When hybrids between closely related species have lower...

When hybrids between closely related species have lower fitness than either parental type, it is known as extrinsic postzygotic isolation. Reinforcement is one form of extrinsic postzygotic isolation that involves assortative mating, as in the Phlox example described in this chapter. But assortative mating is not required for low hybrid fitness to contribute to isolation between groups in the early stages of speciation. For an example from butterflies, see:

Key Concepts

Extrinsic Postzygotic Isolation

Extrinsic postzygotic isolation occurs when hybrids between diverging species have reduced fitness due to environmental or ecological factors rather than intrinsic genetic incompatibilities. This means that the lower fitness is a result of the hybrid's traits being poorly adapted to the parental environments, thus limiting their survival and reproduction.

Reinforcement

Reinforcement is an evolutionary process that strengthens prezygotic isolation mechanisms because of the cost associated with producing unfit hybrids. When natural selection favors individuals that avoid mating with members of another species, reinforcement can lead to increased assortative mating or other behaviors that further reduce gene flow between the diverging groups.

Assortative Mating

Assortative mating refers to a mating pattern where individuals preferentially mate with others that are phenotypically or behaviorally similar. This preferential choice reduces interbreeding between distinct groups, which is particularly important in reinforcing reproductive barriers when hybrids exhibit lower fitness in given ecological contexts.

Hybrid Fitness

Hybrid fitness describes the survival and reproductive success of hybrid offspring relative to their parental species. When hybrids have reduced fitness due to being less adapted to environmental conditions, it can promote divergence and speciation by favoring the evolution of mechanisms that prevent the formation of these suboptimal hybrids.

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