Figure 1. Model scenario.

Graph (a): The ancestral situation is a large mainland population infected with Wolbachia and a small peripheral island population without Wolbachia. Further, the populations have diverged at a phenotypic trait locus T, with local adaptation in the island population of the T ₂ allele. After migration from mainland to island is restored (secondary contact), a new allele at the mating preference locus, P ₂, is introduced at low frequency in the island population. Females with P ₂ prefer T ₂ males relative to T ₁ males. We investigated the stability of infection differences between the populations, probability of local adaptation at the selected locus in the presence of migration, under what conditions mutants at the mate preference locus can invade and result in genetic divergence and prezygotic isolation. Graph (b): The mainland-island model analyzed in this article applies to a situation with a large central population and small peripheral isolates. The peripheral isolates might face migration from the infected mainland or not. Some of the isolates are infected with Wolbachia while others have lost the infection. Our results suggest that it is the ones that lose their Wolbachia that are more likely to diverge into new species.