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In parapatric speciation, two subpopulations of a species evolve
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 offsprin ...
from one another while continuing to exchange
genes In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ...
. This mode 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 with ...
has three distinguishing characteristics: 1) mating occurs non-randomly, 2)
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 ...
occurs unequally, and 3) populations exist in either continuous or discontinuous geographic ranges. This distribution pattern may be the result of unequal dispersal, incomplete geographical barriers, or divergent expressions of behavior, among other things. Parapatric speciation predicts that hybrid zones will often exist at the junction between the two populations. In
biogeography Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevati ...
, the terms parapatric and parapatry are often used to describe the relationship between
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
s whose ranges do not significantly overlap but are immediately adjacent to each other; they do not occur together except in a narrow contact zone. Parapatry is a geographical distribution opposed to
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 s ...
(same area) and
allopatry Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
or
peripatry 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 ...
(two similar cases of distinct areas). Various "forms" of parapatry have been proposed and are discussed below. Coyne and Orr in ''Speciation'' categorise these forms into three groups: clinal (environmental gradients), "stepping-stone" (discrete populations), and stasipatric speciation in concordance with most of the parapatric speciation literature. Henceforth, the models are subdivided following a similar format.
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
was the first to propose this mode of speciation. It was not until 1930, when
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 ...
published ''
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 ...
'' where he outlined a verbal theoretical model of clinal speciation. In 1981,
Joseph Felsenstein Joseph "Joe" Felsenstein (born May 9, 1942) is a Professor Emeritus in the Departments of Genome Sciences and Biology at the University of Washington in Seattle. He is best known for his work on phylogenetic inference, and is the author of ''Inf ...
proposed an alternative, "discrete population" model (the "stepping-stone model). Since Darwin, a great deal of research has been conducted on parapatric speciation—concluding that its mechanisms are theoretically plausible, "and has most certainly occurred in nature".

# Models

Mathematical models, laboratory studies, and observational evidence supports the existence of parapatric speciation's occurrence in nature. The qualities of parapatry imply a partial extrinsic barrier during divergence; thus leading to a difficulty in determining whether this mode of speciation actually occurred, or if an alternative mode (notably,
allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
) can explain the data. This problem poses the unanswered question as to its overall frequency in nature. Parapatric speciation can be understood as a level of gene flow between populations where $m=0$ in allopatry (and peripatry), $m=0.5$ in sympatry, and midway between the two in parapatry. Intrinsic to this, parapatry covers the entire continuum; represented as $0 < m < 0.5$. Some biologists reject this delineation, advocating the disuse of the term "parapatric" outright, "because many different spatial distributions can result in intermediate levels of gene flow". Others champion this position and suggest the abandonment of geographic classification schemes (geographic modes of speciation) altogether.
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 ...
has been shown to be the primary driver in parapatric speciation (among other modes), and the strength of selection during divergence is often an important factor. Parapatric speciation may also result from reproductive isolation caused by
social selection Social selection is a term used with varying meanings in biology. Joan Roughgarden proposed a hypothesis called ''social selection'' as an alternative to sexual selection. Social selection is argued to be a mode of natural selection based on r ...
: individuals interacting altruistically.

Due to the continuous nature of a parapatric population distribution, population niches will often overlap, producing a continuum in the species' ecological role across an environmental gradient. Whereas in allopatric or peripatric speciation—in which geographically isolated populations may evolve reproductive isolation without gene flow—the reduced gene flow of parapatric speciation will often produce a
cline Cline may refer to: Science * Cline (biology), a measurable gradient in a single trait in a species across its geographical range * Cline (hydrology), a fluid layer with a property that varies * Cline (mathematics) or generalised circle, a cir ...
in which a variation in evolutionary pressures causes a change to occur in allele frequencies within the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
between populations. This environmental gradient ultimately results in genetically distinct sister species. Fisher's original conception of clinal speciation relied on (unlike most modern speciation research) the morphological species concept. With this interpretation, his verbal, theoretical model ''can'' effectively produce a new species; of which was subsequently confirmed mathematically. Further mathematical models have been developed to demonstrate the possibility of clinal speciation with most relying on, what Coyne and Orr assert are, "assumptions that are either restrictive or biologically unrealistic". A mathematical model for clinal speciation was developed by Caisse and Antonovics that found evidence that, "both genetic divergence and reproductive isolation may therefore occur between populations connected by gene flow". This research supports clinal isolation comparable to a ring species (discussed below), except that the terminal geographic ends do not meet to form a ring. Doebeli and Dieckmann developed a mathematical model that suggested that
ecological 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 ...
contact is an important factor in parapatric speciation and that, despite gene flow acting as a barrier to divergence in the local population,
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 populati ...
drives assortative mating; eventually leading to a complete reduction in gene flow. This model resembles
reinforcement In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher fr ...
with the exception that there is never a secondary contact event. The authors conclude that, "spatially localized interactions along environmental gradients can facilitate speciation through
frequency-dependent selection Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population. * In positive frequency-dependent selection, the fitness of a phenotyp ...
and result in patterns of geographical segregation between the emerging species." However, one study by Polechová and Barton disputes these conclusions.

