Mating preferences of selfish sex chromosomes.

The evolution of female mating preferences for harmful male traits is a central paradox of sexual selection1-9. Two dominant explanations for this paradox8,10 are Fisher's runaway process, which is based on genetic correlations between preference and trait1,3,4, and Zahavi's handicap principle, in which the trait is an honest costly signal of male quality2,6,8,11. However, both of these explanations require the exogenous initial spread of female preferences before harmful male traits can evolve1-4,6,8,11. Here I present a mechanism for the evolution of female mating preferences for harmful male traits that is based on the selfish evolutionary interests of sex chromosomes. I demonstrate that female-biased genetic elements-such as the W and X sex chromosomes-will evolve mating preferences for males who display traits that reduce their fitness and/or that of their male offspring, but increase fitness in female offspring. In particular, W-linked preferences can cause nearly lethal male traits to sweep to fixation. Sex-linked preferences can drive the evolution of traits such as ornamental handicaps and male parental care, and can explain variation in ornamentation and behaviour across taxa with divergent sex-determining mechanisms.