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Reinforcement is a process of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
where
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charl ...
increases the
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring ...
(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 Hybrid may refer to: Science * Hybrid (biology), an offspring resulting from cross-breeding ** Hybrid grape, grape varieties produced by cross-breeding two ''Vitis'' species ** Hybridity, the property of a hybrid plant which is a union of two dif ...
individuals of low fitness. The idea was originally developed by
Alfred Russel Wallace Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural sele ...
and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from
Theodosius Dobzhansky Theodosius Grigorievich Dobzhansky (russian: Феодо́сий Григо́рьевич Добржа́нский; uk, Теодо́сій Григо́рович Добржа́нський; January 25, 1900 – December 18, 1975) was a prominent ...
. He envisioned a species separated allopatrically, where during
secondary contact Secondary contact is the process in which two allopatricaly distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated the two popul ...
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
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
s). 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 biologist Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life fo ...
s 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 Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar working i ...
, 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
vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...
s and
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
, 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 An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
or the modification of natural
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
s. This has implications for measures of
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') le ...
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 Jerry may refer to: Animals * Jerry (Grand National winner), racehorse, winner of the 1840 Grand National * Jerry (St Leger winner), racehorse, winner of 1824 St Leger Stakes Arts, entertainment, and media * ''Jerry'' (film), a 2006 Indian film ...
and
H. Allen Orr H. Allen Orr (born 1960) is the Shirley Cox Kearns Professor of Biology at the University of Rochester. Education and career Orr earned his bachelor's degree in Biology and Philosophy from the College of William and Mary and his Ph.D. in Biology ...
contend that the theory of reinforcement went through three phases of historical development: # plausibility based on unfit hybrids # implausibility based on hybrids having some fitness # plausibility based on empirical studies and biologically complex and realistic models Sometimes called the Wallace effect, reinforcement was originally proposed by
Alfred Russel Wallace Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural sele ...
in 1889. His hypothesis differed markedly from the modern conception in that it focused on post-zygotic isolation, strengthened by
group selection Group selection is a proposed mechanism of evolution in which natural selection acts at the level of the group, instead of at the level of the individual or gene. Early authors such as V. C. Wynne-Edwards and Konrad Lorenz argued that the behav ...
.
Theodosius Dobzhansky Theodosius Grigorievich Dobzhansky (russian: Феодо́сий Григо́рьевич Добржа́нский; uk, Теодо́сій Григо́рович Добржа́нський; January 25, 1900 – December 18, 1975) was a prominent ...
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 Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...
laid out the first genetic description of the process of reinforcement in ''
The Genetical Theory of Natural Selection ''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher which combines Mendelian genetics with Charles Darwin's theory of natural selection, with Fisher being the first to argue that "Mendelism therefore validates Darwinism" and ...
'', 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 ''Gastrophryne'', the narrowmouth toads (also American narrowmouth toads, North American narrow-mouthed toads), is a genus of microhylid frogs found in the Americas between Honduras and southern United States. Its name means ‘belly-toad’, r ...
'' 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 In vector calculus, divergence is a vector operator that operates on a vector field, producing a scalar field giving the quantity of the vector field's source at each point. More technically, the divergence represents the volume density of t ...
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 The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring ...
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 Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the envir ...
). It necessitates two forces of evolution that act on
mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
:
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charl ...
and
gene flow In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent a ...
. Selection acts as the main driver of reinforcement as it selects against hybrid
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
s 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: *Gene flow between two taxa exists or can be established to have existed at some point. *There is divergence of mating-associated traits between two taxa. *Patterns of mating are modified, limiting the production of low fitness hybrids. *Other selection pressures leading to divergence of the mate-recognition system have not occurred. After speciation by reinforcement occurs, changes after complete reproductive isolation (and further isolation thereafter) are a form of reproductive
character displacement Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not o ...
. 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 Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging g ...
, 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 Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organism ...
, 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 In population genetics, linkage disequilibrium (LD) is the non-random association of alleles at different loci in a given population. Loci are said to be in linkage disequilibrium when the frequency of association of their different alleles is h ...
. Here, selection rejects low fitness Bc or bC
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
combinations while favoring combinations of BC alleles (in the first subpopulation) and bc alleles (in the second subpopulation). The third locus A or a (the assortive mating alleles) have an effect on mating pattern but is not under direct selection. If selection at B and C cause changes in the frequency of allele A, assortive mating is promoted, resulting in reinforcement. Both selection and assortive mating are necessary, that is, that matings of A \times A and a \times a are more common than matings of a \times A and A \times a. 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 A polygene is a member of a group of non-epistatic genes that interact additively to influence a phenotypic trait, thus contributing to multiple-gene inheritance (polygenic inheritance, multigenic inheritance, quantitative inheritance), a type of ...
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 zone A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form ''in situ'' due to the evolution of a new lineage but generally they result from secondary contact ...
s (patchy distributions of parental forms and subpopulations), and in
parapatric In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes. This mode of speciation has three distinguishing characteristics: 1) mating occurs non-randomly, 2) gene ...
populations with narrow contact zones. Population densities are an important factor in reinforcement, often in conjunction with
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
. 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 Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
, as
fertile Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertilit ...
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
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prope ...
s 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 In population genetics, linkage disequilibrium (LD) is the non-random association of alleles at different loci in a given population. Loci are said to be in linkage disequilibrium when the frequency of association of their different alleles is h ...
). The condition of the hybrids under selection can play a role in post-zygotic isolation, as
hybrid inviability Hybrid inviability is a post-zygotic barrier, which reduces a hybrid's capacity to mature into a healthy, fit adult. The relatively low health of these hybrids relative to pure-breed individuals prevents gene flow between species. Thus, hybrid i ...
(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 A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), ...
),
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
responses, the location of breeding grounds, the timing of mating (''e.g.'' seasonal breeding such as in
allochronic speciation Allochronic speciation (also known as allochronic isolation, or temporal isolation) is a form of speciation (specifically ecological speciation) arising from reproductive isolation that occurs due to a change in breeding time that reduces or elimi ...
), or even egg receptivity. Individuals may also discriminate against mates that differ in various traits such as
mating call A mating call is the auditory signal used by animals to attract mates. It can occur in males or females, but literature is abundantly favored toward researching mating calls in females. In addition, mating calls are often the subject of mate choic ...
or
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
. 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 zone A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form ''in situ'' due to the evolution of a new lineage but generally they result from secondary contact ...
s 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 Genetic distance is a measure of the genetic divergence between species or between populations within a species, whether the distance measures time from common ancestor or degree of differentiation. Populations with many similar alleles have s ...
, 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 In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...
. A survey of the rates of speciation in fish and their associated hybrid zones found similar patterns in sympatry, supporting the occurrence of reinforcement.A. R. McCune and N. R. Lovejoy. (1998). The relative rate of sympatric and allopatric speciation in fishes. In D. J. Howard and S. H. Berlocher (eds) ''Endless Forms: Species and Speciation'', Oxford University Press, pp. 172–185.


