EVOLUTION OF CONDITION-DEPENDENT SEX ORNAMENTS AND MATING PREFERENCES: SEXUAL SELECTION BASED ON VIABILITY DIFFERENCES

The possibility that the evolution of mating preferences and secondary sex traits can be based on heritable differences in viability is examined with a three-locus model. Earlier genetic models suggested that viability-based processes alone cannot explain the evolution of mate choice and sex ornaments that reduce survival; a Fisherian mating advantage seemed necessary. The present model is based on a monogamous mating system that precludes such a mating advantage. A key assumption is that ornament development depends on the phenotypic condition and overall genotype of the possessor; there is evidence that secondary sex traits often mirror nutritional status and health, sometimes through hormonal mediation. Ornament and preference can then hitchhike slowly to high frequency with alleles that confer a slight survival advantage, provided that such alleles become available often enough. The evolution of mating preferences and secondary sex traits that reflect overall genotypic constitution therefore can be based solely on viability differences, no Fisherian mating advantage being required. In practice, these and several other mechanisms of sexual selection may occur together.