top

predator species from such a habitat had the effect of reducing the alpha diversity of the

23

consumer species: without a common predator species, a few and perhaps even one

competitively dominant consumer species came to monopolize the living space by

outcompeting other consumer species. Paine’s explanation was that the top or keystone

predators can mediate the coexistence of their prey and maintain the local diversity of a

community at a high level by keeping the density and abundance of competing prey

populations below a level at which the competition would become so severe as to cause

local extinctions.

These and similar experimental findings and explanations of exceptions to CEP

were later generalized and labeled as the intermediate disturbance rule. According to this

rule, intermediate levels of disturbance, such as predators or pathogens, are capable of

mediating the coexistence of competitor species and thus maintain the local diversity of

a community at a high level. Too small or infrequent disturbances lead to local

extinctions of competitively inferior species by competitively dominant ones (CEP thus

holds), whereas too intense or too frequent disturbances allow for the few species that

are the most stress-tolerant to exclude other species from a habitat (this is another

apparent exception to CEP, but without a high level of local diversity). Intermediate

disturbances have the same effect, for instance, through reduction of population densities

of competitor species, which counter the strong competitive effects between species and

provide apparent exceptions to CEP.

Many other empirical and theoretical studies of various so-called coexistence

mechanisms have understood exceptions to CEP as being apparent, not genuine. I

suggest that these coexistence mechanisms are of two kinds, intervening and

constitutive. Both kinds are used to explain the exceptions to CEP as apparent rather

than genuine. This suggests that ecologists know the relevant background conditions

24

under which the principle is not expected to hold, and they have theoretical resources to

explain away the exceptions to CEP as apparent exceptions.

By a constitutive coexistence mechanism, I mean explanations of exceptions to

CEP that show that a certain system does not realize or constitute a competitive system

despite initial appearances. The coexistence of competitors is possible and explained

because the competitive system is only an apparent, not a genuine one, since there are

important ecological differences between the species that allow for coexistence. An

explanation showing that the niches of competing species are less similar than they

originally appeared to be is an instance of a constitutive coexistence mechanism,

because there are ecological differences between species that promote the coexistence of

apparent competitors (Heatwole & Davis 1965). For n species competing for less than n

number of limiting resources, variance in resource levels, such as an unpredictable

amount of rainfall in arid environments, can itself work as a new resource (Tilman

1986). In this case, we are again dealing with an apparent competitive system, because

the number of species is not less than their common limiting resources. Alternatively,

the situation can be described as one in which there are important ecological differences

between species’ niches, with the effect that some species are more capable of utilizing

variance in resource levels than others.

An intervening coexistence mechanism is one that interferes with a system which

constitutes a real competitive system, but prevents competitive exclusion from

occurring. Such an interference affects the system in such a way that its capacity for

competitive exclusion is not manifested because the triggering conditions of the

system’s capacity is affected or the competitive system is prevented from reaching its

equilibrium. A common predator or parasite on a competitor species can rarefy the

population densities of competing species so that the densities remain within the

25

carrying capacity of their environment (Utida 1953, Slobodkin 1964, Paine 1966). In

such a situation, there is a luxury or hoarding competition between the species: A real

competitive system exists with the capacity for competitive exclusion, although the

capacity does not necessarily manifest itself. The reason is that the common resources of

species are not limiting due to an interference coming from outside the competitive

system. The interference rarefies the populations of competing species so that there is an

abundance of common resources allowing coexistence. Environmental heterogeneity

and intermediate disturbance, discussed above, are other examples of intervening

coexistence mechanisms accounting for apparent exceptions to CEP.

I have no argument that CEP lacks any genuine exceptions. Besides, the issue is

empirical rather than philosophical. However, I do have an argument that shows that the

principle is not riddled with genuine and unsystematic exceptions, but rather has

systematic apparent exceptions, which are explainable by using proprietary and

distinctively ecological terminology. This at least is evidence for the claim that the

principle might be universally true and a strict law.

