Dobzhansky-Muller model of hybrid incompatibility

How does genic incompatibility between species evolve without simultaneously causing defects in pure species? A popular explanation is the Dobzhansky-Muller (DM) model of hybrid incompatibility. In the ancestral population, the genotype is AA BB. When the population is split into two, A evolves into a in one population and B evolves into b in the other. a and b are mutually incompatible. As the a-b interaction is not present in the pure species, the evolution of incompatibility is possible. Detailed genetic analysis on hybrid male sterility, however, has shown that hybrid incompatibility often involves conspecific genic interactions as well. The DM model is far too simple in that respect. On the far right (hybrid), the divergence process is indicated by the black double-headed arrows and the incompatibility is indicated by the green double-headed arrow. The relationship between the black and green arrows should be the essence of the evolution of reproductive isolation. A deficiency of the DM model is that it does not consider the divergence process that is indicated by the black arrows but focuses instead on the incompatibility, which is a byproduct of that divergence.