Asymmetry in the evolution of female mating preferences

Abstract

Trivers¹ has suggested that where genetic or developmental constraints on the expression of a trait prevent male and female fitnesses from being maximized simultaneously, female mating preferences should evolve to favour males who exhibit variants of the trait that confer relatively low fitness on males but relatively high fitness on females. This asymmetry is expected because alleles that affect mating preferences are expressed only in females, but are genetically correlated with alleles that differentially affect the fitnesses of the two sexes. Here we describe a two-locus population–genetic model that embodies this idea. The model's qualitative behaviour is exactly like that of previous models^2–10 for the joint evolution of male traits and female mating preferences: evolution is equally likely to proceed in either direction along (or away from) a line of neutral equilibria that relates given frequencies of the preference alleles to corresponding frequencies of the trait alleles. But there is a quantitative asymmetry, of the expected kind, in the shape of the line of equilibria. When we extend the model to include migration between partially isolated demes (breeding groups), ‘selective diffusion’^11,12 moves the demes along the line of equilibria in the direction that increases average female fitness while lowering average male fitness.