REINFORCEMENT OF STICKLEBACK MATE PREFERENCES: SYMPATRY BREEDS CONTEMPT

Detailed studies of reproductive isolation and how it varies among populations can provide valuable insight into the mechanisms of speciation. Here we investigate how the strength of premating isolation varies between sympatric and allopatric populations of threespine sticklebacks to test a prediction of the hypothesis of reinforcement: that interspecific mate discrimination should be stronger in sympatry than in allopatry. In conducting such tests, it is important to control for ecological character displacement between sympatric species because ecological character divergence may strengthen prezygotic isolation as a by-product. We control for ecological character displacement by comparing mate preferences of females from a sympatric population (benthics) with mate preferences of females from two allopatric populations that most closely resemble the sympatric benthic females in ecology and morphology. No-choice mating trials indicate that sympatric benthic females mate less readily with heterospecific (limnetic) than conspecific (benthic) males, whereas two different populations of allopatric females resembling benthics show no such discrimination. These differences demonstrate reproductive character displacement of benthic female mate choice. Previous studies have established that hybridization between sympatric species occurred in the past in the wild and that hybrid offspring have lower fitness than either parental species, thus providing conditions under which natural selection would favor individuals that do not hybridize. Results are therefore consistent with the hypothesis that female mate preferences have evolved as a response to reduced hybrid fitness (reinforcement), although direct effects of sympatry or a biased extinction process could also produce the pattern. Males of the other sympatric species (limnetics) showed a preference for smaller females, in contrast to the inferred ancestral preference for larger females, suggesting reproductive character displacement of limnetic male mate preferences as well.     

Seeking signatures of reinforcement at the genetic level: a hitchhiking mapping and candidate gene approach in the house mouse

Reinforcement is the process by which prezygotic isolation is strengthened as a response to selection against hybridization. Most empirical support for reinforcement comes from the observation of its possible phenotypic signature: an accentuated degree of prezygotic isolation in the hybrid zone as compared to allopatry. Here, we implemented a novel approach to this question by seeking for the signature of reinforcement at the genetic level. In the house mouse, selection against hybrids and enhanced olfactory‐based assortative mate preferences are observed in a hybrid zone between the two European subspecies Mus musculus musculus and M. m. domesticus, suggesting a possible recent reinforcement event. To test for the genetic signature of reinforcing selection and identify genes involved in sexual isolation, we adopted a hitchhiking mapping approach targeting genomic regions containing candidate genes for assortative mating in mice. We densely scanned these genomic regions in hybrid zone and allopatric samples using a large number of fast evolving microsatellite loci that allow the detection of recent selection events. We found a handful of loci showing the expected pattern of significant reduction in variability in populations close to the hybrid zone, showing assortative odour preference in mate choice experiments as compared to populations further away and displaying no such preference. These loci lie close to genes that we pinpoint as testable candidates for further investigation.     

Female mating preference functions predict sexual selection against hybrids between sibling species of cichlid fish

The evolutionary outcome of interspecific hybridization, i.e. collapse of species into a hybrid swarm, persistence or even divergence with reinforcement, depends on the balance between gene flow and selection against hybrids. If female mating preferences are open-ended but sign-inversed between species, they can theoretically be a source of such selection. Cichlid fish in African lakes have sustained high rates of speciation despite evidence for widespread hybridization, and sexual selection by female choice has been proposed as important in the origin and maintenance of species boundaries. However, it had never been tested whether hybridizing species have open-ended preference rules. Here we report the first experimental test using Pundamilia pundamilia, Pundamilia nyererei and their hybrids in three-way choice experiments. Hybrid males are phenotypically intermediate. Wild-caught females of both species have strong preferences for conspecific over heterospecific males. Their responses to F1 hybrid males are intermediate, but more similar to responses to conspecifics in one species and more similar to responses to heterospecifics in the other. We suggest that their mate choice mechanism may predispose haplochromine cichlids to maintain and perhaps undergo phenotypic diversification despite hybridization, and that species differences in female preference functions may predict the potential for adaptive trait transfer between hybridizing species.     

