Role of sexual selection in speciation in Drosophila

Akanksha Singh • Bashisth N. Singh

Received: 20 June 2013 / Accepted: 14 December 2013 / Published online: 22 December 2013

� Springer Science+Business Media Dordrecht 2013

Abstract The power of sexual selection to drive changes

in the mate recognition system through divergence in

sexually selected traits gives it the potential to be a potent

force in speciation. To know how sexual selection can

bring such type of divergence in the genus Drosophila,

comparative studies based on intra- and inter-sexual

selection are documented in this review. The studies pro-

vide evidence that both mate choice and male–male com-

petition can cause selection of trait and preference which

thereby leads to divergence among species. In the case of

intrasexual selection, various kinds of signals play signifi-

cant role in affecting the species mate recognition system

and hence causing divergence between the species. How-

ever, intrasexual selection can bring the intraspecific

divergence at the level of pre- and post-copulatory stage.

This has been better explained through Hawaiian Dro-

sophila which has been suggested a wonderful model

system in explaining the events of speciation via sexual

selection. This is due to their elaborate mating displays and

some kind of ethological isolation persisting among them.

Similarly, the genetic basis of sexually selected variations

can provide yet another path in understanding the specia-

tion genetics via sexual selection more closely.

Keywords Sexual selection � Intra- and inter-sexual

selection � Frequency dependent sexual selection �Speciation � Drosophila

Introduction

Sexual selection has received increasing attention as a

potential factor in speciation. Although natural selection

may often play an important role in the divergence of

populations undergoing speciation (Turelli et al. 2001;

Maan and Seehausen 2011), sexual selection plays an

equally important role in the process of speciation (Sch-

luter 2001). If sexual traits have some or the other adaptive

value, they may also be naturally selected apart from being

sexually selected. Therefore, an organism is subject to both

natural and sexual selection simultaneously but strong

sexual selection is more likely to evolve premating isola-

tion. The idea of sexual selection as a driver of reproduc-

tive isolation received theoretical support only in the early

1980s (Lande 1981; West-Eberhard 1983) which is perhaps

surprising given that sexual selection frequently shapes the

very characters involved in mate preferences and repro-

ductive isolation. In recent years behavioural ecologists

have shown increased interest in sexual selection in

females as well as males. This selection results from dif-

ferential mating success among individuals within a pop-

ulation. Competition for fertilization occurs through direct

competition between members of the same sex (i.e. pre-

mating male–male competition) or through cryptic female

choice (i.e. postcopulatory mechanisms biasing fertiliza-

tion which involves sperm competition). Also, competitive

interactions within a sex may favour the evolution of

diverse, elaborate ornamental traits (Andersson 1994).

Zahavi’s (1975) handicap principle provides evidence to

the above mentioned statement of cryptic female choice,

that females which select males with the most developed

characters are those with best genotypes of the male pop-

ulation. The rapid divergence via sexual selection is

brought about through a parallel change in mate preference

A. Singh � B. N. Singh (&)

Genetics Laboratory, Department of Zoology, Banaras Hindu

University, Varanasi 221005, Uttar Pradesh, India

e-mail: bnsingh@bhu.ac.in; bashisthsingh2004@rediffmail.com

A. Singh

e-mail: singh31bhu@gmail.com

123

Genetica (2014) 142:23–41

DOI 10.1007/s10709-013-9751-4

and secondary sexual traits within a population and this

might lead to prezygotic isolation between populations.

Hence it indicates that sexual selection has the power to

drive rapid divergence and generate reproductive isolation

(Panhuis et al. 2001; Ritchie 2007; Snook et al. 2009; Maan

and Seehausen 2011). Also, there is evidence of rein-

forcement of gametic isolation in Drosophila through

sexual selection (Matute 2010). He has reported the first

case of reinforcement of postmating prezygotic isolation,

which has apparently evolved in natural populations of D.

yakuba sympatric with the sister species D. santomea.

Interest in speciation continues to grow, as evidenced by

an increasing rise in citations of speciation studies over the

last many years (Questiau 1999; Panhuis et al. 2001; Tur-

elli et al. 2001; Orr et al. 2004; Wu and Ting 2004; Ritchie

2007; Sobel et al. 2009; Maan and Seehausen 2011). In the

genus Drosophila, extensive work has been done in the

field of speciation. In these studies, the major focus has

been to understand the role of natural selection in causing

species divergence (Nanda and Singh 2012). Therefore, it

was still unclear whether only natural selection has the

power to drive such type of divergence in sexual traits, or

there is a role of sexual selection. This was answered when

studies were done in Drosophila providing evidence that

not only natural selection but also sexual selection plays a

probable role in enhancing divergence (Ritchie 2007).

There is increasing evidence that genes involved in

reproduction, specifically those showing sex-biased

expression evolve rapidly and are often subject to positive

selection and hence adaptive evolution (Swanson et al.

2001; Proschel et al. 2006). However, what has received

much less attention is exactly how this rapid evolution and

divergence is related to reproductive isolation via sexual

selection. The finding that reproductive genes in males and

females are often subject to positive selection and adaptive

evolution suggests a major role for sexual selection in any

resulting divergence (Clark et al. 1995; Proschel et al.

2006). But some questions still remain unanswered as far

as the process of speciation is concerned: (1) How sexual

selection (inter and intrasexual selection) influence repro-

ductive isolation? (2) Whether it acts directly (i.e. at the

level of sexual traits) or indirectly (i.e. at level of repro-

ductive proteins or both?) (3) Whether reproductive pro-

teins of Drosophila play any kind of role in causing

divergence among species? and (4) If yes, then which

proteins (proteins formed from male biased genes or

female biased genes) show higher level of species diver-

gence? The answers to these questions may provide a way

to understand the different levels at which sexual selection

might play a role in causing divergence.

The rapid divergence between populations (allopatric or

sympatric) might be due to the prevention of gene flow

between them. The potential effect of sexual selection on

speciation are especially evident in allopatric populations

since sexual selection can drive the evolution of signalling

and preference traits in arbitrary divergent directions even

in the absence of environmental differences (Lande 1981;

West-Eberhard 1983; Parker and Partridge 1998; Rice

1998; Gavrilets 2000). However, experimentally evolving

populations of D. melanogaster (Wigby and Chapman

2006) and D. pseudoobscura (Bacigalupe et al. 2007)

showed no such mating discrimination, indicating that

sexual selection is not an obligate promoter of reproductive

isolation in allopatric populations. In sympatric popula-

tions, however, isolation may lead to divergence via its

direct effect on traits that are involved in mate recognition

(Panhuis et al. 2001). This involves a wide range of

modalities like chemical, visual, tactile and auditory sig-

nals (Spieth and Ringo 1983; Ritchie et al. 1999; Rundle

et al. 2005). It is important to point out that the rapid

change between populations as a result of sexual selection

can also play an indirect role in speciation by increasing

the overall rate of change within isolated populations (Li-

gon 1999), this indirect role might be more important than

its direct role. Darwin (1871) noted that elaborate sec-

ondary sexual characters tended to occur in groups that also

had high species richness suggesting that sexually selected

ornamentation and preference is a potent source of selec-

tion and sexual communication can indirectly cause sexual

isolation. Several workers still debate on the use of sec-

ondary sexual traits in same sex competition thus pointing

to their role in sexual selection (Stockley and Bro-Jor-

gensen 2011).

In the 1980s researchers began to emphasize how mate

choice (female and male choice) could cause divergence

between populations thereby leading to speciation. This led

to the natural conclusion that sexual selection within pop-

ulations may lead to sexual isolation between populations.

Lande (1981, 1982) showed in his model that as long as

there is genetic variation for a male trait and a female

preference for that trait, there will be assortative mating

which generates positive covariance between the two.

West-Eberhard (1983) also presented a highly influential

view on the issue suggesting that social evolution including

both intra and inter sexual selection could cause speciation

(Coyne and Orr 2004). Analysis of the frequency of papers

published on sexual selection and speciation shows sig-

nificant upturn from the seminal West Eberhard and Lande

papers (Ritchie 2007). This and subsequent theory has

shown that it is possible for sexual isolation to evolve as

female preference and male traits drift along this line

(Uyeda et al. 2009).

