Sexual Selection In Species Research Articles

Evolution of sexually selected traits across animals

Sexual selection is thought to be a major driver of phenotypic diversity and diversification in animals, but large-scale evolutionary patterns in sexually selected (SS) traits remain largely unknown. Here, we survey and analyze the evolution of these traits across animal phylogeny. We find that female mate choice appears to be the most widespread mechanism of sexual selection, but male-male competition appears to be almost as frequent in chordates and male mate choice is also common in arthropods. Among sensory types, tactile traits appear to be most widespread whereas auditory traits are relatively uncommon. Rather than being ubiquitous or randomly distributed across animals, most of these different types of SS traits are confined to clades in arthropods and chordates, which form “hotspots” for the evolution of these diverse trait types. Thus, different sensory types show accelerated rates of evolution in these clades. Moreover, different types of SS traits are strongly correlated with each other in their evolution across animals. Finally, despite the intensive interest in the role of sexual selection in speciation, we find only limited support for the idea that SS traits drive large-scale patterns of diversification and species richness across all animals.

Evolution of choosiness dictates whether search costs of mate choice enhance speciation by sexual selection

The role of sexual selection in speciation is implicated in both empirical case studies and larger comparative works. However, sexual selection faces two major problems in driving speciation. First, because females with novel preferences search for their initially rare males, search costs are expected to curtail initial sexual divergence. Second, if these populations come back into sympatry, sexual divergence may be erased due to hybridization. A major goal is to understand which conditions increase the likelihood of overcoming these problems. Here we generated a diploid population genetic model of how female search costs and evolution of female 'choosiness' (i.e. preference strength) interact to drive speciation in allopatry and secondary contact. We studied the model using numerical simulations in the context of two different male traits, ecologically 'arbitrary' versus 'magic' traits. First, in allopatry, without female search costs only minor and fluctuating sexual isolation evolved. In contrast, with female search costs, sexual isolation was highly curtailed with arbitrary male traits but was greatly facilitated with magic traits. However, because search costs selected for reduced choosiness, sexual isolation with magic traits was eventually eroded, the rate determined by the genetic architecture of choosiness. These factors also played a key role in secondary contact; with evolvable choosiness and female search costs, pure sexual selection models collapsed upon secondary contact. However, when we added selection against hybrids (i.e. reinforcement) to this model, we found that speciation could be maintained under a wide range of conditions with arbitrary male traits, but not with magic male traits. This surprisingly suggests that arbitrary male traits are in some cases more likely to aid speciation than magic male traits. We discuss these findings and relate them to empirical literature on female choosiness within species and in hybrids.

Body Size, Not Personality, Explains Both Male Mating Success and Sexual Cannibalism in a Widow Spider.

Simple SummaryThe role of personality in sexual selection has mostly been investigated on vertebrate species, in which males provide direct benefits to females or offspring. Less is known about the links between behavioral variation and sexual selection in species where males provide only sperm, while the advantages of female-choice are due to male genes that increase offspring fitness. Our study is centered on a sexually-size dimorphic spider species, the Mediterranean black widow, which is ideal to investigate how sexual selection shapes behavior. In this species, a male-biased operational sex-ratio leads to male-male competition, and aggressive and/or large males should have a selective advantage. Females are selected for fecundity, which should correlate with selection for higher voraciousness. Theory predicts that voracity “spills-over” into the mating context, such that voracity towards prey correlates with voracity towards mates. We tested how body size and two behaviors, male aggression toward rivals and female voracity toward prey, influence mating behavior, mating success, and sexual cannibalism. We show that individual variation in aggression does not play a direct role in the mating behavior of this species. Instead, body size affects male mating success and the occurrence of sexual cannibalism in females.Theory suggests that consistent individual variation in behavior relates to fitness, but few studies have empirically examined the role of personalities in mate choice, male-male competition and reproductive success. We observed the Mediterranean black widow, Latrodectus tredecimguttatus, in the individual and mating context, to test how body size measures and two functionally important aggressive behaviors, i.e., male aggression towards rivals and female voracity towards prey, affect mating behaviors, mating success and sexual cannibalism. We specifically selected voracity towards prey in females to test the “aggressive spillover hypothesis”, suggesting that more voracious females are more sexually cannibalistic. Both females and males exhibit consistent individual differences in the examined aggressive behaviors. While larger males win contests more often and achieve more copulations, neither male nor female size measures correlate to aggression. Female voracity does not correlate with aggression towards mates and sexual cannibalism, rejecting the “spillover hypothesis”. However, occurrence of sexual cannibalism positively relates to longer insertion duration. Furthermore, the smaller the ratio between male and female body length the more likely a female attacked and cannibalized a mate. We show that individual variation in aggression levels plays no direct role in the mating behavior of the Mediterranean black widow. Instead, body size affects male mating success and occurrences of sexual cannibalism in females.