The role of mate‐choice copying in speciation and hybridization

Mate‐choice copying, a social, non‐genetic mechanism of mate choice, occurs when an individual (typically a female) copies the mate choice of other individuals via a process of social learning. Over the past 20 years, mate‐choice copying has consistently been shown to affect mate choice in several species, by altering the genetically based expression of mating preferences. This behaviour has been claimed by several authors to have a significant role in evolution. Because it can cause or increase skews in male mating success, it seems to have the potential to induce a rapid change of the directionality and rate of sexual selection, possibly leading to divergent evolution and speciation. Theoretical work has, however, been challenging this view, showing that copying may decelerate sexual selection and that linkage disequilibrium cannot be established between the copied preference and the male trait, because females copy from unrelated individuals in the population, making an invasion of new and potentially fitter male traits difficult. Given this controversy, it is timely to ask about the real impact of mate‐choice copying in speciation. We propose that a solution to this impasse may be the existence of some degree of habitat selection, which would create a spatial structure, causing scenarios of micro‐allopatry and thus overcoming the problem of the lack of linkage disequilibrium. As far as we are aware, the potential role of mate‐choice copying on fostering speciation in micro‐allopatry has not been tackled. Also important is that the role of mate‐choice copying has generally been discussed as being a barrier to gene flow. However, in our view, mate‐choice copying may actually play a key role in facilitating gene flow, thereby fostering hybridization. Yet, the role of mate‐choice copying in hybridization has so far been overlooked, although the conditions under which it might occur are more likely, or less restricted, than those favouring speciation. Hence, a conceptual framework is needed to identify the exact mechanisms and the conditions under which speciation or hybridization are expected. Here, we develop such a framework to be used as a roadmap for future research at the intersection of these research areas.     

Reinforcement of mate preference among hybridizing Heliconius butterflies

Recent models of mate preference evolution suggest that direct selection on alleles at preference loci and correlated evolution of preference with locally adapted mating cues are more likely to drive the evolution of assortative mate preference than reinforcement. Mate preference evolution in mimetic Heliconius butterflies has been attributed to all three forms of selection, but here we show that reinforcement has been critical. By examining geographical variation in assortative mating and male mate preference among seven populations of three hybridizing Heliconius species from Costa Rica, we found pronounced character displacement of preference such that sexual isolation was enhanced in areas of interspecific contact. Of the different explanations for the evolution of assortative mate preference, only reinforcement is dependent on interspecific contact in this system. Thus, the observed pattern of reproductive character displacement of mate preference is best explained as a product of indirect selection generated by natural selection against nonmimetic hybrids.     

Asymmetrical reproductive character displacement in the house mouse

Our study addressed reproductive character displacement between two subspecies of the house mouse, Mus musculus musculus and Mus musculus domesticus that hybridize in Europe along a zone where selection against hybridization is known to occur. Based on a multi-population approach, we investigated spatial patterns of divergence of mate preference in the two taxa. Mate preference was significantly higher in the contact zone than in allopatry in both subspecies, suggesting that reproductive character displacement occurs. Moreover, patterns of preference were stronger in M. m. musculus than in M. m. domesticus, indicating an asymmetrical divergence between the two. In the context of selection against hybridization, our results may provide empirical support for the hypothesis of reinforcement in a parapatric hybrid zone. We discuss factors that could explain the asymmetrical pattern of divergence and the possible impact of a unimodal structure on the maintenance of premating divergence between the two subspecies.     

Character displacement and the evolution of mate choice: an artificial neural network approach

Interactions with heterospecifics can promote the evolution of divergent mating behaviours between populations that do and do not occur with heterospecifics. This process--reproductive character displacement--potentially results from selection to minimize the risk of mating with heterospecifics. We sought to determine whether heterospecific interactions lead to divergence of female preferences for aspects of conspecific male signals. We used artificial neural network models to simulate a mate recognition system in which females co-occur with different heterospecifics in different populations. Populations that evolved conspecific recognition in the presence of different heterospecifics varied in their preferences for aspects of conspecific male signals. When we tested networks for their preferences of conspecific versus heterospecific signals, however, we found that networks from allopatric populations were usually able to select against heterospecifics. We suggest that female preferences for aspects of conspecific male signals can result in a concomitant reduction in the likelihood that females will mate with heterospecifics. Consequently, even females in allopatry may discriminate against heterospecific mates depending on the nature of their preferences for conspecifics. Such a pattern could potentially explain cases where reproductive character displacement is expected, but not observed.     

