Sexual selection, conflict and isolation: experimental insight into the population genetic consequences of mate competition and the origin of species

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In most sexually reproducing species, males compete for access to mates and/or to fertilize their gametes. Such competition is persistent, widespread, and often intense, and it generates the variation in reproductive success that causes sexual selection. Mate competition is also a key reason why selection differs between males and females, and the resulting sexual conflict can lead to the evolution of traits that benefit males but harm females as a by-product. The evolution of extravagant sexual displays, like the peacock's tail, and exaggerated weapons of sexual rivalry, like a buck's antlers, are well-understood consequences of mate competition; such traits evolve because they increase a male's sexual fitness relative to other males. However, through its effects on sexual selection, sexual conflict, and patterns of mating, mate competition has other evolutionary consequences that are poorly understood, yet arguably fundamental. My research seeks a more comprehensive understanding of these, focussing on the evolutionary consequences of mate competition for: i) nonsexual fitness, via its effects on adaptation to novel environments and the purging of harmful mutations; ii) sex-biased gene expression, which underlies the extensive sexual dimorphism and that constitutes a major component of biological diversity; and iii) sexual isolation, a barrier to gene flow between populations that is important to the origin of new species. Working with fruit flies (Drosophila) and other insects, in both the lab and the field, we use a combination of behavioural and fitness assays, and we rely heavily on the power of experimental evolution, enabling us to compare how the evolutionary process unfolds in replicate populations under different mate competition regimes. We give particular attention to the ecology of the mating environment, studying how it alters the evolutionary consequences of mate competition and whether the highly unusual lab environment of Drosophila studies -a model system for work on this topic- may have biased our perspective. We provide manipulative tests of the underlying mechanisms by which ecology alters mate competition and the effects this has on adaptation and purging. We also quantify how sex-biased gene expression evolves in response to manipulations of mate competition and the environment in which this occurs, and how mate competition and its ecology affect speciation. Over 160 years ago, Darwin suggested that, because sexual selection favours the healthiest and most vigorous males, the broader evolutionary consequence of mate competition is increased adaptation. This, it turns out, was too simplistic; mate competition can have complex and environment-dependent effects with a variety of population genetic consequence that include, but are not limited to, adaptation. This work has broad implications for our understanding of fundamental evolutionary processes and how ecology shapes them.