Laboratory experiments

Laboratory studies that explicitly test for reinforcement are limited, with many of the experiments having been conducted on ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' fruit flies. In general, two types of experiments have been conducted: using artificial selection to mimic natural selection that eliminates the hybrids (often called "destroy-the-hybrids"), and using
disruptive selection Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population ...
to select for a trait (regardless of its function in sexual reproduction). Many experiments using the destroy-the-hybrids technique are generally cited as supportive of reinforcement; however, some researchers such as Coyne and Orr and William R. Rice and Ellen E. Hostert contend that they do not truly model reinforcement, as gene flow is completely restricted between two populations.


Alternative hypotheses

Various alternative explanations for the patterns observed in nature have been proposed. There is no single, overarching signature of reinforcement; however, there are two proposed possibilities: that of sex asymmetry (where females in sympatric populations are forced to become choosy in the face of two differing males) and that of allelic dominance: any of the alleles experiencing selection for isolation should be dominate. Though this signature does not fully account for fixation probabilities or ecological character displacement. Coyne and Orr extend the sex asymmetry signature and contend that, regardless of the change seen in females and males in sympatry, isolation is driven more by females.


Ecological or ethological influences

Ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps w ...
can also play a role in the observed patterns—called ecological character displacement. Natural selection may drive the reduction of an overlap of niches between species instead of acting to reduce hybridization Though one experiment in
stickleback The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more closel ...
fish that explicitly tested this hypotheses found no evidence. Species interactions can also result in reproductive character displacement (in both mate preference or mating signal). Examples include predation and competition pressures, parasites, deceptive
pollination Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, a ...
, and
mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...
. Because these and other factors can result in reproductive character displacement, Conrad J. Hoskin and Megan Higgie give five criteria for reinforcement to be distinguished between ecological and
ethological Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objective ...
influences:
(1) mating traits are identified in the focal species; (2) mating traits are affected by a species interaction, such that selection on mating traits is likely; (3) species interactions differ among populations (present vs. absent, or different species interactions affecting mating traits in each population); (4) mating traits (signal and/or preference) differ among populations due to differences in species interactions; (5) speciation requires showing that mating trait divergence results in complete or near complete sexual isolation among populations. Results will be most informative in a well-resolved biogeographic setting where the relationship and history among populations is known.