Comparative evidence suggests, however, that post-

mating effects promote speciation. The first model to

explicitly address the influence of sexual conflict on spe-

ciation, concerned with conflict over mating at a parapatric

24 Genetica (2014) 142:23–41

123

secondary contact (Parker and Partridge 1998, Partridge

and Parker 1999). This proposed that the effect of conflict

on speciation depended on which sex gained the upper

hand in determining the outcome. If female preference

predominated a mating system, speciation was more likely,

but if male competition overcame female preference then

speciation would be less likely. This is one of a few models

which argue that strong sexual selection can sometimes

inhibit speciation (Panhuis et al. 2001).

The covariance between traits including relevant

behaviours such as male morphology, courtship song or

genitalia and preferences have been predicted to be the

major evolutionary forces that can cause behavioural iso-

lation (Panhuis et al. 2001; Coyne and Orr 2004; Hosken

and Stockley 2004). Evidences such as the one describing

20 sister pairs of passerine birds showing divergence due to

differences in their plumage colour provide evidence that

sexual selection may lead to speciation, in various species

taxa (Barraclough et al. 1995). Divergence also occurs by

differential mating in plant species, according to the

lengths of nectar spurs (Hodges and Arnold 1995). How-

ever, studies on sexual selection with major emphasis on

Drosophila have not been discussed much yet. Here we

provide our perspectives on exciting new research of how

sexual selection unaided by ecological divergence can

drive reproductive isolation in Drosophila. Broadly, we

discuss certain aspects of intersexual selection like role of

different types of stimuli which are the important compo-

nents of mate recognition system that may lead to specia-

tion. In addition to this, we also describe how pre- and

post-copulatory intrasexual selection might play a role in

intraspecific divergence. Further, we discuss the relation-

ship between rapid reproductive protein evolution and

reproductive isolation in the light of sexual selection.

Major emphasis has been laid on how sexual selection play

efficient role in divergence of certain traits in Hawaiian

Drosophila. Keeping these points in view, the main aim of

this review is to give an account of sexual selection oper-

ating to bring about reproductive isolation (pre- and post-

copulatory) and thereby its role in speciation. In addition to

this, how sexual selection plays a role in postzygotic

reproductive isolation thereby leading to speciation will

also be discussed.

Intersexual selection as a mode in driving species

divergence

Intersexual selection is an evolutionary process in which

choice of a mate depends on attractiveness of its traits. This

selection generates or exaggerates precopulatory traits that

improve a male’s mating success. (Darwin 1871; Anders-

son 1994). Paterson (1980) proposed that every species

possesses its own distinct specific mate recognition system

that controls the exchange of sensory information sent and

received by both sexual partners during courtship. Patterns

of mate choice can be altered by changing the costs of

choosiness without altering the preference function. How-

ever, adaptive male mate choice can lead to an important

yet unappreciated cost of sex and sexual selection (Long

et al. 2009). Sexual selection arises due to non random

variation in the mating success of individuals, which often

results from variations in the components of the mate

recognition system. The evolution of a new mate recog-

nition system can cause sexual isolation and hence speci-

ation (Ritchie 2007). However, some questions are still

unanswered, especially those concerned with variation in

mating behaviour that may provide a way to understand

divergence of sexually selected traits. The answers to these

questions can be categorised into five broad areas of

interest. Variation in mating behaviour and costs of

choosiness could:

• Influence the rate and direction of evolution by sexual

selection.

• Provide information about the evolutionary history of

female mating preferences.

• Help to explain inter specific differences in the

evolution of secondary sexual characters by relating

different tactics of mate choice to ecological factors

(time and energy costs of sampling which had a

potential constraint on optimal mate choice, predation

risk which affects female choosiness, territory or

resource quality), social or morphological factors

(interactions between males, variability in male phe-

notypes, female–female competition and female mate

copying).

• Provide information about the level of benefits gained

from mate choice.

• Provide a mechanistic account of the emergence of

mate choice (Jennions and Petrie 1997).

According to the first point, increased variability in

preference might decrease the intensity of female-driven

directional selection. For Fisherian traits (traits which are

selected do not necessarily increase survival) increased

cost may still lead to the evolution of multiple preferences

(Pomiankowski and Iwasa 1993). However, for traits

indicating viability only a preference for a single trait is

stable if assessment of additional traits increases costs

disproportionately (Iwasa and Pomiankowski 1994). The

second point may provide evidence that the Fisherian

model dealing with the evolution of ornaments predicts

considerable heritable variation in female mating prefer-

ence, both within and between populations (Lande 1981).

Turner and Burrows (1995) suggested that the genetic

bases of preferences may lead to different speciation rates

Genetica (2014) 142:23–41 25

123

among lineages. The Fisherian model was validated by the

work of Sharma et al. (2010) who provided evidence that

evolution of a trait occur due to genetic variation in female

preference. In contrast to the Fisherian models, female

choice for males with ‘viability genes’ requires neither

genotypic nor phenotypic variation among females (Grafen

1990). However, polygenic models obviously assume a

heritable basis to preference, and generally predict coevo-

lution of preference and preferred trait (Bakker 1993).

Thomas Getty (2006) in his paper has shown that appli-

cation of the handicap principle to signalling in sexual

selection is not a valid generalization. Although some of

the signalling systems, with additive costs and benefits,

have solutions that resemble sports handicaps, the signal-

ling in sexual selection has multiplicative costs and bene-

fits, and solutions that do not resemble the sports handicap.

Thirdly, most attempts to explain variation in ornamenta-

tion among species invoke strong natural selection against

the elaboration of male traits (Balmford et al. 1993; Win-

quist and Lemon 1994). Factors that inhibit female choice

will reduce selection for elaborate male traits. Variation in

the opportunity for mate choice may also affect other

features of mating behaviour. For example, Slagsvold et al.

(1988) suggested that polygyny in pied flycatchers (Fice-

dula hypoleuca) is partly due to high female sampling costs

leading to limited searches for unpaired males. Sullivan

(1994) has also noted that given severe time constraints on

female choice, females are likely to use static morpho-

logical traits that are quickly assessed. Hence information

on the duration over which females assess males may

explain some interspecific variation in male ornamentation.

Fourthly, given phenotypic plasticity in female mating

preferences it is possible to manipulate the costs of

choosiness and examine the effect on mate choice. This

should provide information about the benefits associated

with discriminatory mating (Jennions and Petrie 1997).

Fifthly, once the importance of variability in female mating

preferences is recognised, a more mechanistic account of

mate choice should emerge. As Ryan (1994) has noted,

knowledge of mechanism provides a stronger base when

explaining why certain traits have evolved (Haines and

Gould 1994).

In Drosophila, the diversity of mating behaviour in

various species and basic similarity between some species

emphasize that mating behaviour has gone through evolu-

tionary changes. Variation of different signals by which two

sexes exchange and their predominance during mating can

contribute to the appearance of the premating isolation

(Butlin and Ritchie 1994). Similarly, Ruedi and Hughes

(2008) studied variation in mating behaviour of D. mela-

nogaster and emphasized the genetics of courtship behav-

iour and various mechanisms underlying sexual selection.

Species interactions causing selection on mating traits play

important role in generating species divergence. This vari-

ation may arise due to genetic differences in developmental

trajectories or proximate environmental factors (Edward

and Chapman 2013). Numerous correlational and experi-

mental studies from many taxa now confirm that males with

increased ornamentation or possessing certain attributes

have a mating advantage (Bradbury and Andersson 1987;

Ryan and Keddy-Hector 1992; Andersson 1994; Moller

1994; Johnstone 1995). In order to maintain female mating

preference three explanations have been proposed:

(a) preferences may be directly selected due to direct ben-

efits which increase female survival or fecundity (Reynolds

and Gross 1990), (b) preferences may be maintained by

indirect selection due to genetic benefits that increase off-

spring fitness (Andersson 1994), (c) preferences may be

maintained as pleiotropic effects of natural selection on

female sensory systems in contexts other than mate choice

such as foraging or predator evasion (Arak and Enquist

1995). For many years, mate choice has been considered to

be expressed predominantly in females resulting in selec-

tion on male displays or courtship characters. However, this

perspective has now undergone both theoretical and

empirical revision (Edward and Chapman 2011). A com-

prehensive set of tests revealed fitness benefits of male mate

choice in D. melanogaster (Edward and Chapman 2012).

