Fine‐scale geographic patterns of gene flow and reproductive character displacement in drosophila subquinaria and d. recens

When two species are incompletely isolated, strengthening premating isolation barriers in response to the production of low fitness hybrids may complete the speciation process. Here we use the sister species Drosophila subquinaria and D. recens to study the conditions under which this reinforcement of species boundaries occurs in natural populations. We first extend the region of known sympatry between these species, and then we conduct a fine-scale geographic survey of mate discrimination coupled with estimates of gene flow within and admixture between species. Within D. subquinaria reinforcement is extremely effective: we find variation in mate discrimination both against D. recens males as well as against conspecific allopatric males on the scale of a few kilometers and in the face of gene flow both from conspecific populations and introgression from D. recens. In D. recens we do not find evidence for increased mate discrimination in sympatry, even where D. recens is rare, consistent...