Figure 1. Mutational pathways for a system with two loci.

There are two different pathways to reach the final mutant. Fitness is represented by the size of the circles denoting the types. Thus the wildtype ab and Ab have a similar fitness whereas AB has a significantly greater fitness compared to the wildtype while aB is much less fit than the wildtype. When all mutation rates are the same, the pathway via aB would be not adaptive, since this type has a low fitness. If the mutation rate is large enough, especially if (indicated by the thick arrow), this pathway becomes accessible.