The phenomenon of intersexual selection was further

revealed through studies on sexual isolation reported by

Vishalakshi and Singh (2006a) between two sibling species,

D. ananassae and D. pallidosa and the results suggest that

there is preferential mating between males and females of

the same species in these two sibling species. Similarly

sexual isolation among three sibling species, D. melano-

gaster, D. simulans and D. mauritiana was studied (Car-

racedo et al. 2000). The results show asymmetrical mating

preferences i.e. D. mauritiana males mate with both D.

melanogaster and D. simulans females and females of D.

mauritiana discriminate strongly against males of these two

species, and D. simulans males mate with D. melanogaster

females but the reciprocal cross is difficult (Watanabe and

Kawanishi 1979; Carracedo and Casares 1985; Coyne

1989). Taylor et al. (2009) reviewed the findings of a series

of investigations on the fitness consequences of female

preference in D. simulans and compared them with its

sibling species D. melanogaster and found stark differences

in the fitness consequences of mating with preferred males

in the two species. This provides evidence for the existence

of assortative mating in D. melanogaster and D. simulans.

Jennings and Etges (2010) studied premating sexual isola-

tion between D. mojavensis and D. arizonae which are the

most important members of the repleta species group. Such

findings were further supported by the studies on sexual

isolation done by Banerjee and Singh (2012) in four species

of the D. bipectinata complex.

26 Genetica (2014) 142:23–41

123

Selection may act on various components of mating

behaviour including rapprochement or pair formation and

courtship behaviour (Alexander et al. 1997). It is thus

crucial to understand how evolution has influenced the

genes that shape courtship leading to species divergence.

One important aspect of this investigation is to evaluate the

biological role of each signal that is used for species dis-

crimination. In this perspective, the co-evolution of male

and female sexual signals and receptors suggests how these

may provide heretofore neglected insight into the mecha-

nism by which isolating barriers may emerge. Heterosexual

courtship in different species of the melanogaster complex

involves a series of behaviours prior to mating. It is thought

that sexual selection operated over millions of years on pre-

existing neuronal pathways, recruiting them for sexual

behaviours and producing the similarity of behavioural

elements common to all members of the genus Drosophila

(Spieth and Ringo 1983). Sturtevant (1915) first described

the courtship of D. melanogaster and attempted to identify

the stimuli involved. During courtship both partners

exchange signals that belong to multiple sensory modali-

ties. These are courtship signals and comprise chemical,

visual, acoustic or tactile stimuli (Liimatainen and Jallon

2007). These stimuli function to inform the female of

species identity of the male and to stimulate the female

beyond her acceptance threshold for accepting the male in

copulation (Spieth and Ringo 1983). Mature virgin females

vary in their acceptance threshold, but the male often has to

repeat his courtship elements often numerous times before

the female is willing to copulate. The female outcome of

courtship appears to be dependent upon the physiological

state of the female and the temporarily summed effect of

the male stimuli. However, Bretman et al. (2011) revealed

the robust mechanisms by which males of D. melanogaster

assess their socio-sexual environment to precisely attune

responses through the expression of plastic behaviour.

When the multiple cues (auditory, olfactory, tactile and

visual) were experimentally removed then the males of D.

melanogaster were unable to detect the rivals which shows

the importance of different types of mating signals in

recognition. Thus different types of stimuli that are key

components of species mate recognition system are dis-

cussed below:

Visual stimuli

The requirement for the perception of visual stimuli for

success in mating behaviour is variable within the genus

Drosophila (Grossfield 1971, 1996) and this variability can

further act as an element in isolation that thereby leads to

speciation. During courtship, some visual stimuli are

dynamic (locomotor activity, wing displays and motion)

whereas others are static (colors, shapes). The level of

interspecific variability can be observed in different species

of this genus. The comparison of the level of insemination

suggests that D. melanogaster can mate in the dark whereas

D. simulans and D. affinis tend to be inhibited in the dark

(Spieth and Hsu 1950). A strong light-effect was noted

between (and sometimes within) the four species of the

melanogaster complex. Investigators have used various

mutations of D. melanogaster to assess the role of visual

stimuli in courtship (Spieth and Ringo 1983). Such mutants

can be divided into two classes (1) those which have an

increased amount of black pigment in the exoskeleton, and

(2) those which have reduced amount of red (pterin) or

brown (ommochrome) pigments in the primary and sec-

ondary pigment cells of the compound eye. Both classes of

mutants have reduced visual acuity (Spieth and Ringo

1983). Crossley (1970) observed that wild type D. mela-

nogaster males and ebony mutants court under darkness in

a similar manner. Males of D. auraria can mate in both

light and darkness but white-eyed mutants, which can

perceive light but lacking visual acuity refuse to mate

under light but readily mate in darkness (Grossfield 1972).

The light dependent species apparently either remain

immobile during darkness or have a key element of their

courtship which is dependent upon visual stimuli. The loss

of visual stimuli might decrease the mating ability. Species

like D. subobscura, D. auraria (Isono et al. 1995) show

almost no mating ability in the dark whereas D. affinis

shows positive response for mating in the presence of light

(McRobert and Tomkins 1987). However, partial isolation

exists between two species of D. auraria complex (D.

auraria and D. triauraria), when both are exposed to light

but the same is not true in the dark as both exhibit complete

isolation (Oguma et al. 1996). Thus, it is quite clear that

visual stimuli often impart its effect in Drosophila mating

system. However, mutations affect such behavioural

aspects. For example there is inhibition of mating of white-

eyed mutants in light conditions in Drosophila and this

could be explained by several hypotheses that eye pigment

deficient mutants exhibit a deficit of optomotor response or

there occurs some neurobehavioral disruption produced by

faulty visual input. Similar type of study was done by

Chatterjee and Singh (1988) in D. ananassae when they

found that white -eyed males are more successful in mating

in the dark than in light. However, such type of mating

deficit in the dark was also found in red-eyed males due to

their reduced locomotor activity. In certain species of

Drosophila i.e. D. suzuki and D. biarmipes, males possess

dark black patch on their wings which serve as a visual

stimulus to the female during courtship. Removal of black

shade in wings reduces mating success in males. Such type

of study was done by Singh and Chatterjee (1987a) in D.

biarmipes suggesting that visual stimuli play an important

role in the mating behaviour of D. biarmipes. These types

Genetica (2014) 142:23–41 27

123

of visual signals may cause differential mating and hence

play an important role in mate recognition system and

thereby lead to species divergence.

Acoustic stimuli

Variation in courtship songs is thought to contribute to

reproductive isolation in various species of Drosophila. It

influences female receptivity during courtship and species

recognition (Ewing and Bennet-Clark 1968; Liimatainen

et al. 1992; Tomaru and Oguma 1994). These songs consist

of two elements: the sine song and the pulse song. The sine

song of D. melanogaster consists of humming sound that is

reminiscent of flight sound. The pulse song exhibits inter

and intra specific variations among the sibling species of

the D. melanogaster subgroup like melanogaster, simulans,

mauritiana, erecta, yakuba and teissieri, in the number of

cycles per pulse, pulse repetition rate and in the duration of

interpulse interval (IPI) (Spieth and Ringo 1983). The male

pulse songs are species specific. Interspecific variations

arise due to involvement of volume and quality of sound

produced (Ewing and Bennet-Clark 1968; Chang and

Miller 1978). While sympatric sibling species typically

have songs that differ significantly in the mean IPI e.g.

melanogaster and simulans, pseudoobscura and persimilis,

allopatric pairs have similar songs i.e. ananassae and

athabasca, pseudoobscura and ambigua (Spieth and Ringo

1983).

