Alycia Lackey, Alyssa Murray, Nadia Mirza, Thomas Powell

The pace of divergence and likelihood of complete speciation may depend how and when different types of reproductive barriers evolve. After initial reproductive barriers evolve, questions remain about how subsequently evolving barriers may facilitate additional divergence and potential speciation. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 generations) sympatric populations, adapted to different host fruits. We found that sexual isolation was significantly stronger than expectations of random mating. Thus, sexual isolation may play an important role in reducing gene flow allowed by earlier-acting ecological barriers. We also tested how warmer temperatures predicted under climate change could alter sexual isolation and found that sexual isolation was markedly asymmetric between the sexes of each population when flies were reared under warmer temperatures. Our findings provide a window into the early divergence process and the role of sexual isolation after initial ecological divergence, in addition to examining how environmental conditions could shape the likelihood of further divergence.