Sexual selection and genital evolution

Male genitalia show patterns of divergent evolution, and sexual selection is recognised as being responsible for this taxonomically widespread phenomenon. Much of the empirical support for the sexual selection hypothesis comes from studies of insects. Here, I synthesise the literature on insect genital evolution, and use this synthesis to address the debate over the mechanisms of selection most likely to explain observed patterns of macroevolutionary divergence in genital morphology. Studies of seven insect orders provide evidence that non-intromittent genitalia are subject to sexual selection through their effects on mating success, while intromittent genitalia are subject to selection through their effects on fertilisation success. However, studies that use quantitative methods to analyse the form of selection are necessary to identify the mechanisms of sexual selection involved. Phylogenetic analyses from diverse taxonomic groups confirm that divergence in male genital morphology can be predicted from variation in the opportunity for sexual selection. Much debate revolves around the importance of female choice and sexual conflict in the evolution of male genitalia, the resolution of which lies in economic studies of mating interactions and in recognising sexual selection as a continuum between male competition, sexual conflict and female choice. The species isolating lock-and-key hypothesis is frequently dismissed as unimportant in genital evolution because in part of a perceived lack of variation in female genitalia across species. Increasingly, however, studies report species-specific variation in female genital morphology and its coevolutionary divergence with male genital morphology. Contemporary views recognise a continuum between female choice that enforces species isolation and female choice that targets variation in male quality within populations, placing lock-and-key processes into the realm of sexual selection. Distinguishing between species-isolating and directional forms of female choice will require studies that examine both the tempo and mode of divergence, both within and among species.