Species recognition is often based on variation in the IPI

or pulse frequency (Bennet-Clark and Ewing 1969; Ewing

and Bennet-Clark 1968; Ritchie et al. 1999). Song evolu-

tion in this group does not always show phylogenetic

trends (Alonso-Pimentel et al. 1995; Etges 2002) suggest-

ing that courtship songs in Drosophila species may often

evolve too rapidly to discern clear pattern of evolution as in

the D. willistoni group (Gleason and Ritchie 1998). When

mating sound was studied in six D. affinis subgroup species

(affinis, algonquin, athabasca, azteca, narragansett and

tolteca), the results showed similar pattern as that of D.

athabasca which is a widespread North American species

and consists of three semi species with different courtship

songs (Miller et al. 1975). Most affinis subgroup species

possess both low and high pulse repetition courtship

sounds. Differences in courtship and mating sounds

between D. affinis subgroup members seem generally

substantial and are most likely to be sufficient for one’s

recognition of these species and semispecies. It has been

reported that D. narragansett and D. tolteca show dis-

tinctive courtship sound patterns i.e. low and high pulse

repetition sounds was found to be present in D. tolteca

whereas D. narragansett shows only high pulse repetition

sound. Absence of interspecific mating between D.

algonquin and D. affinis clearly indicates differences in

their sound patterns (Chang and Miller 1978). However,

inter- and intra-population divergence was also observed in

D. montana for male courtship song (Klappert et al. 2007).

But keeping all these facts under consideration, courtship

song may not be the only factor responsible for species

discrimination. In the affinis group it is evident that closely

related species show distant similarity with respect to

courtship sound whereas in D. athabasca, courtship sound

is found to be very similar to distantly related, D. azteca.

Experiments carried out with song stimulation show that

females are able to recognize homospecific males through

discrimination via IPI, rhythm and pulse length (Kyriacou

and Hall 1982; Isoherranen et al. 1999). D. simulans and D.

mauritiana show difference in IPI hence females of both

species show discriminating nature in selecting their mates.

Kyriacou and Hall (1982) studied the inheritance pattern of

IPI through segregational analysis and found that IPI gene

is said to be located on X chromosome adjacent to per

gene. This was further put into confirmation when per gene

of D. simulans was transferred to per null D. melanogaster

then D. melanogaster confers a characteristic D. simulans

song rhythm. However, this may not be the case always as

IPI of D. auraria complex is controlled by each autosome

(Tomaru and Oguma 1994). The contribution of the non A/

diss gene to evolutionary variation has been shown in

numerous experiments with D. virilis whose song differs

from that of D. melanogaster in many parameters like long

and short pulse etc. (Aspi and Hoikkala 1995; Isoherranen

et al. 1999). The average IPI shows little variability in D.

melanogaster suggesting that this acoustic parameter is

under very strong selection (Ritchie and Kyriacou 1994,

1996). However, there was positive response after one

generation when artificial selection for IPI phenotype was

done. Similarly, Watson et al. (2007) provided the first

description of the song of D. santomea. They reported that

D. yakuba and D. santomea had the largest difference in

IPI between any species of the melanogaster group. They

state that the IPI of secondary and primary song types

differed significantly between species with D. santomea

having much shorter IPI than D. yakuba. Divergence in the

IPIs is large and considerably larger than between other

sibling species of the melanogaster group. This could

indicate that songs also play considerable role in causing

sexual isolation. However, Saarikettu et al. (2005) man-

aged to break down sexual isolation between D. montana

and D. lummei by playing back artificial D. lummei song

modified to have the IPI typical of D. montana to D.

montana females. Similarly Li et al. (2012) studied copu-

latory song in three species of the Drosophila montium

subgroup i.e. D. lini, D. ogumai and D. ohnishii through the

analysis of F1 and backcross generations. D. lini and D.

ogumai produce similar high frequency sine song but a

third species D. ohnishii repels males of the other two

28 Genetica (2014) 142:23–41

123

species (Wen et al. 2011). Therefore, from these evidences

it is understood that acoustic signal divergence plays a very

specific role in reproductive isolation via sexual selection

(Wilkins et al. 2012).

Chemosensory stimuli

Behavioral change may often be the initial trigger for

population divergence (Mayr 1946; Butlin and Ritchie

1994) and altered recognition, processing and response to

chemical cues are expected to be involved in many

behavioural changes. Consequently, understanding the role

and relationship of chemosensory evolution to behaviour is

just for understanding speciation. Chemosensory reception

which includes olfaction and gustation may have a large

role so that differences in pheromones function as mating

signals and can influence sexual isolation (Smadja and

Butlin 2008). Chemical communication is brought by

cuticular hydrocarbons which act as courting contact sig-

nals. Variability in CHC occurs due to differences in chain

length (presence or absence of double bonds). It has been

known that insects frequently employ chemical signals

during courtship. It is not surprising that chemosensory

speciation is documented in many insects e.g. bees (Ver-

eecken et al. 2007); beetles (Peterson et al. 2007); and

walking sticks (Nosil et al. 2007). Divergence of CHC

among species suggests its role in species recognition and

speciation in Drosophila (Etges and Jackson 2001). In D.

melanogaster group, D. simulans and D. mauritiana exist

in sexually dimorphic forms and when asymmetrical

reproductive isolation occurs, it is found that males of

sexually dimorphic species court females of all species,

whereas monomorphic species males will court only con-

specific females. Hence it was predicted that sexual isola-

tion occurs through differences in female CHCs (Coyne

et al. 1994; Coyne 1996). Several types of quantitative and

qualitative differences in CHC blend and thereby give rise

to premating isolation between D. virilis and D. nova-

mexicana (Doi et al. 1996), D. serrata and D. birchii

(Howard et al. 2003) and D. santomea and D. yakuba (Mas

and Jallon 2005). However, in D. pseudoobscura and

D. persimilis there is no role of CHCs in sexual isolation

but mate discrimination in sympatric populations relies on

olfaction (Ortiz-Barrientos et al. 2004). Between popula-

tions, divergence has been observed in D. mojavensis

(Etges and Jackson 2001). In D. melanogaster, the diver-

gence in CHC is likely to occur in different populations

particularly when African and Carribean populations are

compared with the rest of the world populations (CHC 7,

11-HD). Studies on sexual isolation in the Drosophila

species have focussed on the cosmopolitan (M) and Zim-

babwe (Z) races of D. melanogaster. The former race

occurs throughout the world whereas the latter occurs only

in Zimbabwe, Zambia and Botswana. In some parts of

Africa such as Zimbabwe, individuals of both races appear

to be sympatric (Hollocher et al. 1997; Fang et al. 2002;

Takahashi and Ting 2004). These races show marked but

asymmetrical sexual isolation since Zimbabwe type

females has discriminating power for cosmopolitan males

whereas reciprocal mating occurs readily (Wu et al. 1995;

Hollocher et al. 1997). This kind of selection by Zimbabwe

females towards cosmopolitan males is a kind of female

discrimination which may give rise to partial sexual iso-

lation among the population.

Intrasexual selection in species divergence

Intrasexual selection in contrast to intersexual selection

occurs when members of the same sex of a species compete

with each other in order to gain opportunity to mate with

members of the opposite sex e.g. the male–male competi-

tion for females. In the genus Drosophila, pairing and

copulation are synchronous. Bateman (1948) studies in-

trasexual selection in the form of sexual isolation at the

level of subspecies, geographic races and mutants. Usually,

males are in the central arena where intrasexual selection

occurs whereas females are often those which exert choice.

Darwin (1871), however, was unable to explain such type

of sex difference but this was rather an important aspect of

intrasexual selection.

Intrasexual selection has been categorized into pre and

post copulatory intrasexual selection in order to study

species divergence in Drosophila more closely. Precopu-

latory sexual selection is based on an individual’s ability to

physically dominate a rival. Rendel (1944) provide evi-

dence about the role of intrasexual selection in D. su-

bobscura using wild and mutant forms of Drosophila.

These findings revealed that the males court both type of

females but it is the female which show discriminating

nature towards either of the males. Such type of difference

takes place due to difference between the two sexes which

leads to differential mating tendency. Similarly, Tan (1946)

observed such type of sex difference in D. pseudoobscura.