### Ring species

The concept of a ring species is associated with allopatric speciation as a special case; however, Coyne and Orr argue that Mayr's original conception of a ring species does not describe allopatric speciation, "but speciation occurring through the attenuation of gene flow with distance". They contend that ring species provide evidence of parapatric speciation in a non-conventional sense. They go on to conclude that:
Nevertheless, ring species are more convincing than cases of clinal isolation for showing that gene flow hampers the evolution of reproductive isolation. In clinal isolation, one can argue that reproductive isolation was caused by environmental differences that increase with distance between populations. One cannot make a similar argument for ring species because the most reproductively isolated populations occur in the ''same'' habitat.

## Discrete populations

Referred to as a "stepping-stone" model by Coyne and Orr, it differs by virtue of the species population distribution pattern. Populations in discrete groups undoubtedly speciate more easily than those in a cline due to more limited gene flow. This allows for a population to evolve reproductive isolation as either
selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strat ...
or
drift Drift or Drifts may refer to: Geography * Drift or ford (crossing) of a river * Drift, Kentucky, unincorporated community in the United States * In Cornwall, England: ** Drift, Cornwall, village ** Drift Reservoir, associated with the village ...
overpower gene flow between the populations. The smaller the discrete population, the species will likely undergo a higher rate of parapatric speciation. Several mathematical models have been developed to test whether this form of parapatric speciation can occur, providing theoretical possibility and supporting biological plausibility (dependent on the models parameters and their concordance with nature).
Joseph Felsenstein Joseph "Joe" Felsenstein (born May 9, 1942) is a Professor Emeritus in the Departments of Genome Sciences and Biology at the University of Washington in Seattle. He is best known for his work on phylogenetic inference, and is the author of ''Inf ...
was the first to develop a working model. Later, Sergey Gavrilets and colleagues developed numerous analytical and dynamical models of parapatric speciation that have contributed significantly to the quantitative study of speciation. (See the "Further reading" section)

### Para-allopatric speciation

Further concepts developed by Barton and Hewitt in studying 170
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 conta ...
s, suggested that parapatric speciation can result from the same components that cause allopatric speciation. Called para-allopatric speciation, populations begin diverging parapatrically, fully speciating only after allopatry.

## Stasipatric models

One variation of parapatric speciation involves species chromosomal differences. Michael J. D. White developed the stasipatric speciation model when studying Australian morabine grasshoppers ('' Vandiemenella''). The chromosomal structure of sub-populations of a widespread species become underdominate; leading to fixation. Subsequently, the sub-populations expand within the species larger range, hybridizing (with sterility of the
offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ...
) in narrow hybrid zones. Futuyama and Mayer contend that this form of parapatric speciation is untenable and that
chromosomal rearrangement In genetics, a chromosomal rearrangement is a mutation that is a type of chromosome abnormality involving a change in the structure of the native chromosome. Such changes may involve several different classes of events, like deletions, duplicati ...
s are unlikely to cause speciation. Nevertheless, data does support that chromosomal rearrangements can possibly lead to reproductive isolation, but it does not mean speciation results as a consequence.

# Evidence

## Laboratory evidence

Very few laboratory studies have been conducted that explicitly test for parapatric speciation. However, research concerning
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 organis ...
often lends support to the occurrence of parapatry. This is due to the fact that, in symaptric speciation, gene flow within a population is unrestricted; whereas in parapatric speciation, gene flow is limited—thus allowing reproductive isolation to evolve easier. Ödeen and
Florin The Florentine florin was a gold coin struck from 1252 to 1533 with no significant change in its design or metal content standard during that time. It had 54 grains (3.499 grams, 0.113 troy ounce) of nominally pure or 'fine' gold with a purch ...
complied 63 laboratory experiments conducted between the years 1950–2000 (many of which were discussed by
Rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly '' Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesti ...
and Hostert previously) concerning sympatric and parapatric speciation. They contend that the laboratory evidence is more robust than often suggested, citing laboratory populations sizes as the primary shortcoming.

