Sexual selection and the sex ratio: An ESS analysis

Should attractive fathers have more sons? According to widespread verbal arguments, the answer is in the affirma- tive, but formal models are lacking. We investigate the question by means of an ESS analysis of sexual selection and sex ratio selection. The purpose of this paper is twofold: firstly, we show how an ESS approach can be used to model the coevolution of female mate choice and male secondary sexual traits. In the ESS approach, the genetic covariances of traditional genetic models of sexual selection are replaced by covariances between strategies. In com- parison with population genetic and quantitative genetic models of sexual selection, the ESS approach is simpler and yields more insight in the underlying selective forces. Secondly, we show that whether attractive fathers should have more sons depends on the mechanism of sexual selection: sexual selection driven by the Fisherian runaway process alone, without costs of choosiness, does not select for sex ratio specialization according to paternal attractiveness, but the good-genes process causes selection to favor such specialization.