There is a potentially large and heterogeneous set of interfering conditions, such

as earthquakes, celestial bodies hitting the earth, human interventions, fires, and storms,

that are capable of producing exceptions to CEP. Some theoretical arguments have

shown that interfering conditions do not decrease competitive interactions per se, but

affect only the rate at which competitive exclusion is reached (Chesson & Huntly 1997,

Chesson 2000). Thus, interfering conditions are not sufficient to be considered genuine

exceptions to CEP. This suggests that the principle is valid and perhaps even universally

so, because there is no coexistence without “important” niche differences between

competing species. Let us, however, suppose that this is not the case.

26

Lange (2005) has argued that biologists do not need to be worried about all the

counterfactual background conditions that are physically possible and that would violate

biological laws. This suggests that many of the above kinds of interfering conditions are

outside the domain of counterfactual scenarios about which ecologists should be

worried. However, let us suppose that this, too, is not the case.

Let me mark with an “I” conditions interfering with CEP that are physical,

chemical, geological, and so on in nature, that is, conditions that come from the lower

levels. In fact, it does not matter whether the I’s come from levels higher than biology,

and be cultural, social, economic, and so on in nature. Thus, the I’s are interfering

conditions that appear to be outside the domain of biology. The I’s could have various

kinds of effects on competitive systems and the components of such systems. However,

the I’s do not produce unsystematic genuine exceptions to CEP, but are systematically

explainable as apparent exceptions by means of coexistence and intervening coexistence

mechanisms; and we can provide the explanations in terms of proprietary ecological

terms. Below, I will use a cursive typeface to highlight the explanations of exceptions to

CEP given in distinctively ecological explanantia.

The I’s might affect the limiting resources of species and result in the coexistence

of competitors that would not otherwise have coexisted. The I’s might make certain

limiting resources superabundant (human interventions/geological factors that, for

instance, result in eutrophication of water). This would be explained, however, as a case

in which there is a luxury or hoarding competition, that is, the resources were no longer

limiting. The exception to CEP by the I’s is thus an apparent one. The I’s might

negatively affect population densities of competing species (celestial bodies falling to

earth, earthquakes, etc.) and result in coexistence that would not have happened without

the I’s. This would be a case that could be explained as an apparent exception to the

27

principle: the species are not at their environment’s carrying capacity and/or the

resources are not limiting. The I’s could destroy some of the species from the system

(earthquakes, storms, etc.) and a coexistence of competitors might follow. If, as a

consequence, the number of species in the competitive system is fewer in number than

the common limiting resources of the species, the exception would be explainable: the

competitive system is only an apparent one.

The I’s could affect the variance in resource levels (climatic and weather

conditions, storms, fires, etc.) and result in competitive coexistence that would not have

happened without the I’s. I have already given reasons above how this case can be

explained as an apparent exception to the principle: coexistence happened, for instance,

because the niches of the species are different and/or the variance in resource levels

counts as a new resource itself. The I’s could affect the environment in such a way that

it becomes spatially or temporally heterogeneous (earthquakes, storms, fires, climatic

and weather conditions, human interventions, etc.) and result in coexistence that would

not have happened without the I’s. Again, this can be explained, for instance, by

claiming that the competitive rankings between species change so that a competitive

equilibrium is not reached. Thus, there seem to be good reasons to believe that all sorts

of I’s – lower- or higher-level ones along with those within the domain of biology and

those that are not – can be explained as producing systematic apparent exceptions to

CEP by using proprietary and distinctive ecological terminology.

Proponents of ceteris paribus laws typically hold that the explanations for

exceptions to regularities in the special sciences and the specification of their application

domains come from lower levels of mechanisms and/or are stated in the vocabulary of

the lower-level sciences (Fodor 1991 and Carrier 1998). However, in the case of CEP,

ecologists are capable of explaining exceptions using distinctive and proprietary

28

ecological concepts, such as common predators, pathogens, differences in niches or

resource use, resources not being limiting, and so on, which produce apparent exceptions

to the principle. There is no need to refer to the lower-level sciences when ecologists

employ intervening and constitutive coexistence mechanisms as explanatia to the

exceptions to CEP and specifying under which background conditions the principle

should hold.

