Breaking the Chain – The role of intersexual genetic correlations in sexual dimorphism and the consequences and limits of their evolution

In many sexually reproducing species, males and females often differ in countless ways beyond their primary sexual organs. This phenomenon is known as sexual dimorphism, and it is generally considered to be an adaptive response to differences in the selection pressures experienced by males and females. Despite the advantages associated with sexual dimorphism, it does not evolve completely unhindered – there are plenty of biological effects that can limit the extent and rate of divergence between the sexes. This research project focusses on the potential role of the intersexual genetic correlation (rmf­) – which describes the degree to which brothers and sisters are phenotypically similar to each other – as a limiter on the evolution of sexual dimorphism, a topic of considerable recent debate. We examined the extent to which the intersexual genetic correlation for body size (a sexually dimorphic trait) could be experimentally evolved in replicate populations of fruit flies, Drosophila melanogaster, via selection acting at the family level over the course of five generations. We observed a dramatic (but short-lived) response in rmf­ to selection, which suggests that under the right circumstances that the intersexual genetic correlation is more evolvable than previously thought. A follow-up fitness assay conducted at the end of the experimental evolution period also revealed that some of the intersexual conflict over fitness could be overcome given the right kind of mating patterns. This project provides fresh insight into the relative evolvability of intersexual genetic correlations, as well as empirical evidence on ways in which selection can facilitate the adaptive evolution of sexual dimorphism