How can sexual selection promote population divergence?

Sexual selection is a competition between conspecific individuals to acquire mates and maximize their reproductive success. This can lead to the development of conspicuous secondary sexual traits under several possible mechanisms. Since these sexual characters can be used as potential discriminant features in species recognition, it seems legitimate to think of a potential role of sexual selection in population divergence. Here I review several empirical studies and models that try to show how important sexual selection can be in speciation processes. The emergence of differences in secondary sexual traits occurs generally in allopatric populations triggered by random processes and reinforced by runaway selection. In a contact zone, this can "indirectly" result in assortative mating. In sympatry, cases of speciation by means of sexual selection are scarce unless ecological barriers are adduced. The conclusion is that sexual selection acts in a somewhat indirect fashion, more by enhancing previous evolutionary directions than by initiating divergence.