The problem of ceteris paribus laws is providing truth conditions to the proviso

clause “when some other unknown background conditions remain absent and/or the

same” that prescribes the domain of a law outside of which the law does not necessarily

hold. If the interfering background conditions were known in advance, there would be

no need for ceteris paribus clauses nor any problem of semantics for ceteris paribus

laws; in other words, there would be only an epistemic problem of finding out what

known background conditions could interfere with a law. And with regard to CEP, there

is an epistemic, not a semantic, problem, because we are familiar with different kinds of

mechanisms producing apparent exceptions to the principle, namely, intervening and

constitutive coexistence mechanisms, as discussed above.

The above points suggest that CEP should be interpreted as a strict law rather than

as a ceteris paribus law. There is no semantic problem of ceteris paribus clauses in the

case of CEP, but rather an epistemic problem of knowing which of the known

coexistence mechanisms are present when there is a prima facie exception to the

principle. The competitive exclusion principle has only the appearance of a ceteris

paribus law, while in fact it is a strict law, because it can be strictly complemented: CEP

holds except when intervening and constitutive coexistence mechanisms are in place.

There is no empirical vacuity, semantic inaccuracy, ad hocness, or open-endness in the

proviso clause of CEP, there is only the epistemic problem of figuring out which of the

29

different instances of known exception-making mechanisms is responsible for

exceptional instances to the principle, not a semantic problem of spelling out in detail

the proviso clause of the principle in terms of ceteris paribus clauses. The proviso clause

of CEP is eliminable by a known, complete, and finite list of external factors capable of

producing apparent exceptions to the principle. Moreover, the conditions under which

CEP holds can be explained by utilizing distinctively biological or ecological

terminology, suggesting that CEP is autonomous or distinctively biological as a law.

The above concludes the argument that CEP is neither weakly nor strongly

contingent in the sense of ESCT2 as a regularity, but rather is a strict and distinctive and

autonomous biological law that might be universally true and holds in all the relevant

background conditions.

Sober (1997) responded to Beatty’s thesis by claiming that Beatty failed to

recognize that laws do not hold outside their domain of application. When this domain is

added to the statement of a law, the result is a statement that is not contingent, but

instead is necessary. The Hardy-Weinberg rule is restricted to situations in which “there

are no evolutionary forces present.” Let “I” denote this applicability clause, which

defines a domain of application. When there are evolutionary forces present, the Hardy-

Weinberg rule does not apply. Let this rule be written as a statement of a universal form,

x(FxGx); the probabilistic formulation would not affect the point being made here.

According to Sober, we get a non-contingent statement if we add the clause “I” as an

antecedent to the statement of the Hardy-Weinberg rule. In other words, the statement “I

(x(FxGx))” as a whole is not contingent, although both of its components “I” and

“x(FxGx)” could be. In this way, the necessity of biological generalizations is

redeemed.

30

Sober’s response could be criticized as being a trivial logical exercise. It is similar

to another reply to Beatty’s thesis: the idea that biological regularities are ceteris paribus

laws (Carrier 1995). However, Sober seemed to distinguish his reply from that of the

ceteris paribus law. In this section, I have shown how Sober’s response can be fleshed

out and how CEP can be necessitated without referring to ceteris paribus clauses, which

many think of as being problematic because of empirical, semantic, and epistemic issues

involved in ceteris paribus clauses.

6 Conclusions

This article has provided several new results insofar as contingency and its implications

for the existence of biological laws are concerned.

Beatty was correct in arguing for the contingency of biological regularities in the

sense that such regularities lack maximal or high degrees of constancy ESCT2, but he

was less correct about trajectory stability aspects of this contingency ESCT1, as far as

the implications of contingency for the existence of lawlikeness of biological regularities

are concerned. ESCT2 provides a more general, applicable, and substantial argument

against biological laws than ESCT1. ESCT2, in contrast to ESCT1, does not presuppose

the truth of the multiple realizability thesis of biological properties. I have suggested

above and many others have argued elsewhere that the truth of this thesis, if not

debatable, is at least such as that its scope is more restricted than was traditionally

supposed. In other words, instead of there being widely heterogeneous realization bases

for biological properties at lower levels, there might in many cases be realization

necessities showing no multiple realizability.

