Process of increasing reproductive isolation
Reinforcement is a process of speciation where natural selection increases the reproductive isolation (further divided to pre-zygotic isolation and post-zygotic isolation) between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation (differences in behavior or biology that inhibit formation of hybrid zygotes). Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. This aspect has been particularly appealing among evolutionary biologists. 
The support for reinforcement has fluctuated since its inception, and terminological confusion and differences in usage over history have led to multiple meanings and complications. 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, though its prevalence in nature remains unknown.
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: both vertebrates and invertebrates, fungi, and plants. The secondary contact of originally separated incipient species (the initial stage of speciation) 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.
History.
Reinforcement has had a complex history in that its popularity among scholars has changed over time. Jerry Coyne and H. Allen Orr contend that the theory of reinforcement went through three phases of historical development:
Sometimes called the Wallace effect, reinforcement was originally proposed by Alfred Russel Wallace in 1889. His hypothesis differed markedly from the modern conception in that it focused on post-zygotic isolation, strengthened by group selection. Theodosius Dobzhansky was the first to provide a thorough description of the process in 1937, though the term itself was not coined until 1955 by W. Frank Blair. In 1930, Ronald Fisher laid out the first genetic description of the process of reinforcement in "The Genetical Theory of Natural Selection", and in 1965 and 1970 the first computer simulations were run to test for its plausibility. Later population genetic and quantitative genetic studies were conducted showing that completely unfit hybrids lead unequivocally to an increase in prezygotic isolation.
Dobzhansky's idea gained significant support; he suggested that it illustrated the final step in speciation, for example after an allopatric population comes into secondary contact. In the 1980s, many evolutionary biologists began to doubt the plausibility of the idea, based not on empirical evidence, but largely on the growth of theory that deemed it an unlikely mechanism of reproductive isolation. A number of theoretical objections arose at the time and are addressed in the Arguments against reinforcement section below.
By the early 1990s, reinforcement saw a revival in popularity among evolutionary biologists; due primarily from a sudden increase in data—empirical evidence from studies in labs and largely by examples found in nature. Further, computer simulations of the genetics and migration patterns of populations found, "something "looking" like reinforcement". The most recent theoretical work on speciation has come from several studies (notably from Liou and Price, Kelly and Noor, and Kirkpatrick and Servedio) using highly complex computer simulations; all of which came to similar conclusions: that reinforcement is plausible under several conditions, and in many cases, is easier than previously thought.
Terminology.
Confusion exists around the meaning of the term reinforcement. It was first used to describe the observed mating call differences in "Gastrophryne" frogs within a secondary contact hybrid zone. The term secondary contact has also been used to describe reinforcement in the context of an allopatrically separated population experiencing contact after the loss of a geographic barrier. The Wallace effect is similar to reinforcement, but is rarely used. Roger Butlin demarcated incomplete post-zygotic isolation from complete isolation, referring to incomplete isolation as reinforcement and completely isolated populations as experiencing reproductive character displacement. Daniel J. Howard considered reproductive character displacement to represent either assortive mating or the divergence of traits for mate recognition (specifically between sympatric populations). Reinforcement, under his definition, included prezygotic divergence and complete post-zygotic isolation. Servedio and Noor include any detected increase in prezygotic isolation as reinforcement, as long as it is a response to selection against mating between two different species. Coyne and Orr contend that, "true reinforcement is restricted to cases in which isolation is enhanced between taxa that can still exchange genes".
Models.
One of the strongest forms of reproductive isolation in nature is sexual isolation: traits in organisms involving mating. This pattern has led to the idea that, because selection acts so strongly on mating traits, it may be involved in the process of speciation. This process of speciation influenced by natural selection is reinforcement, and can happen under any mode of speciation ("e.g." geographic modes of speciation or ecological speciation). It necessitates two forces of evolution that act on mate choice: natural selection and gene flow. Selection acts as the main driver of reinforcement as it selects against hybrid genotypes that are of low-fitness, regardless if individual preferences have no effect on survival and reproduction. Gene flow acts as the primary opposing force against reinforcement, as the exchange of genes between individuals leading to hybrids cause the genotypes to homogenize.
Butlin laid out four primary criteria for reinforcement to be detected in natural or laboratory populations:
After speciation by reinforcement occurs, changes after complete reproductive isolation (and further isolation thereafter) are a form of reproductive character displacement. A common signature of reinforcement's occurrence in nature is that of reproductive character displacement; characteristics of a population diverge in sympatry but not allopatry. One difficulty in detection is that ecological character displacement can result in the same patterns. Further, gene flow can diminish the isolation found in sympatric populations. Two important factors in the outcome of the process rely on: 1) the specific mechanisms that causes prezygotic isolation, and 2) the number of alleles altered by mutations affecting mate choice.
