Figure 1: Divergent selection and the evolution of intrinsic isolation. Time 0: Two populations, connected by migration inhabit different environments. Both are initially fixed for the AB genotype. Time 1: Divergent ecological selection has caused divergence at the A locus, with the a allele evolving to high frequency in Population 2. Time 2: The modifier b evolves in Population 2, either because it ameliorates a pleiotropic cost of the a allele (Model 1) or because the B locus also experiences divergent ecological selection (Model 2).