The effect of intrasexual fitness differences on genotype frequency stability at Fisherian sex ratio equilibrium

Fisherian sex ratio evolution is based on a set of common assumptions which , if met, will result in the maintenance of an equilibrium sex ratio of 0.5 . One of these assumptions is the absence of intrasexual fitness difference s. To investigate Fisherian sex ratio selection, variation is provided by t hree factor sex determination systems like that found in the platyfish, Xip hophorus maculatus. Here, populations with genetically-based intrasexual fi tness differences were established at a sex ratio predicted to be at Fisher ian equilibrium. The frequency of earlier versus later maturing genotypes c hanged in these populations, resulting in a sex ratio of 0.5, but at a diff erent point along a curve of Fisherian sex ratio equilibria. Thus it appear s that genetic differences in fitness which are in link-age with sex-determ ining factors can result in evolution along the sex ratio equilibrium curve .