Reinforcement (speciation)

Reinforcement (sometimes called secondary contact) is a process whereby pre-zygotic isolation is enhanced between two populations of incipient species by natural selection. This increase in isolation is due to selection that acts against the production of hybrid individuals with lower fitness. The idea was originally developed by Alfred Russel Wallace and is still often referred to as the Wallace effect. The modern conception of reinforcement originates from Theodosius Dobzhansky, where he envisioned a species separated in allopatry, wherein secondary contact produces unfit hybrids, with natural selection favoring enhanced pre-zygotic isolation. The support for reinforcement has fluctuated since its inception. Various objections have been raised by evolutionary biologists as to the plausibility of its occurrence. Since the 1990s, data from theory, experiments, and nature have overcome many of the past objections; rendering reinforcement widely accepted. However, its ubiquity remains unknown as well as its prevalence in nature. Furthermore, terminological confusion and term usage over history has led to multiple meanings and complications.

Reinforcement is one of the few cases in which selection can favor an increase in pre-zygotic isolation, influencing the process of speciation directly. This aspect having been particularly appealing among evolutionary biologists. Numerous models have been developed to understand its operation in nature, most relying on several facets: genetics, population structures, influences of selection, and mating behaviors.

Empirical support for reinforcement exists, both in the laboratory and in nature. Documented examples are found in a wide range of organisms: amphibians, birds, crustaceans, echinoderms, fish, fungi, insects, mammals, molluscs, and plants. The secondary contact of originally separated incipient species is increasing due to human activities such as the introduction of invasive species or the modification of natural habitats. This has implications for measures of biodiversity and may become more relevant in the future.

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