The evolution of host preference in allopatric vs. parapatric populations of Timema cristinae walking-sticks

Divergent habitat preferences can contribute to speciation, as has been observed for host-plant preferences in phytophagous insects. Geographic variation in host preference can provide insight into the causes of preference evolution. For example, selection against maladaptive host-switching occurs only when multiple hosts are available in the local environment and can result in greater divergence in regions with multiple vs. a single host. Conversely, costs of finding a suitable host can select for preference even in populations using a single host. Some populations of Timema cristinae occur in regions with only one host-plant species present (in allopatry, surrounded by unsuitable hosts) whereas others occur in regions with two host-plant species adjacent to one another (in parapatry). Here, we use host choice and reciprocal-rearing experiments to document genetic divergence in host preference among 33 populations of T. cristinae. Populations feeding on Ceanothus exhibited a stronger preference for Ceanothus than did populations feeding on Adenostoma. Both allopatric and parapatric pairs of populations using the different hosts exhibited divergent host preferences, but the degree of divergence tended to be greater between allopatric pairs. Thus, gene flow between parapatric populations apparently constrains divergence. Host preferences led to levels of premating isolation between populations using alternate hosts that were comparable in magnitude to previously documented premating isolation caused by natural and sexual selection against migrants between hosts. Our findings demonstrate how gene flow and different forms of selection interact to determine the magnitude of reproductive isolation observed in nature.     

Geographical variation in reproductive character displacement in mate choice by male sailfin mollies

Female Amazon mollies, Poecilia formosa, are a unisexual species that reproduce by gynogenesis. They must coexist and mate with males of other species (usually the mollies Poecilia latipinna or Poecilia mexicana) to induce embryogenesis, but inheritance is strictly maternal. We examined the mating preference of the male sailfin molly, P. latipinna, for female sailfin mollies versus Amazon mollies, P. formosa. We compared the mating preferences of sympatric and allopatric populations collected throughout the Gulf Coast of North America. Male P. latipinna from six populations sympatric with Amazon mollies showed a significantly greater strength of preference for conspecific sailfin females than males from five populations that were allopatric with Amazon mollies. These results provide strong evidence for reproductive character displacement of male mate choice in sympatry. Furthermore, the large geographical range of populations that we tested revealed variation among populations within sympatry and allopatry, indicating that it is important to evaluate a large number of populations when examining reproductive character displacement.     

Learning to speciate: The biased learning of mate preferences promotes adaptive radiation

Bursts of rapid repeated speciation called adaptive radiations have generated much of Earth's biodiversity and fascinated biologists since Darwin, but we still do not know why some lineages radiate and others do not. Understanding what causes assortative mating to evolve rapidly and repeatedly in the same lineage is key to understanding adaptive radiation. Many species that have undergone adaptive radiations exhibit mate preference learning, where individuals acquire mate preferences by observing the phenotypes of other members of their populations. Mate preference learning can be biased if individuals also learn phenotypes to avoid in mates, and shift their preferences away from these avoided phenotypes. We used individual‐based computational simulations to study whether biased and unbiased mate preference learning promotes ecological speciation and adaptive radiation. We found that ecological speciation can be rapid and repeated when mate preferences are biased, but is inhibited when mate preferences are learned without bias. Our results suggest that biased mate preference learning may play an important role in generating animal biodiversity through adaptive radiation.     