In instances of peripatric speciation, reinforcement is unlikely to complete speciation in the case that the peripherally isolated population comes into secondary contact with the main population. In sympatric speciation, selection against hybrids is required; therefore reinforcement can play a role, given the evolution of some form of fitness trade-offs. In sympatry, patterns of strong mating discrimination are often observed—being attributed to reinforcement. Reinforcement is thought to be the agent of gametic isolation.
Genetics.
The underlying genetics of reinforcement can be understood by an ideal model of two haploid populations experiencing an increase in linkage disequilibrium. Here, selection rejects low fitness formula_1 or formula_2 allele combinations while favoring combinations of formula_3 alleles (in the first subpopulation) and formula_4 alleles (in the second subpopulation). The third locus formula_5 or formula_6 (the assortive mating alleles) have an effect on mating pattern but is not under direct selection. If selection at formula_7 and formula_8 cause changes in the frequency of allele formula_5, assortive mating is promoted, resulting in reinforcement. Both selection and assortive mating are necessary, that is, that matings of formula_10 and formula_11 are more common than matings of formula_12 and formula_13. A restriction of migration between populations can further increase the chance of reinforcement, as it decreases the probability of the differing genotypes to exchange.
An alternative model exists to address the antagonism of recombination, as it can reduce the association between the alleles that involve fitness and the assortive mating alleles that do not. Genetic models often differ in terms of the number of traits associated with loci; with some relying on one locus per trait and others on polygenic traits.
Population structures.
The structure and migration patterns of a population can affect the process of speciation by reinforcement. It has been shown to occur under an island model, harboring conditions with infrequent migrations occurring in one direction, and in symmetric migration models where species migrate evenly back and forth between populations.
Reinforcement can also occur in single populations, mosaic hybrid zones (patchy distributions of parental forms and subpopulations), and in parapatric populations with narrow contact zones.
Population densities are an important factor in reinforcement, often in conjunction with extinction. It is possible that, when two species come into secondary contact, one population can become extinct—primarily due to low hybrid fitness accompanied by high population growth rates. Extinction is less likely if the hybrids are inviable instead of infertile, as fertile individuals can still survive long enough to reproduce.
Selection.
Speciation by reinforcement relies directly on selection to favor an increase in prezygotic isolation, and the nature of selection's role in reinforcement has been widely discussed, with models applying varying approaches. Selection acting on hybrids can occur in several different ways. All hybrids produced may be equality low-fitness, conferring a broad disadvantage. In other cases, selection may favor multiple and varying phenotypes such as in the case of a mosaic hybrid zone. Natural selection can act on specific alleles both directly or indirectly. In direct selection, the frequency of the selected allele is favored to the extreme. In cases where an allele is indirectly selected, its frequency increases due to a different linked allele experiencing selection (linkage disequilibrium).
The condition of the hybrids under selection can play a role in post-zygotic isolation, as hybrid inviability (a hybrid unable to mature into a fit adult) and sterility (the inability to produce offspring entirely) prohibit gene flow between populations. Selection against the hybrids can even be driven by any failure to obtain a mate, as it is effectively indistinguishable from sterility—each circumstance results in no offspring.
Mating and mate preference.
Some initial divergence in mate preference must be present for reinforcement to occur. Any traits that promote isolation may be subjected to reinforcement such as mating signals ("e.g." courtship display), signal responses, the location of breeding grounds, the timing of mating ("e.g." seasonal breeding such as in allochronic speciation), or even egg receptivity. Individuals may also discriminate against mates that differ in various traits such as mating call or morphology. Many of these examples are described below.
Evidence.
The evidence for reinforcement comes from observations in nature, comparative studies, and laboratory experiments.
Nature.
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. Comparative studies of this allow for determining large-scale patterns in nature across various taxa. Mating patterns in hybrid zones can also be used to detect reinforcement. Reproductive character displacement is seen as a result of reinforcement, so many of the cases in nature express this pattern in sympatry. Reinforcement's ubiquity is unknown, but the patterns of reproductive character displacement are found across numerous taxa and is considered to be a common occurrence in nature. Studies of reinforcement in nature often prove difficult, as alternative explanations for the detected patterns can be asserted. Nevertheless, empirical evidence exists for reinforcement occurring across various taxa and its role in precipitating speciation is conclusive.
Comparative studies.
Assortive mating is expected to increase among sympatric populations experiencing reinforcement. This fact allows for the direct comparison of the strength of prezygotic isolation in sympatry and allopatry between different experiments and studies. Coyne and Orr surveyed 171 species pairs, collecting data on their geographic mode, genetic distance, and strength of both prezygotic and postzygotic isolation; finding that prezygotic isolation was significantly stronger in sympatric pairs, correlating with the ages of the species. Additionally, the strength of post-zygotic isolation was not different between sympatric and allopatric pairs. This finding supports the predictions of speciation by reinforcement and correlates well with a later study that found 33 studies expressing patterns of strong prezygotic isolation in sympatry. A survey of the rates of speciation in fish and their