Reinforcement is the process by which natural selection increases reproductive isolation.

Reinforcement can occur as follows:

When two populations which have been kept apart, come back into contact, the reproductive isolation between them might be complete or incomplete.

If it is complete, speciation has occurred.

If it is incomplete, hybrids would be produced.

If the hybrids had lower fitness than either parental form, selection would act to increase the reproductive isolation because each form would do better not to mate with the other and form the disadvantageous hybrids. Speciation might then be speeded up by favoring genes which caused individuals to avoid mating with hybrids.

Reinforcement is a necessary requirement for both the parapatric and sympatric theories of speciation: it is the process by which a hybrid zone develops into a full species barrier.

• Secondary reinforcement:

Reinforcement is known as secondary reinforcement if the reproductive isolation has partly evolved allopatrically, and is then reinforced when the two populations come into secondary contact. Reinforcement could occur whenever two forms coexist, and the hybrids between them have lower fitness than crosses within each form.

• Artificial selection experiments:

Reinforcement can be simulated by artificial selection experiments. By continually selecting for assortative mating it has been possible to obtain significant changes in prezygotic isolation mechanisms. However, the theoretical conditions for speciation to take place by reinforcement are difficult and it is controversial whether the process takes place in nature.

Does reinforcement exist in nature?