Likewise, aristapedia mutant in Drosophila reduces the

mating ability of females whereas Bare Curly mutants

enhance the mating ability of females. Intrasexual selection

plays potent role in causing sexual isolation between two

subspecies of D. virilis i.e. D. v. virilis and D. v. ameri-

cana. This was experimentally proved when males were

confined with females of the opposite subspecies it shows

discrimination (Stalker 1942). The above finding was fur-

ther supported by the work done by Singh and Chatterjee

(1987b) in D. ananassae. They studied the variation in the

mating propensity and fertility in five laboratory strains of

D. ananassae, established from single females collected

Genetica (2014) 142:23–41 29

123

from different geographical localities and the results

clearly indicate the presence of intraspecific variations

which is due to the difference in sexual activity of males,

that means males are more subject to intrasexual selection.

Similar type of results were obtained from the study done

by Singh and Sisodia (1995) in the laboratory strains of

D. bipectinata where variation in mating propensity occurs

due to difference in the sexual activity of both the sexes.

Therefore, these evidences suggest that though the dis-

crimination is sometimes found in males but it is more or

less restricted to females. This is further put into relevance

by the study of mating ability of homo- and hetero-kary-

otypes of D. ananassae from natural populations and the

study suggests that chromosomal polymorphism in D.

ananassae may have a partial behavioural basis and males

are inherently more subject to intrasexual selection (Singh

and Chatterjee 1986). However, the fact that males are

more subject to intrasexual selection was still debated by

the study of Singh and Singh (1999) for the mating success

on six wild type strains of D. ananassae. The results clearly

implicate that the variation among the strains for the

mating success in different geographical strain is due to

variation in receptivity of females than sexual activity of

males which reveals that females also play significant role

in intrasexual selection. Such intra-specific variation is due

to variation in their genetic constitution. Similarly, diver-

gence in body size, underlie the evolution of incipient

reproductive isolation between a set of D. melanogaster

populations which provide an example how selection acts

on body traits (Ghosh and Joshi 2012). Therefore, keeping

this in view, Vishalakshi and Singh (2008) studied whether

there is any relationship between mating success and size

and asymmetry of different morphological traits, using two

geographical strains of D. ananassae. The results suggest

that the size of the sexual trait is a more reliable indicator

of individual quality in sexual selection rather than fluc-

tuating asymmetry (FA) in D. ananassae. The mating

system of a species was considered by Darwin (1871) to be

an important element in determining sexual selection. The

only mating system in which intra-sexual selection is

ineffective is strict monogamy with numerical equality of

both sexes.

However, post copulatory (post mating) sexual selection

is also considered as a driving force where differential

selection by females (cryptic female choice) (Eberhard

1996) or competition between males (sperm competition)

leads to rapid divergence between species. Piscedda and

Rice (2012) have clearly provided evidence that the post

copulatory process bears potential to drive evolution of

promiscuous mating system similar to that of female mate

choice. One of the important aspects of post copulatory

sexual selection is female remating since it determines the

patterns of sexual selection and sexual conflict. Female

remating has been studied in various species of Drosophila

under both natural and laboratory conditions (Singh et al.

2002). Female remating is fundamental to evolutionary

biology as it determines the patterns of sexual selection via

sperm competition. The patterns of remating may provide

an insight into the phylogenetic relationship shared by the

four closely related members of the D. bipectinata complex

(Singh and Singh 2013). Sperm competition is another

aspect of intrasexual selection and is considered as an

outcome of female remating. In this context sperm com-

petition can also drive selection of offensive and defensive

male traits. In one way forces will favour males that may

affect the storage of another male’s sperm or that can use

his own sperm in such a way that his own fertilization

success is maximized. While the other way round males

that are able to prevent or reduce subsequent competition

from sperm of other males gains an advantage. Hence the

impact of sperm competition on male fitness (Gromko and

Pyle 1978) exhibits an excellent example of sexual selec-

tion. Sperm competition offers a unique opportunity to

study adaptations shaped by the interacting forces of nat-

ural, sexual and antagonistic selection (Rice 1996). The

occurrence of sperm competition in Drosophila is depicted

through the proportion of progeny produced by second

male in double mating experiments (Singh et al. 2002).

This approach has been used to quantify genetic variation

underlying sperm competition and thereby elucidate the

dependence of different male competitive abilities on the

genotypes of the females with which they mate in order to

discern the potential role of sperm competition in species

isolation (Civetta 1999). Manier et al. (2013 a, b) studied

species specific sperm precedence mechanism in D. simu-

lans and D. mauritiana by expressing GFP or RFP in sperm

heads of these sister species. This experimental approach

illustrates how sperm precedence mechanism can be used

to predict the mechanisms and extent of reproductive iso-

lation between populations and species. Similarly, rapid

evolution of reproductive traits has been attributed to

sexual selection arising from interaction between sexes. At

the post-copulatory level, intra- and inter-specific size co-

evolution between male sperm and female sperm storage

organs have been documented in Drosophila (Pitnick et al.

1999, Miller and Pitnick 2002). In the extreme case, it has

been suggested that females may mate with a number of

males and then select the sperm that will be used to fertilize

the eggs (Eberhard 1996). The selective pressure arising

from sperm competition has led to numerous adaptations to

assist males in gaining fertilization. These adaptations may

be (1) pre- and post-copulatory guarding behaviour, (2)

mating plugs, (3) chemical or physical characteristics of

the ejaculate which reduce receptivity to remating, (4)

sperm displacement, and (5) sperm precedence. The fore-

most problem which is faced at the time of sperm

30 Genetica (2014) 142:23–41

123

competition is that which sperm should get access to fer-

tilize the eggs of female. Sperm competition may be

intense when the sperm of several males is stored simul-

taneously within specialized storage organs of the female

reproductive tract before fertilization. It has also been

conjectured that males have evolved to produce large

quantities of sperm in order to confuse or confound

female’s cryptic system of selection (Wigby and Chapman

2004). Recent investigations on sperm precedence mech-

anisms in three closely related species of Drosophila

revealed how postcopulatory sexual selection leads to

divergence in male and female reproductive traits (Manier

et al. 2010, 2013c; Lupold et al. 2011). In many insect

species, costs of multiple mating are offset via direct

benefits due to: (1) the replenishment of sperm stores, and

(2) nutrient donations found in the ejaculate from each of

the mates. However, evolutionary maintenance of polyan-

dry in insects can be understood as direct benefits. This

could be explained in D. melanogaster whereby males

exposed to rivals subsequently mate for longer and thus

accrue fitness benefits under increased competition (Bret-

man et al. 2009). Similar type of study in D. melanogaster

revealed that remating by small bodied low fecundity

females resulted in the production of daughters of rela-

tively higher fecundity, whereas the opposite pattern was

observed for large–bodied females. This shows the direct

and indirect benefits of polyandry on the fitness of an

organism (Long et al. 2010).

Role of sexual conflict in speciation

In sexual reproduction there are two individuals who may

actually have no genetic interest in each other’s future, the

parents, but who nevertheless have a joint genetic interest

in the other individual or group of individuals. A conflict

that expresses courtship acceptance or refusal usually

arises because each parent’s fitness is generally maximized

if it invests less and the other parent invests more than

would maximize the other parent’s fitness. This conflict has

been referred to as ‘‘the battle of the sexes’’. Most of the

conflict between mates is the result of post-copulatory

sexual selection that is sperm competition and cryptic

female choice (Eberhard 1996). Traditionally post copu-

latory sexual selection generates sexual conflict through

three discrete processes: (1) there may be conflict over how

many gametes are dedicated to each mate (Parker 1970).

