Sexual Selection and Mate Choice: Insights from Neutralist Perspectives

Darwin's concept of sexual selection has been an area of intense research interest for the past half-century. Research has mainly focused on intersexual selection (selection arising from mate choice), and has particularly focused on the hypothesis that mates are chosen on the basis of "genetic quality" which is "honestly" signaled by sexually dimorphic traits. I discuss these models in the light of evidence that most genetic variation in real populations is either selectively neutral or slightly deleterious. Since several well-known models have focused on the immune system as a source of heritable variation in fitness, I examine evidence from studies of the vertebrate major histocompatibility complex and its interaction with pathogens. Finally, I discuss alternative hypotheses for the evolution of secondary sexual characteristics that are consistent with the prevalence of purifying selection rather than positive selection in most populations. One such model, the random walk model, relies only on the well-attested processes of mutation, purifying selection, and genetic drift, thereby providing an attractive alternative to models that assume ubiquitous positive selection.