To accept or reject heterospecific mates: behavioural decisions underlying premating isolation.

Premating isolation in animals involves decision-making processes that affect whether individuals accept or reject heterospecific mates. An integrative understanding of the behavioural processes underlying heterospecific acceptance can clarify the conditions under which premating isolation evolves. As an illustration, we review how Reeve's (Reeve 1989 Am. Nat.133, 407-435. (doi:10.1086/284926)) acceptance threshold model can help make sense of patterns of premating isolation in nature. This model derives a threshold trait value for acceptance for rejection of recipients of an action (e.g. mating) based on the fitness consequences of these decisions. We show that the maintenance of partial reproductive isolation can be an outcome of optimal acceptance thresholds, even in the face of reinforcement. We also use this model to clarify how the composition of multispecies communities can shape premating isolation. The acceptance threshold model can also be viewed as the behavioural underpinning of reproductive character displacement and cascading reinforcement. Finally, we highlight potential limitations of the acceptance threshold model with respect to investigating the role of sexual selection in speciation, and we propose that integration of behavioural models in speciation research will help us gain a full picture of the mechanisms underlying premating isolation. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Divergent male and female mate preferences do not explain incipient speciation between lizard lineages.

Diversification in sexual signals is often taken as evidence for the importance of sexual selection in speciation. However, in order for sexual selection to generate reproductive isolation between populations, both signals and mate preferences must diverge together. Furthermore, assortative mating may result from multiple behavioral mechanisms, including female mate preferences, male mate preferences, and male–male competition; yet their relative contributions are rarely evaluated. Here, we explored the role of mate preferences and male competitive ability as potential barriers to gene flow between 2 divergent lineages of the tawny dragon lizard, Ctenophorus decresii, which differ in male throat coloration. We found stronger behavioral barriers to pairings between southern lineage males and northern lineage females than between northern males and southern females, indicating incomplete and asymmetric behavioral isolating barriers. These results were driven by both male and female mate preferences rather than lineage differences in male competitive ability. Intrasexual selection is therefore unlikely to drive the outcome of secondary contact in C. decresii, despite its widely acknowledged importance in lizards. Our results are consistent with the emerging view that although both male and female mate preferences can diverge alongside sexual signals, speciation is rarely driven by divergent sexual selection alone.

Endless forms of sexual selection.

In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a “stock-taking” workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely depends on integration of temporally variable social and physiological costs that are challenging to measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the ‘aesthetic sense’ proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate ‘null model’ of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

Sex differences in song and plumage color do not evolve through sexual selection alone: new insights from recent research

Recent studies show that traits long recognized as sexually selected ornaments, such as the bright plumage colors and melodious vocal displays of songbirds, have evolved in surprising ways. Male plumage colors are generally more elaborate and diverse than female colors, and in many species only males sing, so both of these dimorphisms are widely presumed to have evolved through sexual selection on males. Yet, in multiple widely divergent songbird clades, sexual differences in colors and songs have evolved through losses in females rather than gains in males. Indeed, changes in female phenotypes may be the principle driving factor affecting such sexual dimorphisms in most songbirds. Sexual dichromatism and song dimorphism also tend to be associated with life history traits other than mating system, such as seasonal migration or territorial behaviors. Altogether, these recent findings support the view that sexual dimorphism is an evolutionary byproduct of multiple, different selection pressures acting on each sex rather than just sexual selection on males. Researchers should therefore be cautious to avoid assuming that dimorphism itself is a character solely reflecting levels of sexual selection in species.