Males and females contribute differently to the evolution of habitat segregation driven by hybridization

Costly heterospecific mating interactions, such as hybridization, select for prezygotic reproductive isolation. One of the potential traits responding to the selection arising from maladaptive hybridization is habitat preference, whose divergence results in interspecific habitat segregation. Theoretical studies have so far assumed that habitat preference is a sexually shared trait. However, male and female habitat preferences can experience different selection pressures. Here, by combining analytical and simulation approaches, we theoretically examine the evolution of sex-specific habitat preferences. Habitat segregation can have demographic consequences, potentially generating eco-evolutionary dynamics. We thus explicitly consider demography in the simulation model. We also vary the degrees of species discrimination to examine how mate choice influences the evolution of habitat preferences. Results show that both sexes can reduce hybridisation risk by settling in the habitats where abundant conspecific mates reside. However, when females can discriminate species, excess conspecific male aggregation intensifies male–male competition for mating opportunities, posing an obstacle to conspecific aggregation. Meanwhile, conspecific female aggregation attracts conspecific males, by offering the mating opportunity. Therefore, under effective species discrimination, females play a leading role in initiating habitat use divergence. Simulations typically result in either the coexistence with established habitat segregation or the extinction of one of the species. The former result is especially likely when the species differ to some extent in habitat preferences upon secondary contact. Our results disentangle the selection pressures acting on male and female habitat preferences, deepening our understanding of the evolutionary process of habitat segregation due to hybridization.     

Mate preference across the speciation continuum in a clade of mimetic butterflies

Premating behavioral isolation is increasingly recognized as an important part of ecological speciation, where divergent natural selection causes the evolution of reproductive barriers. A number of studies have now demonstrated that traits under divergent natural selection also affect mate preferences. However, studies of single species pairs only capture a snapshot of the speciation process, making it difficult to assess the role of mate preferences throughout the entire process. Heliconius butterflies are well known for their brightly colored mimetic warning patterns, and previous studies have shown that these patterns are also used as mate recognition cues. Here, we present mate preference data for four pairs of sister taxa, representing different stages of divergence, which together allow us to compare diverging mate preferences across the continuum of Heliconius speciation. Using a novel Bayesian approach, our results support a model of ecological speciation in which strong premating isolation arises early, but continues to increase throughout the continuum from polymorphic populations through to "good," sympatric ecologically divergent species.     

Female mating preference for own males on species and population level in Chromaphyosemion killifishes (Cyprinodontiformes, Nothobranchiidae)

Chromaphyosemion killifishes are a very promising taxon for the examination of general principles and mechanisms underlying speciation. The polygamous mating system and high degree of sexual dimorphism in this group suggest that sexual selection and female mating preferences play a crucial role in the current radiation process. Because the emergence of mating preferences for individuals of the own population during allopatry is a necessary precondition for speciation by sexual selection, either via diverging mate recognition traits or reinforcement, we examined whether females of different nominal species (Chromaphyosemion bivittatum vs. Chromaphyosemion volcanum) or populations (C. bivittatum “Toko” vs. C. bivittatum “Ilor”) prefer their own males as mating partners. For this purpose, two different methods were used. First, we conducted simultaneous choice tests where females could choose between two different males, and second, we counted the eggs produced by females in consecutive matings with different males. Both approaches produced the same result: females of the populations under examination preferred to mate with their own males. Preference for own males was symmetrical in all tests. Males, on the other hand, do not discriminate between females, not even on the level of nominal species. Thus, our results are in accordance with the hypothesis that female choice is an important factor for speciation in Chromaphyosemion. Additionally, we present some preliminary data on post-zygotic isolation between the populations.     

The effects of sexual selection on trait divergence in a peripheral population with gene flow

The unique aspects of speciation and divergence in peripheral populations have long sparked much research. Unidirectional migration, received by some peripheral populations, can hinder the evolution of distinct differences from their founding populations. Here, we explore the effects that sexual selection, long hypothesized to drive the divergence of distinct traits used in mate choice, can play in the evolution of such traits in a partially isolated peripheral population. Using population genetic continent‐island models, we show that with phenotype matching, sexual selection increases the frequency of an island‐specific mating trait only when female preferences are of intermediate strength. We identify regions of preference strength for which sexual selection can instead cause an island‐specific trait to be lost, even when it would have otherwise been maintained at migration‐selection balance. When there are instead separate preference and trait loci, we find that sexual selection can lead to low trait frequencies or trait loss when female preferences are weak to intermediate, but that sexual selection can increase trait frequencies when preferences are strong. We also show that novel preference strengths almost universally cannot increase, under either mating mechanism, precluding the evolution of premating isolation in peripheral populations at the early stages of species divergence.     

