The Evolution of Male Sexual Traits Through Differential Allocation

Differential allocation (DA) is one form of post-copulatory mate choice in which females increase their reproductive investment when mated to an attractive male. Despite its widespread occurrence in the animal kingdom and its importance as an agent of sexual selection, most empirical studies are still focussing on demonstrating its existence and underlying proximate mechanisms, rather than how DA contributes to the evolution of male sexual traits. One reason for the lack of such studies is that, until recently, a coherent theoretical framework has not existed. Since DA operates through the interaction between the phenotypes of a male and an unrelated, female partner, we propose that indirect genetic effect (IGE) theory based on the 'interacting phenotype' provides a useful framework to examine how female DA influences the evolution of male sexual traits. Theoretically, IGEs can influence the rate and/or direction of evolution and generate large time-lags in the response of phenotypic traits to selection. Despite the potential importance of this approach, more empirical research is required to establish its plausibility. Our proposed research will combine the modern methods of multivariate selection analysis, life-history theory, sexual selection and quantitative genetics to empirically test the importance of female DA to the evolution of male sexual pheromones in the ovoviviparous cockroach Nauphoeta cinerea - a species with strong evidence for female DA based on male sexual pheromones. Consequently, our proposed research will provide the first compelling study to move beyond simply quantifying the existence of DA to examining how this process influences the evolutionary trajectories of male sexual traits.