Table 1: Simple Dobzhanksy-Muller incompatibility. With this type of genetic architecture, it is possible to evolve from one high fitness genotype (AB) to another (ab) without passing through any fitness valleys (by AB → aB → ab). Matings between two high fitness types yield unfit hybrids (i.e., AB × ab produces the unfit type Ab).

AA

BFitFit
bUnfitFit