Fig. 2.

A model for the buildup of postzygotic isolation between 2 populations descended from a common ancestor adapting to distinct ecological environments, after ref. 28. 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 red arrow segments.