Rapid evolution towards equal sex ratios in a system with heterogamety

The equal sex ratios found in many species with heterogametic sex determina tion may be a consequence of selection for equality or the result of the Me ndelian segregation of the two sex chromosomes. A lack of genetic variation in sex ratio in species with heterogamety has been the major obstacle in d istinguishing between these two hypotheses. We overcome this obstacle by ge nerating hybrids between two species of Drosophila. The resulting hybrid li nes had biased sex ratios, allowing us to observe the evolution of sex rati o in replicate populations. Sex ratio converged towards 1:1 after 16 genera tions of natural selection. These changes in sex ratio were not due to diff erences in viability between the sexes and the loci underlying the variatio n in sex ratio were not sex-linked. Equal sex ratios may therefore be the r esult of natural selection as Fisher predicted.