The Depletion of Genetic Variance by Sexual Selection

Sexually selected traits display substantial genetic variance 1, 2, in conflict with the expectation that sexual selection will deplete it 3, 4, 5. Condition dependence is thought to resolve this paradox 5, 6, 7, but experimental tests that relate the direction of sexual selection to the availability of genetic variance are lacking. Here, we show that condition-dependent expression is not sufficient to maintain genetic variance available to sexual selection in multiple male sexually selected traits. We employed an experimental design that simultaneously determined the quantitative genetic basis of nine male cuticular hydrocarbons (CHCs) of Drosophila bunnanda, the extent of condition dependence of these traits, and the strength and direction of sexual selection acting upon them. The CHCs of D. bunnanda are condition dependent, with 18% of the genetic variance in male body size explained by genetic variance in CHCs. Despite the presence of genetic variance in individual male traits, 98% of the genetic variance in CHCs was found to be orientated more than 88° away from the direction of sexual selection and therefore unavailable to selection. A lack of genetic variance in male traits in the direction of sexual selection may represent a general feature of sexually selected systems, even in the presence of condition-dependent trait expression.