The maintenance of phenotypic divergence through sexual selection: An experimental study in barn swallows Hirundo rustica.

Previous studies have shown that sexual signals can rapidly diverge among closely related species. However, we lack experimental studies to demonstrate that differences in trait-associated reproductive performance maintain sexual trait differences between closely related populations, in support for a role of sexual selection in speciation. Populations of Northern Hemisphere distributed barn swallows Hirundo rustica are closely related, yet differ in two plumage-based traits: ventral color and length of the outermost tail feathers (streamers). Here we provide experimental evidence that manipulations of these traits result in different reproductive consequences in two subspecies of barn swallow: (H. r. erythrogaster in North America and H. r. transitiva in the East Mediterranean). Experimental results in Colorado, USA, demonstrate that males with (1) darkened ventral coloration and (2) shortened streamers gained paternity between two successive reproductive bouts. In contrast, exaggeration of both traits improved reproductive performance within H. r. transitiva in Israel: males with a combination treatment of darkened ventral coloration and elongated streamers gained paternity between two successive reproductive bouts. Collectively, these experimental results fill an important gap in our understanding for how divergent sexual selection maintains phenotype differentiation in closely related populations, an important aspect of the speciation process.

Ecological divergence and conservatism: spatiotemporal patterns of niche evolution in a genus of livebearing fishes (Poeciliidae: Xiphophorus).

BackgroundEcological factors often have a strong impact on spatiotemporal patterns of biodiversity. The integration of spatial ecology and phylogenetics allows for rigorous tests of whether speciation is associated with niche conservatism (constraints on ecological divergence) or niche divergence. We address this question in a genus of livebearing fishes for which the role of sexual selection in speciation has long been studied, but in which the potential role of ecological divergence during speciation has not been tested.ResultsBy combining reconstruction of ancestral climate tolerances and disparity indices, we show that the earliest evolutionary split in Xiphophorus was associated with significant divergence for temperature variables. Niche evolution and present day niches were most closely associated with each species’ geographic distribution relative to a biogeographic barrier, the Trans-Mexican Volcanic Belt. Tests for similarity of the environmental backgrounds of closely related species suggested that the relative importance of niche conservatism and divergence during speciation varied among the primary clades of Xiphophorus. Closely related species in the two swordtail clades exhibited higher levels of niche overlap than expected given environmental background similarity indicative of niche conservatism. In contrast, almost all species of platyfish had significantly divergent niches compared to environmental backgrounds, which is indicative of niche divergence.ConclusionThe results suggest that the relative importance of niche conservatism and divergence differed among the clades of Xiphophorus and that traits associated with niche evolution may be more evolutionarily labile in the platyfishes. Our results ultimately suggest that the taxonomic scale of tests for conservatism and divergence could greatly influence inferences of their relative importance in the speciation process.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0593-4) contains supplementary material, which is available to authorized users.

Experimental limitation of oviposition sites affects the mating system of an arachnid with resource defence polygyny

The availability and spatial distribution of reproductive resources determine female distribution, thus affecting the organization of mating systems and the intensity of sexual selection. Females of the harvestman Serracutisoma proximum lay eggs on specific plant species, which are defended by males of the large morph (majors). After oviposition, females remain inside the major's territory, forming a harem, which may be invaded by males of the small morph (minors) to sneak copulations. We experimentally manipulated the availability of the plants used as oviposition sites, and investigated how it affected the mating system of S. proximum. Our results show that females and minors moved preferentially to plots with high plant availability. Plots with high plant availability had more harems, which were established on the preferred plants. In plots with low plant availability, many females laid eggs on plants where S. proximum clutches had never been observed previously, rather than choosing to be a part of a harem on a preferred plant. Harem invasions were accomplished by both minors and majors, and the frequency of harem invasions was not influenced by plant availability. Finally, male–male competition for harems and the potential for sexual selection were also not influenced by plant availability. Female plasticity in oviposition site use probably attenuates competition among majors, allowing males with poorly developed weaponry to establish a territory and achieve copulations. Minors apparently adjust their distribution in response to harem density, but the income of minors does not increase the frequency of harem invasions in plots with high plant availability. Plasticity in the major mating tactic probably accounts for the high frequency of harem invasions, regardless of plant availability. We argue that behavioural plasticity may thus profoundly affect our ability to make predictions about the effects of oviposition site limitation on mating systems and the intensity of sexual selection in species exhibiting resource defence polygyny.