Male versus female mate choice: sexual selection and the evolution of species recognition via reinforcement

Male mate choice, expressed through courtship preferences, sometime occurs even under the mating system of polygyny, when the operational sex ratio is skewed toward males. The conditions under which male mate choice may be expected during polygyny are not well established. Servedio and Lande (2006, Evolution 60:674-685), assuming strict polygyny where all females have equal mating success, show that when having a preference does not increase the amount of energy that a male can put into courtship, male preferences for "arbitrary" female ornaments should not be expected to evolve; direct selection acts against them because they place males that carry them into situations in which there is high competition for mates. Here I explore in detail two situations under which logic dictates that this effect may be overcome or reversed. First I determine the contributions that direct and indirect selection place on male versus female preferences for traits that increase viability, using notation that allows the exact expression of these measures of selection. I find that direct selection against male preferences still predominates in the male mate choice model, causing less evolution by male than female preferences under these conditions. Second I address whether male mate choice is likely to evolve as a mechanism of premating isolation leading to species recognition, driven by the process of reinforcement. Reinforcement is compared under male and female mate choice, using a variety of models analyzed by both analytical techniques assuming weak selection and numerical techniques under broader selective conditions. I demonstrate that although under many conditions stronger premating isolation evolves under female mate choice, reinforcement may indeed occur via male mate choice alone.     

Comparing complex fitness surfaces: among-population variation in mutual sexual selection in Drosophila serrata

Despite a dramatic increase in empirical estimates of phenotypic selection over the past two decades, we remain remarkably ignorant about variation in the multivariate fitness surfaces that shape the adaptive landscape. We develop a novel approach for quantifying patterns of spatial and/or temporal variation in multivariate selection that directly compares vectors of linear selection gradients (beta) and matrices of nonlinear selection gradients (gamma) that describe the multivariate fitness surface in each population. We apply this approach to estimates of sexual selection on a suite of cuticular hydrocarbons (CHCs) in males and females from nine geographic populations of Drosophila serrata. In males, variation in linear sexual selection was associated with the presence of the related species Drosophila birchii, suggesting that female mate preferences for male CHCs differ between sympatry and allopatry. This is consistent with previous experimental results suggesting that reproductive character displacement of male CHCs has resulted from selection caused by the presence of D. birchii. No significant associations were found for nonlinear sexual selection in males. In females, large-scale variation in both linear and nonlinear sexual selection was negatively associated with assumed-neutral population genetic structure, suggesting a key role for chance events in male mate preference divergence.     

The evolution of female mating preferences: differentiation from species with promiscuous males can promote speciation

Females of many species are frequently courted by promiscuous males of their own and other closely related species. Such mating interactions may impose strong selection on female mating preferences to favor trait values in conspecific males that allow females to discriminate them from their heterospecific rivals. We explore the consequences of such selection in models of the evolution of female mating preferences when females must interact with heterospecific males from which they are completely postreproductively isolated. Specifically, we allow the values of both the most preferred male trait and the tolerance of females for males that deviate from this most preferred trait to evolve. Also, we consider situations in which females base their mating decisions on multiple male traits and must interact with males of multiple species. Females will rapidly differentiate in preference when they sometimes mistake heterospecific males for suitable mates, and the differentiation of female preference will select for conspecific male traits to differentiate as well. In most circumstances, this differentiation continues indefinitely, but slows substantially once females are differentiated enough to make mistakes rare. Populations of females with broader preference functions (i.e., broader tolerance for males with trait values that deviate from females' most preferred values) will evolve further to differentiate if the shape of the function cannot evolve. Also, the magnitude of separation that evolves is larger and achieved faster when conspecific males have lower relative abundance. The direction of differentiation is also very sensitive to initial conditions if females base their mate choices on multiple male traits. We discuss how these selection pressures on female mate choice may lead to speciation by generating differentiation among populations of a progenitor species that experiences different assemblages of heterospecifics. Opportunities for differentiation increase as the number of traits involved in mate choice increase and as the number of species involved increases. We suggest that this mode of speciation may have been particularly prevalent in response to the cycles of climatic change throughout the Quaternary that forced the assembly and disassembly of entire communities on a continentwide basis.     

