different way. This can result in the populations having different genotypes, that can cause Dobzhansky–Muller incompatibilities. Genes that are incompatible according to the Dobzhansky–Muller model require three criteria. 1. The gene reduces the fitness of the hybrid, 2. The gene has functionally diverged in each of the hybridising species and, 3. The hybrid incompatibility is only present in combination with a partner gene. Whether the genes are actually incompatible is also dependent on whether the genes are dominant or recessive. Incompatibility will only occur if both alleles are expressed and not if one is recessive. The genetic changes that are accumulated when populations diverge from a common ancestor will not severely decrease viability or fertility because natural selection influences these strongly deleterious alleles. However, natural selection cannot act when alleles have never occurred together, as they would in the genome of a hybrid. Therefore, it is possible that when these alleles interact, these alleles prove to be incompatible. An incompatible gene prevents the populations from successfully hybridising. These Dobzhansky–Muller incompatibilities can therefore also increase the chance of speciation. Certain patterns in the Dobzhansky–Muller incompatibilities can provide information of modes of divergence. For instance, if divergence is due to different selection pressures, thus causing natural selection to act, or to random genetic drift. Therefore, Dobzhansky–Muller incompatibilities can also provide information on the time and type of divergence which can help in phylogenetic studies. References Evolutionary biology Speciation