Dissecting processes underlying reinforcement and its consequences for speciation

Abstract

A primary focus of evolutionary biology is the origin of biodiversity, particularly how new species arise and how they remain distinct. Hybrid zones, where recently diverged species come into contact, provide a window into the processes driving the early stages of speciation since the balance of selection and gene flow between species is crucial to determining whether species remain distinct or fuse back into one. When hybrids are sterile or unfit, selection can act to increase pre-mating barriers to reinforce species boundaries. Whether this reinforcement occurs will depend on the strength of this selection and gene flow both between and within species. This reinforcement can also have secondary effects, where selection in the contact zone drives divergence between populations in this zone and those outside it. In two species of fruit fly in North America, such a process is occurring. Drosophila subquinaria and D. recens share a contact zone east of the Rocky Mountains. In D. subquinaria reinforcing selection has strengthened pre-mating isolation, and D. subquinaria females from this zone reject D. recens males as potential mates. These females also reject males of their own species from populations outside this zone. In contrast, D. recens does not show either of these patterns in mating behavior. Here, I dissect what forces are contributing to divergence both between and within species. I found that strong selection to avoid hybridization coupled with reduced gene flow from populations outside the contact zone has facilitated the evolution of premating isolation in D. subquinaria. Additionally, the genetic basis of mate choice between and within species may be shared or closely linked, which may have facilitated the evolution of isolation within this species. On the other hand, a lower cost to hybridization, coupled with abundant gene flow from populations outside the contact zone appears to have hindered the evolution of reinforced premating isolation in D. recens. Thus, the outcome of secondary contact can differ for each species involved, depending on the conditions of gene flow and selection, and divergence within species as a result of this contact may be facilitated by genetic linkage.