Barton (2001) used the Fisher-Orr geometric model of adaptation (Orr, 1998) to demonstrate that both intrinsic and extrinsic postzygotic isolation should evolve as a by-product of adaptation to contrasting environments. To illustrate, he assumed that divergent selection acted on just a single trait between populations (Fig. 3.2), whereas multiple additional traits were under stabilizing selection, favoring the same mean in both environments. Mutations were assumed to be pleiotropic, which means that while they change the population mean for the trait under directional selection in each environment, they have the side-effect of changing the mean in other traits, too. As each population attains its local adaptive peak by a sequence of mutational steps, advantageous mutations fixing later compensate for deleterious side-effects of advantageous mutations that fixed earlier (Fig. 3.2). These compensatory mechanisms fail in hybrids containing a sample of mutations from each of the parent populations, resulting in a phenotype that deviates from the optimum in the secondary traits (Barton, 2001). Thus, as populations adapt to different environments, the fitness of hybrids between them evolves below that predicted from the hybrid’s phenotype for the trait under divergent selection, the amount depending on genetic details such as

FIGURE 3.2 A model for the buildup of postzygotic isolation between 2 populations descended from a common ancestor adapting to distinct ecological environments, after Barton (2001). The perimeter of each circle represents a contour of equal fitness; fitness in each environment is higher inside the circle than outside. Trait x is under divergent natural selection, represented by separate adaptive peaks. Other traits, here represented by a single dimension y, are under stabilizing selection in both environments, as indicated by identical optima along this axis. A population adapts by fixing new advantageous mutations that bring the mean of trait x toward the optimum. An example of a sequence of adaptive steps is shown for each population by the linked arrow segments.