Noisy neighbors can hamper the evolution of reproductive isolation by reinforcing selection.

Reinforcement is the process by which selection against hybridization leads to an increase in reproductive isolation. The influence of reinforcing selection can be detected when sympatric individuals (those from areas of secondary contact) show a higher degree of prezygotic isolation than allopatric individuals (those from areas outside each other's range). In areas of secondary contact with Drosophila santomea, Drosophila yakuba females show reinforcement of gametic isolation but not behavioral isolation, despite the fact that both behavioral and gametic isolation evolve in D. yakuba in experimental sympatry. Using behavioral assays and experimental evolution, I studied how both gametic and behavioral isolation are affected by biotic factors that the two species encounter in their natural environment. I show that if D. yakuba females are in environments where D. yakuba, D. santomea, and males from other species coexist, these females cannot fully discern between conspecific and heterospecific males. In such complex environments, gametic but not behavioral isolation evolves. The presence of nonhybridizing species can constrain the effect of reinforcement on behavioral isolation.