Evidence for speciation by reinforcement

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Reinforcement is a process within speciation where natural selection increases the reproductive isolation between two populations of species by reducing the production of hybrids.^[1][2] Evidence for speciation by reinforcement has been gathered since the 1990s, and along with data from comparative studies and laboratory experiments, has overcome many of the objections to the theory.^{[3]: 354 [4][5]} Differences in behavior or biology that inhibit formation of hybrid zygotes are termed prezygotic isolation. Reinforcement can be shown to be occurring (or to have occurred in the past) by measuring the strength of prezygotic isolation in a sympatric population in comparison to an allopatric population of the same species.^[3]: 357 Comparative studies of this allow for determining large-scale patterns in nature across various taxa.^[3]: 362 Mating patterns in hybrid zones can also be used to detect reinforcement.^[6] Reproductive character displacement is seen as a result of reinforcement,^[7] so many of the cases in nature express this pattern in sympatry. Reinforcement's prevalence is unknown,^[4] but the patterns of reproductive character displacement are found across numerous taxa (vertebrates, invertebrates, plants, and fungi), and is considered to be a common occurrence in nature.^[6] Studies of reinforcement in nature often prove difficult, as alternative explanations for the detected patterns can be asserted.^[3]: 358 Nevertheless, empirical evidence exists for reinforcement occurring across various taxa^[7] and its role in precipitating speciation is conclusive.^[8]