Role of sexual selection in speciation in Drosophila

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 provide evidence that both mate choice and male-male competition 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 significant role in affecting the species mate recognition system and hence causing divergence between the species. However, intrasexual selection can bring the intraspecific divergence at the level of pre- and post-copulatory stage. This has been better explained through Hawaiian Drosophila 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 speciation genetics via sexual selection more closely.

Quantifying Reproductive Barriers in a Sympatric Pair of Darter Species

Quantifying and comparing the strengths of different reproductive barriers between diverging lineages is especially useful for determining the evolutionary mechanisms driving speciation. Etheostoma barrenense and Etheostoma zonale are closely related sympatric species of darters that are sexually dimorphic and exhibit clear differences in male nuptial coloration. Prior studies demonstrated that these species exhibit complete behavioral isolation, and that both intraspecific and interspecific variation in male coloration play a role in female choice, all consistent with speciation by sexual selection on male nuptial color. Remaining unclear, however, is whether behavioral isolation is the strongest reproductive barrier between these species or, alternatively, whether additional reproductive barriers are equally strong, which could implicate mechanisms other than sexual selection in speciation. Here, we compare the relative strengths of multiple reproductive barriers between the two focal species, measuring: (1) ecological isolation, (2) gametic incompatibility, (3) hybrid inviability, (4) conspecific sperm precedence, and comparing these measures to a previously estimated strength of behavioral isolation. We find that behavioral isolation is the strongest reproductive barrier measured to date and suggest it may be the only barrier that has evolved to completion. This result provides additional empirical evidence for speciation driven by sexual selection and provides insight into the maintenance of sympatric species in nature.

Sexual selection acting on a speciation trait in darters (Percidae: Etheostoma)

Despite theoretical support linking sexual selection for elaborate coloration and behavioral isolation, few empirical examples clearly demonstrate that the sexually dichromatic signals responsible for behavioral isolation between species are, or have been, subject to sexual selection within species. The present study investigates sexual selection on male nuptial coloration in Etheostoma barrenense, a darter species for which male color has been shown in a previous study to contribute to behavioral isolation from a sympatric congener. Through the use of motorized model fish, we demonstrate that female E. barrenense discriminate between the orange and red body hues of conspecific males and exhibit an association preference for orange over red models. Combined with the results from a previous study, these data provide evidence for a link between sexual selection and behavioral isolation and by extension for a role of sexual selection in speciation. A post hoc analysis of the colors modeled in darter visual space suggests that quantifying signal values as they are perceived, rather than expressed, provide a promising way of linking sexual selection and behavioral isolation.

Variance in mating success does not produce strong sexual selection in a polygynous songbird

AbstractThe sexually dimorphic ornaments and songs of birds are generally thought to be the result of sexual selection. For some time, variation in dimorphism among taxa was attributed to differences in social mating patterns. However, extrapair mating provides an alternative mechanism for sexual selection in both monogamous and polygynous birds. In monogamous systems, extrapair mating typically increases variance in mating success and hence sexual selection. However, it is less clear how extrapair mating influences sexual selection in polygynous species. We measured sexual selection acting on 6 sexually dimorphic male traits in a polygynous songbird, the dickcissel (Spiza americana). We also examined patterns of association between social and extrapair mating success, between male traits and reproductive success within each mating context, and the contribution of each type of mating to overall reproductive success. Despite high variance in both mating success and male traits, we found no evidence for current sexual selection in dickcissels. Indeed, variance among males in dimorphic traits did not significantly contribute to male success in any mating context. These results add to an array of studies uncovering relatively weak sexual selection in polygynous species. It is possible that, in contrast to monogamous systems, extrapair paternity weakens sexual selection in polygynous systems. In addition, natural selection arising from high predation and interspecific brood parasitism, stochastic processes, or annual variation in selective pressures may also weaken sexual selection in this species.

