Local Adaptation of Male Sexual Fitness in Drosophila melanogaster

Abstract

Darwin first proposed sexual selection as a process to explain the evolution of extravagant morphological traits in males. Despite being potentially detrimental to individual survival, such traits evolve because they increase a male's reproductive success, and provide a net benefit to their fitness. Mate competition is the source of sexual selection, and healthier, more vigorous males are likely to be superior competitors. Because most genes are likely to impact an individual’s health/vigour, sexual selection should act across much of the genome to favour the same alleles as natural selection, thereby promoting adaptation. On the other side of the coin, adaptation to an environment should enhance male sexual fitness, since it is likely to increase the overall health/vigour of individuals within a population, though tests of this prediction are rare and results are mixed. Taking advantage of a long-term evolution experiment involving replicate populations of Drosophila melanogaster, I performed a reciprocal transplant in which the sexual fitness of males was compared when raised in an environment to which they are well adapted and in one to which they are not. I improved on past tests via a comprehensive measure of male sexual fitness that included pre- and post-copulatory reproductive success in a competitive assay under conditions that closely mirrored those to which the populations have been evolving. I found that sexual fitness was higher in locally-adapted males from these experimental populations, a result that was consistent across environments that also manipulated the context in which mate competition occurred.