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 years) 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 found that sexual isolation was markedly asymmetric between the sexes of each population. Lastly, we tested how warmer temperatures predicted under climate change could alter sexual isolation and found that mating interactions were sensitive to temperature experienced during development. Our findings provide a window into the early divergence process and the role of sexual isolation after initial ecological divergence, in addition to examining multiple factors that could shape the likelihood of further divergence.

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The pace of divergence and likelihood of speciation often depends on how and when different types of reproductive barriers evolve. Questions remain about how reproductive isolation evolves after initial divergence. 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 (hawthorn and apple). We found that flies from both populations were more likely to mate within than between populations. Thus, sexual isolation may play an important role in reducing gene flow allowed by early‐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 under warmer temperatures – apple males and hawthorn females mated randomly while apple females and hawthorn males mated more within populations than between. Our findings provide a window into the early speciation 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.