The role of linkage disequilibrium in the evolution of premating isolation

The suggestion that speciation may often occur, or be completed, in the presence of gene flow has long been contentious, due to an appreciation of the challenges to maintaining population- or species-specific gene combinations when gene flow is occurring. Linkage disequilibrium between loci involved in postzygotic and premating isolation must often be built and maintained as the source of these species-specific genotypes. Here, I discuss proposed solutions to facilitate the establishment and maintenance of this linkage disequilibrium. I concentrate primarily on two such factors: one-allele versus two-allele mechanisms of premating isolation, and the form of selection against hybrids as it relates to its effect on the pathway between postzygotic and prezygotic isolation. The goal of this discussion is not to thoroughly review these factors, but instead to concentrate on aspects and implications of these solutions that are currently underemphasized in the speciation literature.