This type of conflict will generate selection for traits such

as copulatory plugs (Polak et al. 2001), antiaphrodisiacs

and mate guarding by males, which invest in gametes

monopolized through direct intervention of mate behav-

iour, (2) conflict may evolve through physiological trade-

offs between traits contributing to reproductive success,

this is common because one sex invests predominantly in

offspring while the other sex (males) invests predominantly

in fertilization opportunities (Bateman 1948) and (3) sexual

conflict may arise when traits that are adaptive for one sex

in reproductive competition have negative effects on the

opposite sex for example the toxicity of male seminal fluid

proteins in Drosophila (Chapman et al. 1995). Any devi-

ation from monogamy increases sexual conflict because an

individual’s lifetime reproductive interests will not coin-

cide (Rice and Holland 1997). Therefore, sexual conflict

should increase with multiple mating as does the potential

for sperm competition as a result, sperm competition

should enhance sexual conflict and thus lead to the evo-

lution of characters that increases reproductive success in

one sex, while they are costly to other. There are two

arguments on the path of sexual conflict i.e. (1) The one

who experience the strongest selection pressure will win

the conflict and (2) those that are in a superior position to

manipulate the other will win the evolutionary race. The

male will also feel any costs to females from mating with

males through reduced reproductive success which

improves the competitive ability of sperm. Thus, selection

for male paternity assurance is expected to be stronger than

selection for female resistance to mating or male adapta-

tions (Parker 1970). Hosken and Snook (2005) in their

review suggest that sexual conflict generates sexually

antagonistic evolution. Alteration of the operational sex

ratio of adult Drosophila over just a few tens of generations

can lead to altered ejaculate allocation patterns and the

evolution of resistance in females to the costly effects of

elevated mating rates. Manipulation of the relative inten-

sity of intra- and inter-sexual selection can lead to repli-

cable and repeatable effects on mating systems and reveal

potential for significant contemporary evolutionary change

(Edward et al. 2010). The co-evolution between males and

females that can be caused by sexual conflict can result in

several types of evolutionary dynamics (Parker 1970). One

of these is reproductive isolation that might lead to speci-

ation. Two contributions investigate this experimentally.

Gay et al. (2009) test the prediction that under models of

sexual conflict, larger rather than smaller population size

may lead to more rapid reproductive isolation. Hosken

et al. (2009) illustrate using data from two experimental

evolution studies in flies that the experimental manipula-

tion of sexual conflict may provide evidence for repro-

ductive isolation in some species. Recently, Pitnick et al.

(2001) demonstrated that sexual selection favours larger

males which invest a greater production of their total

energy produced in sperm production. While observing the

greater reproductive success of females with monogamous-

line males they suggested that male and female reproduc-

tive success interest’s do not naturally counteract in D.

melanogaster. However, studies carried out by Somashekar

Genetica (2014) 142:23–41 31

123

and Krishna (2011) revealed that females of D. bipectinata

prefer to mate with older males giving a very good example

of intrasexual selection. In contrast to this, similar type of

study revealed that attractive males do not sire superior

daughters which contradict the good gene model (Taylor

et al. 2010).

There is increasing evidence that genes involved in

reproduction, specifically those showing sex-biased

expression evolve rapidly and are often subject to positive

selection (Swanson et al. 2001; Proschel et al. 2006). For

example male sperm competition success in D. melano-

gaster is associated with certain male genotypes of seminal

accessory gland proteins (Acps) (Clark et al. 1995; Fiumera

et al. 2005) and certain features of the evolution of these

Drosophila Acps are also consistent with sexual selection.

Hence sexual selection might explain the rapid evolution of

reproductive protein in theory leading to speciation. The

Acps of Drosophila which comprise the bulk of the non-

sperm part of the male ejaculate, have been subjected to the

most intense investigation (Swanson et al. 2001; Mueller

et al. 2005) and it is estimated that about 10 % of them

show some evidence of positive selection (Swanson et al.

2001).

Acp genes in D. melanogaster do not appear to have

homologues in D. pseudoobscura because in at least some

cases D. melanogaster Acp genes lacking homologues in D.

pseudoobscura show evidence of directional selection and

hence adaptive evolution (Mueller et al. 2005). Moreover,

there is accumulating data to show that non-homologous and

sometimes very different, genes can encode very similar

seminal fluid traits across species (Mueller et al. 2005; Begun

et al. 2006; Braswell et al. 2006; Davies and Chapman 2006).

A fundamental question regarding rapid reproductive pro-

tein evolution is whether such changes simply result in

secondary isolating barriers. Alterations in reproductive

proteins can indeed result in reproductive isolation and could

theoretically cause speciation in allopatry or sympatry (Sa-

inudiin et al. 2005). Furthermore, in Drosophila there is

some experimental evidence that Acps are associated with

reproductive isolation. This can be proved when a cross

between male D. pulchrella and female D. suzukii is sterile

even though sperm are transferred (Fuyama 1983). However,

the cross can be made fertile if the female is given a dose of

D. suzukii Acps. This suggests that the isolation is at least

partly maintained by the actions of Acps. If reproductive

isolation is associated with rapid evolution in reproductive

proteins, there is also merit in asking how exceptionally

labile novel reproductive genes evolve. It is evident that new

Acp gene could be created by means other than gene dupli-

cation possibly from non coding regions of DNA (Begun

et al. 2006).

This was further revealed by Singh and Jagadeeshan

(2012) who suggested that Drosophila sex and reproduction-

related (SRR) genes evolve faster than non reproductive

proteins unveiling the importance of SRR molecules in

speciation research. When genes expressed in testis, ovary,

and non reproductive tissues were screened for rates of

evolution it became proportion of genes in reproductive

tissue evolved more rapidly than genes expressed in non

reproductive tissues. FA is often used as a measure of

developmental instability resulting from perturbations in

developmental pathways. This is explained by the work of

Polak et al. (2004) who have demonstrated that the sex comb

in D. bipectinata is subject to intraspecific sexual selection

suggesting that the sex comb differences seen across popu-

lations throughout the species geographic range are at least in

part the result of adaptive diversification driven by differ-

ential mating success. The data also indicates a potential

constraint on the evolution of sex comb size driven by the

combined effects of selection and a genetic interaction with

comb asymmetry which could promote the maintenance of

heritable genetic variation underlying expression of this

sexual ornament. In D. ananassae it has been found that

magnitude of FA differs significantly among morphological

traits being lowest for non-sexual traits and highest for sexual

traits suggesting that sexual traits are better indicator of

developmental stress (Vishalakshi and Singh 2006b). Like-

wise, Therefore, a divergence trend of tes-

tis [ ovary [ somatic genes emerged suggesting male and

female SRR genes evolve under different selective

pressures.

Frequency dependent sexual selection in Drosophila

Natural selection is seldom constant and changes with

abiotic and biotic factors in the environment. In the field of

population genetics the maintenance of genetic variability

in a given population is of foremost importance. Here the

importance of phenomenon of frequency-dependent selec-

tion may be mentioned as it maintains the genetic vari-

ability in a population. It has been experimentally

demonstrated that the selective value of a given genotype is

often dependent on the function of its frequency in the

population. Frequency dependence may be positive (i.e. in

favour of the common type), or negative (i.e. in favour of

the rare type). Rare-male mating advantage or minority

male mating advantage is one of the interesting and best

studied examples of this frequency-dependent selection.

When two variants of the same species are present together

the rare type is more successful in mating than the common

type. The phenomenon of rare-male mating advantage is of

considerable evolutionary significance as it plays an

important role in the maintenance of high levels of genetic

variability (Singh 1999). An important consequence of

rare-male mating advantage is that it promotes outbreeding

32 Genetica (2014) 142:23–41

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because an occasionally visiting male from another popu-

lation tends to be favoured by the female. The rare-male

effects have been reported when Drosophila males differ at

single loci affecting external somatic traits, when males

come from different laboratory strains, are of different

karyotypes, or carry different isozyme variants (Singh and

Sisodia 2000). Petit (1951) was the first to report the

occurrence of rare-male mating advantage in multiple

choice mating between Bar eye and wild type Drosophila

melanogaster. Ehrman (1966) demonstrated frequency-

dependent selection between populations of different geo-

graphic origin in D. pseudoobscura and D. paulistorum.

Minority male mating advantages have so far been reported

in 9 species of Drosophila: melanogaster, pseudoobscura,

persimilis, willistoni, tropicalis, equinoxialis, funebris,

ananassae and bipectinata (Singh 1999). Singh and Chat-

terjee (1989) studied this phenomenon in D. anannassae by

using sepia and cardinal mutant stocks and wild type stock

in order to detect rare-male effect and they found that both

types of males are more successful in mating when they are

in minority. Thus, the results provide evidence for the

existence of a minority male mating advantage in D.

ananassae. Likewise, density and frequency-dependent

selection on the signed locus was studied in D. melano-

gaster. The results clearly revealed that at higher density

the frequency of either of the two genotypes (wild type and

mutant) is low, its viability increases. While, when the

frequency of either of the two genotypes is high its viability

decreases which suggest the inverse relationship between

frequency and adaptive value (Singh and Sisodia 2000).

