Interactions between the sexes: new perspectives on sexual selection and reproductive isolation
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Understanding the processes underlying the origin of new species is a fundamental problem in evolutionary research. Whilst it has long been recognised that closely related taxa often differ markedly in reproductive characteristics, only relatively recently has sexual selection been evoked as a key promoter of speciation through its ability to generate reproductive isolation (RI). Sexual selection potentially can influence the probability that individuals from the same or different populations will reproduce successfully since it shapes precisely those traits involved in mating and reproduction. If reproductive characters diverge along different trajectories, then sexual selection can impact on the evolution of reproductive barriers operating both before and after mating. In this perspective, we consider some new advances in our understanding of the coevolution of male and female sexual signals and receptors and suggest how these developments may provide heretofore neglected insights into the mechanisms by which isolating barriers may emerge. Specifically, we explore how selfish genetic elements (SGEs) can mediate pre- and post-copulatory mate choice, thereby influencing gene flow and ultimately population divergence; we examine evidence from studies of intracellular sperm–egg interactions and propose that intracellular gametic incompatibilities may arise after sperm entry into the egg, and thus contribute to RI; we review findings from genomic studies demonstrating rapid, adaptive evolution of reproductive genes in both sexes and discuss whether such changes are causal in determining RI or simply associated with it; and finally, we consider genetic, developmental and functional mechanisms that might constrain reproductive trait diversification, thereby limiting the scope for reproductive barriers to arise via sexual selection. We hope to stimulate work that will further the understanding of the role sexual selection plays in generating RI and ultimately speciation.