Figure 1.

Alternative views of potential protein adaptive landscapes. In (A), the protein adaptive landscape is viewed as being like an arrêt ridge, with only a single narrow path leading from the current adaptive peak in the foreground to a new adaptive peak in the distance. This landscape is conducive to convergence. In (B), the adaptive landscape is viewed as being like rolling hills, with many alternative routes to nearby adaptive hilltops that are not substantially different from one another. With so many alternative paths and alternative similar hilltops, under this scenario sequences would be unlikely to converge (i.e., follow the same path) even under similar adaptive pressure.