However, Markow et al. (1978) did not find a rare-male

effect for the mutant sepia competing with a wild type in D.

melanogaster. However, similar type of studies was done

by Singh and Sisodia (1997) in D. bipectinata by using

wild type and cut wing mutants and the results clearly

illustrate that both type of males were more successful in

mating when they are in minority. The genetic basis was

given by Som and Singh (2004) by studying such type of

selection on the alpha inversion in the left arm of the

second chromosome (2L) in D. ananassae by using two

strains: ST/ST standard gene arrangement and AL/AL

alpha inversion in 2L. The results clearly illustrate the

presence of minority male mating advantage and prefer-

ential mating found in the AL/AL strain which shows

inversion karyotype also plays role in rare-male mating

advantage. When similar study was carried out by Som and

Singh (2005) in D. ananassae by using three pairs of wild

type i.e. Mysore, Pune and Tirupati and three types of

mutant strains i.e. yellow body colour, claret eye colour

and cut wing and they found one sided rare-male mating

advantages one for claret eye colour males and other wild

type males (Tirupati). However, no advantage was found

for rare males with Mysore and yellow body colour. Hence

this study provides evidence for minority male mating

success and minority female mating disadvantage in D.

ananassae. Rare-male mating advantage was also studied

at an enzyme locus in D. pseudoobscura. The study

revealed that Amy locus has an effect on the mating

behaviour which includes some degree of rare-male mating

advantage (Singh and Sisodia 2000). Therefore it is well

proven from the above findings that a rare-male effect

seems to occur for mutants, inversion karyotype, isozyme

variants, and geographic strains, strains reared at different

temperatures and having behavioural differences. In order

to explain the rare-male effect, Ehrman and Spiess (1969)

suggested sampling and habituation hypothesis. According

to this hypothesis nature of cue is different for different

male types. The females become conditioned against

mating with the males that first court them during their

unreceptive period after eclosion. Since these males would

usually be the more frequent type, the rare-male type

would gain mating advantage when the females become

sexually active as they are able to break through the

habituation by its slightly different cues. Thus from an

evolutionary point of view rare-male mating advantage

bears a great importance in the field of population genetics.

Initially the rare genotype will increase in frequency if

there are no other selective forces operating against it but

as soon as this rare-male becomes common its advantage

decreases. Thus as a result of frequency-dependent sexual

selection, a balanced polymorphism can be maintained by

sexual selection in the absence of heterosis in the hetero-

zygotes. If such type of phenomenon is at all widespread in

natural populations, it may play a considerable role in

maintaining genetic diversity. Hence rare-male mating

advantage is of great importance in genus Drosophila as

the number of genes and chromosomal polymorphism in

Drosophila is maintained by such frequency dependent

selection (Singh and Sisodia 2000). Therefore, it is well

understood from the above facts that the cause of rare-male

effect is not yet fully resolved. Nonetheless, this effect is

likely to play an important facet in the process of sexual

selection and speciation.

Sexual selection and speciation in Hawaiian Drosophila

Recently, studies on sexual selection and speciation in the

Hawaiian species have been enhanced, since Hawaiian

Drosophila has stimulated considerable thought about the

role of sexual selection in speciation. As far as we know

the Hawaiian Drosophila have astonishing diversity i.e.

they represent about 20 % of the described species in a

world-wide distributed genus, despite the fact that

Hawaiian Islands have such a small land area (Carson

1982). The closely related species are however not

Genetica (2014) 142:23–41 33

123

ecologically different but they differ in secondary sexual

traits such as courtship pheromones (Tomkins et al. 1993),

the number of tibial bristles (Carson and Bryant 1979),

head width (Boake et al. 1997), acoustic signals and

courtship behavioural response (Hoikkala and Kaneshiro

1993). Several extraordinarily close and apparently newly

evolved species pairs have been identified on the newest

island, Hawaii. Among them the three best studied mem-

bers of the genus in Hawaii, D. planitibia, D. silvestris and

D. heteroneura inhabit cloud forest on the flanks of vol-

canoes on Maui (D. planitibia) and Hawaii (D. silvestris,

D. heteroneura). The flies of Hawaii are highly suitable for

investigation of the role of sexual selection in speciation

due to two possible reasons. First they have elaborate

mating displays and show some degree of ethological

isolation. Second, the crosses between them are usually

fertile (Craddock 1974; Ahearn and Templeton 1989).

Ringo (1977) proposed that sexual selection was the main

reason for the great diversity of this group. However,

Templeton (1979) disagreed arguing that the sexual

selection is generally stabilizing and hence could not lead

to divergence. Hence the debate between the two raises the

question that sexual selection is ever directional and spe-

cifically whether directionality is found in the Hawaiian

Drosophila.

To answer these questions, Kaneshiro (1976, 1980)

proposed a hypothesis based on the studies done on the

members of the planitibia group. He observed that there is

existence of behavioural isolation among the four most

recently evolved species of the planitibia group. Behav-

ioural isolation is often asymmetrical with females of more

ancestral species being unlikely to mate with males of the

more derived species. This could lead to a pattern of

asymmetrical behavioural isolation, with ancestral females

being narrower in their preference (Boake 2005). Another

model was put forward which states that the males do not

provide resources to females in relation to mating and that

females visit many males and compare their mating dis-

plays which helps them in finding the most appropriate

mate. This shows how sexual selection influences the

divergence of phenotype and hence leads to reproductive

isolation. Ahearn et al. (1974) studied sexual isolation

among three species of Drosophila i.e. D. heteroneura, D.

silvestris and D. planitibia and found that D. heteroneura

and D. silvestris are sympatric while D. planitibia shows

allopatric distribution. In order to understand behavioural

isolation among Hawaiian species, studies based on mor-

phological traits such as head width, shows differential

pattern between the two species. It was revealed that the

females prefer males with a broad head, which are also

more likely to win the fights. This shows that head width is

sexually selected through both female mating preferences

and male–male aggression and that selection is directed in

favour of broader heads. However, no significant difference

for the mating success was found for the two types of

hybrid males with D. heteroneura females. This shows that

head width was not involved in intraspecific mate choice.

Also, secondary sexual traits and number of tibial bristles

were compared across populations of D. silvestris and

significant differences were found (Carson and Bryant

1979; Carson et al. 1982). Similarly, flies of Hawaiian

region possess spectacular diversity in male foreleg mod-

ifications which thereby represent the results of sexual

selection (Stark and O’Grady 2009). Recently, qualitative

and quantitative chemical compositions of cuticular

hydrocarbons (CHCs) in 138 flies belonging to 27

Hawaiian Drosophila species, picture winged and non-

picture-winged were analyzed regarding sexual dimor-

phism, differences in saturation, branching position and

length of CHCs. The study shows significant variation in

CHCs pattern i.e. new species show decrease in unsaturated

hydrocarbons and gradual increase in branched com-

pounds, monomethylalkanes and dimethylalkanes (Alves

et al. 2010).

However, Boake and Konigsberg (1998) studied in

detail the genetics of sexual selection and speciation with

major emphasis on genetic analysis of male sexually

selected traits in D. silvestris. Among all the traits

observed, wing vibration seems to play role in behavioural

isolation. These examples clearly illustrate that Hawaiian

Drosophila are excellent model to study the role of sexual

selection in causing species divergence.

Genetics of sexual selection

What type of genetic changes brings about speciation is

one of the most basic questions in biology. The concept of

sexual selection involves identification of genes that are

involved in the divergence of sexually selected traits.

Usually, it has been observed that traits that are related to

mating behaviour and fertilization have a direct role in

species formation. Hence one will expect for high diver-

gence between species in those genes whose products are

linked to sexuality. Studies on the molecular evolution of

genes may help us to understand the role played by dif-

ferent evolutionary forces during evolution. The rapidly

accumulating number of gene sequences provides an

opportunity to study the molecular nature of sex related

gene evolution. These genes show an interesting pattern of

high divergence between related species. Clark et al.