Increased genetic divergence between two closely related fir species in areas of range overlap

Because of introgressive hybridization, closely related species can be more similar to each other in areas of range overlap (parapatry or sympatry) than in areas where they are geographically isolated from each other (allopatry). Here, we report the reverse situation based on nuclear genetic divergence between two fir species, Abies chensiensis and Abies fargesii, in China, at sites where they are parapatric relative to where they are allopatric. We examined genetic divergence across 126 amplified fragment length polymorphism (AFLP) markers in a set of 172 individuals sampled from both allopatric and parapatric populations of the two species. Our analyses demonstrated that AFLP divergence was much greater between the species when comparisons were made between parapatric populations than between allopatric populations. We suggest that selection in parapatry may have largely contributed to this increased divergence.     

REPRODUCTIVE CHARACTER DISPLACEMENT OF MALE STICKLEBACK MATE PREFERENCE: REINFORCEMENT OR DIRECT SELECTION?

Reproductive isolation can evolve between species as a byproduct of adaptation to different niches, through reinforcement, and by direct selection on mating preferences. We investigated the potential role of direct selection in the reproductive isolation between sympatric species of threespine sticklebacks. Each sympatric pair consists of a small-bodied limnetic species and large-bodied benthic species. We compared the mate preferences and courtship behavior of males from one sympatric limnetic population and two allopatric populations. We used limnetic-like allopatric populations to control for the effects of ecological character displacement and adaptation to different niches on mate preferences. The sympatric limnetic males preferred the small limnetic females, whereas the allopatric limnetic-like males preferred the large benthic females, suggesting that adaptation to the limnetic niche does not automatically confer a preference for small limnetic females. This reproductive character displacement of male preference is consistent with the predictions of both reinforcement and direct selection on mate preferences. To test for direct selection, we assessed a prediction of one proposed mechanism: predation by benthic females on eggs guarded by limnetic males. The allopatric males come from populations in which there is no egg predation. Sympatric limnetic males were more aggressive toward benthic females than toward limnetic females, whereas the allopatric limnetic-like males did not treat the two types of females differently. The contrast in male behavior suggests that egg predation has shaped male preferences. Direct selection is potentially more effective than indirect selection via reinforcement, and it is likely that it has been important in building up reproductive isolation between limnetic and benthic sticklebacks.     

Counteracting selective regimes and host preference evolution in ecotypes of two species of walking-sticks

The evolution of ecological specialization has been a central topic in ecology because specialized adaptations to divergent environments can result in reproductive isolation and facilitate speciation. However, the order in which different aspects of habitat adaptation and habitat preference evolve is unclear. Timema walking-stick insects feed and mate on the host plants on which they rest. Previous studies of T. cristinae ecotypes have documented divergent, host-specific selection from visual predators and the evolution of divergent host and mate preferences between populations using different host-plant species (Ceanothus or Adenostoma). Here we present new data that show that T. podura, a nonsister species of T. cristinae, has also formed ecotypes on these host genera and that in both species these ecotypes exhibit adaptive divergence in color-pattern and host preference. Color-pattern morphs exhibit survival trade-offs on different hosts due to differential predation. In contrast, fecundity trade-offs on different hosts do not occur in either species. Thus, host preference in both species has evolved before divergent physiological adaptation but in concert with morphological adaptations. Our results shed light onto which traits are involved in the initial stages of ecological specialization and ecologically based reproductive isolation.