Effects of foraging and sexual selection on ecomorphology of a fish with alternative reproductive tactics

The foraging ecology of fish is often considered to be the primary determinant of body shape due to tight links between morphology, swimming performance, and foraging efficiency. Fish foraging on littoral benthic macroinvertebrates typically have a deeper body shape than those foraging on pelagic zooplankton in the water column. However, morphological traits often have multiple ecological functions, which could result in performance trade-offs between functions. Here, we provide the first examination of body shape and diet in a species with alternative reproductive tactics, in this case, bluegill sunfish (Lepomis macrochirus Rafinesque, 1819). Bluegill males mature into either “parental” or “cuckolder” reproductive tactics. Parentals build nests and provide sole parental care and defense of the young. Cuckolders instead act as “sneakers” darting into the nests of parental males while mating is occurring and then later in life become “satellites,” mimicking female appearance and behavior. Using stable carbon and nitrogen isotopic analysis of diet, we found that parentals and females consumed primarily pelagic zooplankton yet were the deepest in body shape. Sneakers consumed more littoral resources but were the most streamlined. Satellite males also consumed predominately littoral resources but had a deeper body form that was more similar to females than to size-matched juveniles. Our results differ from past studies of foraging ecomorphology and suggest that other selection pressures, such as sexual selection in species with alternative reproductive tactics, may also be an important factor influencing shape.

Sexual selection and magic traits in speciation with gene flow

Abstract The extent to which sexual selection is involved in speciation with gene flow remains an open question and the subject of much research. Here, we propose that some insight can be gained from considering the concept of magic traits (i.e., traits involved in both reproductive isolation and ecological divergence). Both magic traits and other, “non-magic”, traits can contribute to speciation via a number of specific mechanisms. We argue that many of these mechanisms are likely to differ widely in the extent to which they involve sexual selection. Furthermore, in some cases where sexual selection is present, it may be prone to inhibit rather than drive speciation. Finally, there are a priori reasons to believe that certain categories of traits are much more effective than others in driving speciation. The combination of these points suggests a classification of traits that may shed light on the broader role of sexual selection in speciation with gene flow. In particular, we suggest that sexual selection can act as a driver of speciation in some scenarios, but may play a negligible role in potentially common categories of magic traits, and may be likely to inhibit speciation in common categories of non-magic traits.

The relationship between sexual selection and speciation

The intriguing relationship between sexual selection and speciation has sparked an increasing number of papers in recent years that have attempted to draw connections between the two processes. These include a myriad of theoretical and empirical studies, as well as several influential reviews (e.g. Price, 1998; Panhuis et al., 2001, Kirkpatrick and Ravigne, 2002; Ritchie, 2007; Maan and Seehausen, 2011). In particular, many researchers have speculated on whether sexual selection can lead to speciation, either alone, or in conjunction with ecological processes. Such speculation is tempting in part because of the obvious differences in secondary sexual characters that often exist between closely related species. Indeed one of the earliest and most influential of all sexual selection models, Lande’s (1981) paper “Models of speciation by sexual selection on a polygenic trait” centered its title on the role that sexual selection may play in speciation. What, however, is the evidence of a causal relationship between sexual selection and speciation? It turns out to be surprisingly ambiguous. Many have attempted to address a relationship by comparative studies, a process that can be problematic because of both methodological concerns and the fact that the presence of sexual selection is inferred by proxy (Kraaijeveld et al., 2011). Kraaijeveld et al. (2011) point out that these studies only sometimes support a correlation between sexual selection and speciation, and using a meta–analysis find only weak support for a relationship across multiple datasets. A large number of individual experimental studies have also examined various aspects of the intersection of sexual selection and speciation, but it is often unclear what would constitute a true test of the role of sexual selection in speciation, and unambiguous tests of this role are essentially absent (large scale literature analysis by Safran, Mendelson, Scordato and Symes, unpublished data). In particular, the relative roles of ecological factors and sexual selection often appear very intertwined and difficult to tease apart (case studies reviewed in Ritchie, 2007; Maan and Seehausen, 2011). Some studies have attempted to assess whether there is a difference between mate choice and species recognition (e.g. Kozak et al., 2009), but these have also found mixed results. Theoretical studies, which focus on the case of speciation with gene flow in particular, have also demonstrated a very mixed relationship between sexual selection and speciation. Many studies examining the evolution of premating isolation couch the language in terms of sexual selection, simply assuming that species recognition and sexual selection are one and the same in this regard. While this is strictly accurate by the definition of sexual selection as differential mating success, it does not address the spirit of the question of whether sexual selection processes within a species are ultimately responsible for speciation. Several theoretical studies have focused on whether sexual selection alone could account for sympatric speciation, but the general conclusion is that this is unlikely (e.g., van Doorn et al., 2004). Intriguingly, the theoretical literature has also in recent years developed an idea largely absent from the empirical literature, namely that the role of sexual selection in speciation with gene flow may often be inhibitory rather than driving (see discussion in Servedio and Kopp, 2012, this issue). Specifically, sexual selection may often eliminate variation necessary for speciation (e.g., Kirkpatrick and Nuismer, 2004), or lead to stabilizing selection that can counter divergent ecological selection to prevent speciation with gene flow (e.g., Matessi et al., 2001; Otto et al., 2008; Pennings et al., 2008).

