The Dobzhansky-Muller Model

Chromosome mutation. Getty/Chris Dascher

The Dobzhansky-Muller Model is a scientific explanation of why natural selection influences speciation in such a way that when hybridization occurs between species, the resulting offspring is genetically incompatible with other members of its species of origin.

This occurs because there are several ways that speciation occurs in the natural world, one of which is that a common ancestor can break off into many lineages due to reproductive isolations of certain populations or parts of populations of that species. In this scenario, the genetic makeup of those lineages changes over time through mutations and natural selection choosing the most favorable adaptations for survival. Once the species have diverged, many times they are no longer compatible and can no longer sexually reproduce with each other.

The natural world has both prezygotic and postzygotic isolation mechanisms that keep species from interbreeding and producing hybrids, and the Dobzhansky-Muller Model helps to explain how this occurs through the exchange of unique, new alleles and chromosomal mutations.

A New Explanation for Alleles

Theodosius Dobzhansky and Hermann Joseph Muller created a model to explain how new alleles arise and are passed down in the newly formed species. Theoretically, an individual that would have a mutation at the chromosomal level would not be able to reproduce with any other individual.

The Dobzhansky-Muller Model attempts to theorize how a brand new lineage can arise if there is only one individual with that mutation; in their model, a new allele arises and becomes fixed at one point. In the other now diverged lineage, a different allele arises at a different point on the gene. The two diverged species are now incompatible with each other because they have two alleles that have never been together in the same population.

This changes the proteins that are produced during transcription and translation, which could make the hybrid offspring sexually incompatible; however, each lineage can still hypothetically reproduce with the ancestral population, but if these new mutations in the lineages are advantageous, eventually they will become permanent alleles in each population—when this occurs, the ancestral population has successfully split into two new species.

Further Explanation of Hybridization

The Dobzhansky-Muller Model is also able to explain how this may happen at a large level with whole chromosomes. It is possible that over time during evolution, two smaller chromosomes may undergo centric fusion and become one large chromosome. If this happens, the new lineage with the larger chromosomes is no longer compatible with the other lineage and hybrids cannot happen.

What this essentially means is that if two identical yet isolated populations start with a genotype of AABB, but the first group evolves to aaBB and the second to AAbb, meaning that if they crossbreed to form a hybrid, the combination of a and b or A and B occurs for the first time in the population's history, making this hybridized offspring unviable with its ancestors.

The Dobzhansky-Muller Model states that incompatibility, then, is most likely caused by what's known as alternative fixation of two or more populations instead of just one and that the hybridization process yields a co-occurrence of alleles in the same individual that is genetically unique and incompatible with others of the same species.