A model that describes how changes at multiple genetic loci can lead to subpopulations of a species becoming unable to interbreed successfully. This reproductive isolation eventually results in the formation of new species. The model assumes that subpopulations of an ancestral species become separated, evolve independently, and over time accumulate new alleles for many different genes. Subsequent breeding between individuals from the different subpopulations will produce sterile hybrids because of incompatibility between the products of each parental set of genes. Incompatibility between related lineages can also arise owing to rearrangements in the chromosomes (e.g. centric fusion) within a subpopulation, resulting in the chromosomes’ inability to pair normally during sex cell formation (meiosis) in the hybrid offspring. The model is named after Theodosius Dobzhansky (1900–75) and Hermann Joseph Muller (1890–1967).