31

Beatty’s other reason why trajectory instability ESCT1 holds for biological

regularities is that drift and mutations affect evolution by making it a contingent

outcome or result. This claim is true to a certain degree. However, there are functional

necessities or generatively entrenched traits that resist evolutionary change. The

existence of generatively entrenched traits along with the questioning of the truth of the

multiple realizability thesis suggest that, for certain traits and features of organisms,

replaying life’s tape might result in a similar evolutionary history, because functional

necessities might be realization necessities as well; that is, evolution and biological

phenomena might display trajectory stability. Finally, it was argued that regularities that

have a high degree of constancy are those that display a high degree of trajectory

stability. Regularities that hold in many or all the different background conditions are the

regularities that would have held had the initial historical background conditions been

different. In other words, a high degree of trajectory stability follows from a high degree

of constancy. As was argued above, however, the converse does not necessarily hold.

Generatively entrenched traits (and realization necessities) are capable of showing high

degrees of trajectory stability despite being instable in the sense of constancy.

There is thus the possibility that evolutionary history might show repeatable and

generalizable regularities and patterns and yet be non-lawlike, in contrast to what Beatty,

some of his commentators, and some paleobiologists have suggested. An example of a

regularity displaying a high or moderate degree of trajectory stability, but with a low

degree of constancy was suggested above, namely, Bergmann’s rule. Other putative

examples of non-lawlike frozen accidents include the Krebs cycle, Mendel’s rules,

hereditary information, and regularities concerning other functional necessities

mentioned above.

32

A counterexample to Beatty’s thesis was also suggested by the competitive

exclusion principle (CEP). CEP is not riddled with genuine exceptions, only apparent

exceptions, and it might even be universally true. Thus, CEP is not weakly contingent.

Ecologists recognize two types of mechanisms, intervening and constitutive, capable of

producing apparent exceptions to the principle. The two mechanisms specify the relevant

background or constancy stability conditions under which the principle does not

necessarily hold. When the two coexistence mechanisms are not in place, the principle

has a maximal or a very high degree of constancy stability. Thus, CEP is not strongly

contingent. Finally, the principle is distinctively biological or autonomous and strict as a

law, because ecologists are capable of explaining away its exceptions and specifying its

domain by using proprietary ecological explanantia.

Of course, the presence of one counterexample does not prove that Beatty’s thesis

does not apply to biology in general, meaning that biology is deficient in laws. There are

other responses to Beatty’s thesis, whereby a putative biological law is presented as a

counterexample to the thesis. Morgan (2009) suggested that the Caspar-Klug theory of

virus structure – a hypothesis concerning the structure and organization of capsids in

viruses – is such a counterexample. There are problems with Morgan’s suggestion. It

could be argued that the example is a biochemical regularity, lacking constancy, but

having a high or moderate degree of trajectory stability, and that its exceptions cannot be

explained in terms of of proprietary or distinctively biological explanantia. This

suggests that Morgan’s example is not distinctively biological or a strict law, in contrast

to the competitive exclusion principle discussed above.

Finally, Mitchell’s (1997, 2000) analysis of the evolutionary contingency thesis is

different from the above analysis. Mitchell treats stability as a monolithic thing, and

consequently thinks that stability has general or uniform implications for the existence of

33

biological laws. I argued that not only are there distinct forms of stability, but also that

distinct forms of stability have different implications for the existence of biological laws.

Acknowledgments The work was supported financially by the Academy of Finland as a

part of the project Causal and Mechanistic Explanations in the Environmental Sciences

(project no. 1258020). I am grateful to the anonymous referees and to the editor of this

journal, Helmut Pulte, who provided helpful comments and suggestions. N. Emrah

Aydinonat, Markus Eronen, Till Grüne-Yanoff, Andrew Hamilton, Tomi Kokkonen,

Jaakko Kuorikoski, Aki Lehtinen, Caterina Marchionni, Uskali Mäki, Anna-Mari

Rusanen, Petri Ylikoski, Tero Ijäs, Petri Turunen, Rami Koskinen, Miles MacLeod, and

Ilkka Pättiniemi all provided helpful comments, discussions, and suggestions on

previous drafts of this paper.

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