The Rapid Evolution of Male Genitalia

The male genitalia of internally fertilizing species evolve very rapidly, typically faster than any other physical character, and even in species that otherwise look identical, male genitals differ greatly. However selection on male genitalia does not seem to be particularly strong, which is paradoxical - fast evolution, yet (relatively) weak selection. There are several ways to resolve this apparent paradox: perhaps selection has not been measured in sufficient detail, perhaps genitals are relatively free from constraining genetic correlations, or perhaps selection and genetic variation in genitals are aligned such that rapid evolution is facilitated. At present no study has investigated all these possibilities. The genital arch of Drosophila simulans is the only trait showing consistent differences between D. simulans and its sibling species (D. melanogaster & D. mauritiana), and must therefore have evolved relatively rapidly, like male genitals generally. The genital arch also influences male fertilization success in D. simulans and is easy to measure. In this study we will characterise the genetic variance and covariance matrix and the strength and form of selection acting on the genital arch of D. simulans to assess potential explanations for the apparent mismatch between rates of genital evolution and the strength of selection on them. We will then test our findings by artificially selecting on the genital arch and assessing the responses to and fitness consequences of this selection. Our results will provide us with fundamental insights into one of the most pervasive patterns across the animal kingdom, the rapid evolution of male genitalia. Furthermore, because genital mismatches prevent successful fertilization, this work also has consequences for our understanding of reproductive isolation, and hence speciation and the generation of biodiversity.