How sensory drive can promote speciation

Some of the most spectacular and diverse traits in animals are the signals used to attract mates. Closely related species often differ dramatically in signaling traits, in spite of similarity in other morphological traits. The idea that reproductive isolation arises when male mating signals and female preferences differ among populations is an old one. However, until recently, there was almost no information on what generates diversity in mating signals and preferences. This is beginning to change, with emerging results that highlight the importance of habitat differences in generating this diversity. Such differences in ecology are at the root of one hypothesis for divergence in sexual signaling – sensory drive. The sensory drive hypothesis focuses on how communication systems adapt to local environments and predicts that divergence in communication systems will occur when environments differ. Reproductive isolation can arise as a byproduct of this adaptive divergence in behavior.     

Intraspecific sperm competition genes enforce post-mating species barriers in Drosophila

Sexual selection and sexual conflict are considered important drivers of speciation, based on both theoretical models and empirical correlations between sexually selected traits and diversification. However, whether reproductive isolation between species evolves directly as a consequence of intrapopulation sexual dynamics remains empirically unresolved, in part because knowledge of the genetic mechanisms (if any) connecting these processes is limited. Here, we provide evidence of a direct mechanistic link between intraspecies sexual selection and reproductive isolation. We examined genes with known roles in intraspecific sperm competition (ISC) in D. melanogaster and assayed their impact on conspecific sperm precedence (CSP). We found that two such genes (Acp36DE and CG9997) contribute to both offensive sperm competition and CSP; null/knockdown lines both had lower competitive ability against D. melanogaster conspecifics and were no longer able to displace heterospecific D. simulans sperm in competitive matings. In comparison, Sex Peptide (Acp70A)—another locus essential for ISC—does not contribute to CSP. These data indicate that two loci important for sperm competitive interactions have an additional role in similar interactions that enforce post-mating reproductive isolation between species, and show that sexual selection and sexual isolation can act on the same molecular targets in a gene-specific manner.     

Does courtship behavior contribute to species-level reproductive isolation in field crickets?

Reproductive behavior influences gene flow within and amongspecies; thus, sexual selection may be a major contributor tothe maintenance of species, and possibly their formation. HereI experimentally manipulate the courtship interactions of thefield crickets Gryllus rubens and G. texensis to examine thepotential of close-range courtship interactions to limit interspecificgene flow. Responses of males to females and of females to malecourtship song and males per se were examined for four pairedsympatric and allopatric populations. Male G. rubens were morelikely to court conspecific females, but male G. texensis courtedfemales of both species equally. If paired with conspecificmales, female G. rubens and G. texensis both preferred conspecificcourtship song. In none of these comparisons were the responsesof males or females from allopatry different from those in sympatry.There was an asymmetry of courtship response across both sexand species: male G. rubens were more discriminating than maleG. texensis, whereas female G. texensis were more discriminatingthan female G. rubens. Despite significant preferences for conspecifics,the net effect of courtship interactions would appear to limitinterspecific gene flow only weakly. These results are consistentwith courtship behavior evolving under the influence of sexualselection and only indirectly promoting species integrity.     

Darwin's ‘mystery of mysteries’: the role of sexual selection in plant speciation

Sexual selection is considered one of the key processes that contribute to the emergence of new species. While the connection between sexual selection and speciation has been supported by comparative studies, the mechanisms that mediate this connection remain unresolved, especially in plants. Similarly, it is not clear how speciation processes within plant populations translate into large-scale speciation dynamics. Here, we review the mechanisms through which sexual selection, pollination, and mate choice unfold and interact, and how they may ultimately produce reproductive isolation in plants. We also overview reproductive strategies that might influence sexual selection in plants and illustrate how functional traits might connect speciation at the population level (population differentiation, evolution of reproductive barriers; i.e. microevolution) with evolution above the species level (macroevolution). We also identify outstanding questions in the field, and suitable data and tools for their resolution. Altogether, this effort motivates further research focused on plants, which might potentially broaden our general understanding of speciation by sexual selection, a major concept in evolutionary biology.     

Sexual isolation evolves faster than hybrid inviability in a diverse and sexually dimorphic genus of fish (Percidae: Etheostoma)

Abstract .Theory predicts that sexual (or behavioral) isolation will be the first form of reproductive isolation to evolve in lineages characterized by sexual selection. Here I directly compare the rate of evolution of sexual isolation with that of hybrid inviability in a diverse and sexually dimorphic genus of freshwater fish. The magnitude of both sexual isolation and hybrid inviability were quantified for multiple pairs of allopatric species. Rates of evolution were inferred by comparing genetic distances of these species pairs with the magnitude of each form of reproductive isolation: the slope of the regression of genetic distance on the magnitude of reproductive isolation represents the rate of evolution. Of the two forms of isolation, the magnitude of sexual isolation exhibited the steeper slope of regression, indicating that sexual isolation will tend to evolve to completion earlier than hybrid inviability, strictly as a by-product of evolution in geographically isolated populations. Additional evidence from the literature is used to qualitatively compare rates of evolution of sexual isolation with that of other forms of reproductive isolation. Preliminary comparisons support the prediction that sexual isolation will evolve more rapidly than other forms. Because Etheostoma is characterized by striking sexual dimorphism, these results are consistent with the hypothesis that sexual selection for exaggerated mate-recognition characters causes the relatively rapid evolution of sexual isolation.     

Does reproductive isolation evolve faster in larger populations via sexually antagonistic coevolution?

Sexual conflict over reproductive investment can lead to sexually antagonistic coevolution and reproductive isolation. It has been suggested that, unlike most models of allopatric speciation, the evolution of reproductive isolation through sexually antagonistic coevolution will occur faster in large populations as these harbour greater levels of standing genetic variation, receive larger numbers of mutations and experience more intense sexual selection. We tested this in bruchid beetle populations (Callosobruchus maculatus) by manipulating population size and standing genetic variability in replicated lines derived from founders that had been released from sexual conflict for 90 generations. We found that after 19 generations of reintroduced sexual conflict, none of our treatments had evolved significant overall reproductive isolation among replicate lines. However, as predicted, measures of reproductive isolation tended to be greater among larger populations. We discuss our methodology, arguing that reproductive isolation is best examined by performing a matrix of allopatric and sympatric crosses whereas measurement of divergence requires crosses with a tester line.