(1995) studied the role of reproductive protein in the genus

Drosophila and revealed that the male accessory gland

proteins affect female postmating behaviour and sperm

precedence. For example, Acp26A has shown high diver-

gence between species (Aguade et al. 1992; Tsaur and Wu

34 Genetica (2014) 142:23–41

123

1997) and this is most probably due to directional selection

(Aguade 1998). This type of evolution has been also

detected for Acp29 and Acp70A (Aguade 1999). Karotam

et al. (1993) studied the divergence of Esterase-6 which is

an ejaculatory duct protein between D. melanogaster and

D. simulans. Similarly, gene called transformer (tra) which

involves in sexual differentiation in Drosophila shows poor

conservation between D. melanogaster, D. simulans, D.

erecta, D. hydei and D. virilis (O’Neil and Belote 1992).

However, certain sets of genes that have a direct as well as

indirect role in courtship behaviour have been suggested to

be involved in variations in mating preference like

cacophony, fruitless, Voila, courtless, desat as well as per,

nonA/dissonance and dissatisfaction. Also, sex specific

transcripts of two loci, fruitless (fru) and double sex (dsx)

determine male versus female identity (Williams and

Carroll 2009). For sexual signal traits, such as wing song

and pheromones, sex specific neuron development is crit-

ical (Kurtovic et al. 2007; Yamamoto 2008). Expression of

the male dsx and fru variants are necessary for develop-

ment of the region of the brain (P1) and associated den-

drites that are associated with male multimodal sensory

processing and courtship behaviour (Kimura et al. 2008).

Role of fru in wing song demonstrated that neural com-

mands for song are absent in females because they depend

on neurons expressing male fru transcripts (Clyne and

Miesenbock 2008). Similarly, genomic response to court-

ship song stimulation in female D. melanogaster provides

novel insight into specific molecular changes in females in

response to courtship song stimulation (Immonen and

Ritchie 2012). Nanda and Singh (2012) reviewed the

genetic basis of mate recognition between D. simulans and

D. sechellia which revealed that the majority of quantita-

tive trait loci responsible for both male mating behaviour

and pheromone concentration are located on the third

chromosome. In their review it has been shown that the

genes affecting cuticular hydrocarbons that differ between

D. simulans and D. sechellia may cause sexual isolation.

Elicitation of male courtship by female D. melanogaster

is strongly dependent on cuticular hydrocarbon (CHC)

pheromones especially dienes which depend on female-

specific expression of the desaturase locus i.e. desatF

(Shirangi et al. 2009). Disruption of expression of the de-

sat1 locus in D. melanogaster has phenotypic effects on

both the production of CHCs and mating decisions in both

sexes (Grillet et al. 2006; Marcillac et al. 2005) suggesting

possible pleiotropic control of trait and preference. Simi-

larly, role of fru in wing song demonstrated that neural

commands for song are absent in females because they

depend on neuron expressing male free transcripts (Clyne

and Miesenbock 2008). In the case of post copulatory

sexual selection, female remating is proved to be an

important aspect of sexual selection and its genetic control

has been proved to be yet another path in understanding the

genetics of sexual selection. Similarly, various studies

provide evidence that the genes on X-chromosome play

role in affecting remating speed of females of D. melano-

gaster. However, chromosome substitution analysis, bio-

metrical and planned comparison analysis, and

recombination analysis of experiments for remating speed

demonstrate the involvement of chromosome II which

contribute significantly to the differences in remating speed

in two selected lines i.e. fast and slow lines (Singh et al.

2002). Genes found in simulans clade of melanogaster i.e.

Odysseus has been proved to influence sperm production

and potentially sperm competition in D. simulans so post

copulatory sexual selection may have driven its divergence

and indirectly contributed to hybrid sterility (Sun et al.

2004). Similarly, results of Singh and Singh (2001) selec-

tion experiment in D. ananassae revealed that the remating

speed, mating propensity and fertility are under polygenic

control. Hence from all these findings it is quite obvious

that genes play role in driving species divergence via

sexual selection.

Conclusion

There is no doubt that sexual selection has the potential to

play a major role in speciation. Examples from inter- and

intra-sexual selection studies in Drosophila revealed that

sexual selection has power to drive rapid divergence via

choice and competition that may lead to reproductive iso-

lation. But sexual selection may not be the only cause of

speciation and more than one force may operate to bring

about speciation. Any one study is insufficient to prove

what forces have operated to bring about speciation. This

problem stems from our inability to observe the whole

process, forcing us either to infer the most probable future

course of events (when the process of speciation is not yet

complete) or to separate different possible histories. It has

been revealed that sexual selection should be demonstrated

directly from the effect of variation in the trait on mating

success rather than simply being inferred from sexual

dimorphism. However, divergence under sexual selection

does not necessarily result in a substantial barrier to gene

exchange. Usually, prezygotic isolation is the direct result

of changes in sexually selected traits or evolutionary his-

tory. Although Darwin (1871) in his work on sexual

selection ‘‘The Descent of Man, and Selection in Relation

to Sex’’ appreciated the importance of mating preferences

in sexual selection he did not clearly identify the evolution

of mate choice as a key topic in its own right. It is now

clear that the evolution of mate choice is one of the most

important topics in sexual selection research. The Fisherian

process is probably operating in some system, but we do

Genetica (2014) 142:23–41 35

123

not know how ubiquitous it is. On the other hand,

depending on the evolution and maintenance of genetic

correlations between traits and preferences, the possibility

remains that the Fisherian process explains very little with

respect to the evolution of female preferences.

However, special emphasis has been made in this review

is to know how sexual selection causes rapid divergence

among different species of Hawaiian Drosophila. This has

been revealed by the fact that divergence in some of the

traits may cause differences in female preferences which

are sexually selected and propagate among the species.

However, rapid radiation in the Hawaiian Drosophila can

be explained by other evolutionary forces such as drift

followed by natural selection, or that the sexually selected

traits are not involved in species recognition. Thus sexual

selection might not be as important in the origin of some of

the Hawaiian Drosophila species pairs. While our under-

standing of the processes that facilitate reproductive iso-

lation and the genetics of speciation has advanced

enormously over the past few years (Coyne and Orr 2004),

this has not been matched by an increased understanding of

the molecular processes underlying reproductive isolation.

It is exceptionally difficult to determine whether sexual

selection is responsible for phenomenon associated with

reproductive isolation solely by examining patterns of

evolutionary change in traits. However, understanding

rapid evolution of reproductive proteins has helped in

solving this problem to a great extent. To determine the

respective roles of selection and drift one has to determine

whether the effect of divergence in a particular sequence

benefits males, females or both. Such information would

perhaps allow us to detect whether sexual selection is

predominantly responsible. The role of sexual selection

could also be revealed by associations between reproduc-

tive protein variants and mating systems. Thus future

studies investigating how sexual selection might result in

reproductive isolation need to consider whether reproduc-

tive traits are evolutionary constrained.

Despite the triumph of modern sexual selection

research, there are still many related topics that need to be

addressed. For example, some models of the evolution of

mate choice enjoy limited support and for most part we are

not sure which model explains the majority of choice

evolution within or between systems. Studies of factors

determining intensity of sexual selection are still more

confusing. We are still in the process of building connec-

tions between reproductive ecology and selection differ-

entials. Finally, there seems to be a lack of connection

between theory related to mate choice evolution and theory

related to sexual selection intensity. Overall, our review on

sexual selection elucidates how selection acts at the level

of reproduction that may lead to evolution of different

types of traits. However, still we are far from resolving

many issues so the next several decades should be at least

as exciting as the recent past in the field of speciation

research.

Acknowledgments Financial assistance in the form of Meritorious

Fellowship to AS and UGC-BSR Faculty Fellowship Award to BNS

from the University Grant Commission, New Delhi is gratefully

acknowledged. We also thank two anonymous reviewers for their

helpful comments on the original draft of the manuscript.

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