Assortative mating and the genic view of speciation

Abstract

1,3 4 We welcome Wu’s (2001) review of speciation from a genic perspective, and in particular its focus on the adaptive processes underlying the generation of new species, rather than the nature of the species themselves. However, we wish to stress that even from this perspective, the evolution of assortative mating should continue to be seen as a process of special importance in speciation. Although adaptive divergence in mating behaviour or habitat preference is implicitly incorporated into a genic perspective, we believe that assortative mating should have particular emphasis because it can cause dramatic changes in the fitness landscape, as the behaviour of different genotypes rapidly generates many epistatic interactions that did not exist under conditions of random mating. Such selection on multilocus genotypes could rapidly create the holes in the fitness landscape envisaged by recent models of speciation (Gavrilets, 1999). These increased levels of epistasis will further increase the size of the genomic regions where linkage disequilibria can be maintained by selection, and where new epistatic alleles can spread to fixation, eventually contributing to post as well as further premating isolation. However, in contrast to the generation of such post-mating incompatibilities, behavioural or signalling traits may evolve rapidly under direct selection, in particular during adaptation to divergent habitats, under sexual selection, or under direct selection for increased assortment (reproductive character displacement or reinforcement). A key divergence in our viewpoint is illustrated by Wu’s comments on transformed Drosophila lines