Fusion

It is possible that the pattern of enhanced isolation could simply be a temporary outcome of secondary contact where two allopatric species already have a varying range of prezygotic isolation: with some exhibiting more than others. Those that have weaker prezygotic isolation will eventually fuse, losing their distinctiveness. This hypothesis does not explain the fact that individual species in allopatry, experiencing consistent gene flow, would not differ in levels of gene flow upon secondary contact. Furthermore, patterns detected in ''Drosophila'' find high levels of prezygotic isolation in sympatry but not in allopatry. The fusion hypothesis predicts that strong isolation should be found in both allopatry and sympatry. This fusion process is thought to occur in nature, but does not fully explain the patterns found with reinforcement.


Sympatry

It is possible that the process of
sympatric speciation Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organism ...
itself may result in the observed patterns of reinforcement. One method of distinguishing between the two is to construct a phylogenetic history of the species, as the strength of prezygotic isolation between a group of related species should differ according to how they speciated in the past. Two other ways to determine if reinforcement occurs (as opposed to sympatric speciation) are: * if two recently speciated taxa do not show signs of post-zygotic isolation of both sympatric and allopatric populations (in sympatric speciation, post-zygotic isolation is not a prerequisite); * if a cline exists between two species over a range of traits (sympatric speciation does not require a cline to exist at all).


Sexual selection

In a runaway process (not unlike Fisherian runaway selection), selection against the low-fitness hybrids favors assortive mating, increasing mate discrimination rapidly. Additionally, when there is a low cost to female mate preferences, changes in male phenotypes can result, expressing a pattern identical to that of reproductive character displacement. Post-zygotic isolation is not needed, initiated simply by the fact that unfit hybrids cannot get mates.


Arguments against reinforcement

A number of objections were put forth, mainly during the 1980s, arguing that reinforcement is implausible. Most rely on theoretical work which suggested that the antagonism between the forces of natural selection and gene flow were the largest barriers to its feasibility. These objections have since been largely contradicted by evidence from nature.


Gene flow

Concerns about hybrid fitness playing a role in reinforcement has led to objections based on the relationship between selection and recombination. That is, if gene flow is not zero (if hybrids aren't completely unfit), selection cannot drive the fixation of alleles for prezygotic isolation. For example: If population X has the prezygotic isolating allele A and the high fitness, post-zygotic alleles B and C; and population Y has the prezygotic allele a and the high fitness, post-zygotic alleles b and c, both ABC and abc genotypes will experience recombination in the face of gene flow. Somehow, the populations must be maintained. In addition, specific alleles that have the selective advantage within the overlapped populations are only useful within that population.J. A. Moore. (1957). An embryologist's view of the species concept. In Ernst Mayr (eds) ''The Species Problem'', American Association for the Advancement of Science, pp. 325–338. However, if they are selectively advantageous, gene flow should allow the alleles to spread throughout both populations. To prevent this, the alleles would have to be deleterious or neutral. This is not without problems, as gene flow from the presumably large allopatric regions could overwhelm the area when two populations overlap. For reinforcement to work, gene flow must be present, but very limited. Recent studies suggest reinforcement can occur under a wider range of conditions than previously thought and that the effect of gene flow can be overcome by selection. For example, the two species ''Drosophila santomea'' and '' D. yakuba'' on the African island
São Tomé São Tomé is the capital and largest city of the Central African island country of São Tomé and Príncipe. Its name is Portuguese for " Saint Thomas". Founded in the 15th century, it is one of Africa's oldest colonial cities. History Álvar ...
occasionally hybridize with one another, resulting in fertile female offspring and sterile male offspring. This natural setting was reproduced in the laboratory, directly modeling reinforcement: the removal of some hybrids and the allowance of varying levels of gene flow. The results of the experiment strongly suggested that reinforcement works under a variety of conditions, with the evolution of sexual isolation arising in 5–10 fruit fly generations.


Rapid requirements

In conjunction with the fusion hypothesis, reinforcement can be thought of as a race against both fusion and extinction. The production of unfit hybrids is effectively the same as a heterozygote disadvantage; whereby a deviation from genetic equilibrium causes the loss of the unfit allele. This effect would result in the extinction of one of the populations. This objection is overcome by when both populations are not subject to the same ecological conditions. Though, it is still possible for extinction of one population to occur, and has been shown in population simulations. For reinforcement to occur, prezygotic isolation must happen quickly.


References

{{DEFAULTSORT:Speciation Ecology Evolutionary biology concepts Speciation