DobzhanskyMuller

Dobzhansky–Muller incompatibility is a genetic mechanism by which reproductive isolation between diverging populations can arise without any single population showing reduced fitness in its own environment. The concept was developed independently by Theodosius Dobzhansky (1937) and Hermann Muller (1942) as an explanation for postzygotic isolation observed between species that have diverged in allopatry.

In its simplest form, the model involves two loci and two alleles in a common ancestor with alleles A at locus 1 and B at locus 2. In one population, A mutates to a, while B remains unchanged; in the other population, B mutates to b, while A remains A. Each derived allele is compatible with the ancestral background within its own lineage, so both lineages appear fit. However, when individuals from the two populations hybridize, the combination of the two derived alleles (a and b) can interact negatively through epistasis, reducing hybrid fitness. The result is postzygotic isolation between the populations even though neither population carries the deleterious interaction in its own genetic background.

The Dobzhansky–Muller framework has been extended to involve multiple loci and complex networks of epistatic interactions. It remains a central explanatory model for how genetic divergence can translate into reproductive barriers and is frequently used to interpret genomic patterns of limited introgression during speciation.