The role of sexual isolation during rapid ecological divergence: evidence for a new dimension of isolation in Rhagoletis pomonella

The pace of divergence and likelihood of complete speciation may depend on
how and when different types of reproductive barriers evolve. Questions
remain about the evolution of reproductive barriers 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 within
population than between. 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.