A robust new metric of phenotypic distance to estimate and compare multiple trait differences among populations

Abstract Whereas a rich literature exists for estimating population genetic divergence, metrics of phenotypic trait divergence are lacking, particularly for comparing multiple traits among three or more populations. Here, we review and analyze via simulation Hedges’ g, a widely used parametric estimate of effect size. Our analyses indicate that g is sensitive to a combination of unequal trait variances and unequal sample sizes among populations and to changes in the scale of measurement. We then go on to derive and explain a new, non-parametric distance measure, “Δp”, which is calculated based upon a joint cumulative distribution function (CDF) from all populations under study. More precisely, distances are measured in terms of the percentiles in this CDF at which each population’s median lies. Δp combines many desirable features of other distance metrics into a single metric; namely, compared to other metrics, p is relatively insensitive to unequal variances and sample sizes among the populations sampled. Furthermore, a key feature of Δp—and our main motivation for developing it—is that it easily accommodates simultaneous comparisons of any number of traits across any number of populations. To exemplify its utility, we employ Δp to address a question related to the role of sexual selection in speciation: are sexual signals more divergent than ecological traits in closely related taxa? Using traits of known function in closely related populations, we show that traits predictive of reproductive performance are, indeed, more divergent and more sexually dimorphic than traits related to ecological adaptation.

Post‐mating prezygotic barriers to gene exchange between hybridizing field crickets

Studies of sexual selection in speciation have traditionally focused on mate preference, with less attention given to traits that act between copulation and fertilization. However, recent work suggests that post-mating prezygotic barriers may play an important role in speciation. Here, we evaluate the role of such barriers in the field crickets, Gryllus firmus and Gryllus pennsylvanicus. Gryllus pennsylvanicus females mated with G. firmus males produce viable, fertile offspring, but when housed with both species produce offspring sired primarily by conspecifics. We evaluate patterns of sperm utilization in doubly mated G. pennsylvanicus females and find no evidence for conspecific sperm precedence. The reciprocal cross (G. firmus female × G. pennsylvanicus male) produces no progeny. Absence of progeny reflects a barrier to fertilization rather than reduced sperm transfer, storage or motility. We propose a classification scheme for mechanisms underlying post-mating prezygotic barriers similar to that used for premating barriers.

A Cost of Sexual Attractiveness to High-Fitness Females

Author SummaryIn many species, females are frequently subject to harassing courtship from males attempting to mate with them. These persistent male behaviors can result in females incurring substantial direct fitness costs. We set out to examine how these costs may influence adaptive potential in a species that also exhibits male mate choice, i.e., a preference by males for females exhibiting certain traits. We found that harmful courtship behaviors were directed predominantly towards females of greater reproductive potential (and away from females of lesser potential), resulting in a reduction in the variation of lifetime reproductive successes among females in the population. This change in distribution of realized fitnesses represents a previously unappreciated consequence of sexual conflict–adaptive male mate preference can slow the rate of accumulation of beneficial mutations and speed the rate of accumulation of harmful mutations, thereby creating a “sexual conflict adaptive load” within a species.