Frequency-dependent sexual selection as a result of variations in fitness at breeding time

Summary

Darwin's model of sexual selection by variations in fitness at breeding time is generalised to allow for the sexual advantage of the favoured males to be only partial and not complete. If the earlier breeding pairs are the fitter, as Darwin assumed, then the favoured males will always gain a selective advantage; computer simulations show that this selection is frequency-dependent. If a female mating preference determines the sexual advantage of the males, then the proportion α of the females who exercise the mating preference will determine whether the frequency dependence is positive or negative. When α is less than 0·3, the overall selective advantage of the favoured males declines as they increase in frequency: the selection is thus negatively frequency-dependent. Polymorphic equilibria may also become established if natural selection is acting against the sexually favoured character of the males. When α is greater than 0·7, the selective advantage increases rapidly as the favoured males increase in frequency. When earlier pairs are not fitter and fitness at breeding time is distributed symmetrically round the optimum, males favoured as a result of a female mating preference can still gain an advantage even though a proportion of these males will be chosen by the earlier females and form pairs with low fitnesses. This model of sexual selection does not therefore depend on Darwin's assumption that the earlier pairs are the fitter.