## Observational evidence

Parapatric speciation is very difficult to observe in nature. This is due to one primary factor: patterns of parapatry can easily be explained by an alternate mode of speciation. Particularly, documenting closely related species sharing common boundaries does not imply that parapatric speciation was the mode that created this geographic distribution pattern. Coyne and Orr assert that the most convincing evidence of parapatric speciation comes in two forms. This is described by the following criteria: * ''Species populations that join, forming an
ecotone An ecotone is a transition area between two biological communities, where two communities meet and integrate. It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and gras ...
can be interpreted as convincingly forming in parapatry if'': ** No evidence exists for a period of geographic separation between two closely related species ** Different loci are not in agreement along the cline **
Phylogenies A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
including sister groups support different divergence times * ''An
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...
species that exists within a specialized habitat next to its sister species that does not reside in the specialized habitat strongly suggests parapatric speciation.'' This has been exemplified by the grass species '' Agrostis tenuis'' that grows on soil contaminated with high levels of copper leeched from an unused mine. Adjacent is the non-contaminated soil. The populations are evolving reproductive isolation due to differences in flowering. The same phenomenon has been found in '' Anthoxanthum odoratum'' in lead and zinc contaminated soils. Speciation may be caused by allochrony.
Clines Clines is a surname. Notable people with the surname include: *Gene Clines Eugene Anthony Clines (October 6, 1946 – January 27, 2022) was an American professional baseball Professional baseball is organized baseball Basebal ...
are often cited as evidence of parapatric speciation and numerous examples have been documented to exist in nature; many of which contain hybrid zones. These clinal patterns, however, can also often be explained by allopatric speciation followed by a period of secondary contact—causing difficulty for researchers attempting to determine their origin. Thomas B. Smith and colleagues posit that large ecotones are "centers for speciation" (implying parapatric speciation) and are involved in the production 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'') ...
in tropical rainforests. They cite patterns of morphologic and genetic divergence of the
passerine A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped'), which includes more than half of all bird species. Sometimes known as perching birds, passerines are distinguished from other orders of birds by t ...
species '' Andropadus virens''. Jiggins and
Mallet A mallet is a tool used for imparting force on another object, often made of rubber or sometimes wood, that is smaller than a maul or beetle, and usually has a relatively large head. The term is descriptive of the overall size and propor ...
surveyed a range of literature documenting every phase of parapatric speciation in nature positing that it is both ''possible'' and ''likely'' (in the studied species discussed). A study of tropical cave snails ('' Georissa saulae'') found that cave-dwelling population descended from the above-ground population, likely speciating in parapatry. '' Partula'' snails on the island of
Mo'orea Moorea ( or ; Tahitian: ), also spelled Moorea, is a volcanic island in French Polynesia. It is one of the Windward Islands, a group that is part of the Society Islands, northwest of Tahiti. The name comes from the Tahitian word , meaning ...
have parapatrically speciated ''in situ'' after a single or a few colonization events, with some species expressing patterns of ring species. In the Tennessee cave salamander, timing of migration was used to infer the differences in
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 ...
between cave-dwelling and surface-dwelling continuous populations. Concentrated gene flow and mean migration time results inferred a heterogenetic distribution and continuous parapatric speciation between populations. Researchers studying '' Ephedra'', a genus of
gymnosperms The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, ''Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμνό ...
in North American, found evidence of parapatric niche divergence for the sister species pairs ''E. californica'' and ''E. trifurca''. One study of Caucasian rock lizards suggested that habitat differences may be more important in the development of reproductive isolation than isolation time. ''
Darevskia ''Darevskia'' is a genus of wall lizards of the family Family (from la, familia) is a group of people related either by consanguinity Consanguinity ("blood relation", from Latin Latin (, or , ) is a classical language belonging t ...
rudis'', ''D. valentini'' and ''D. portschinskii'' all hybridize with each other in their
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 conta ...
; however, hybridization is stronger between ''D. portschinskii'' and ''D. rudis'', which separated earlier but live in similar habitats than between ''D. valentini'' and two other species, which separated later but live in climatically different habitats.

### Marine organisms

It is widely thought that parapatric speciation is far more common in oceanic species due to the low probability of the presence of full geographic barriers (required in allopatry). Numerous studies conducted have documented parapatric speciation in marine organisms. Bernd Kramer and colleagues found evidence of parapatric speciation in
Mormyrid The Mormyridae, sometimes called "elephantfish" (more properly freshwater elephantfish), are a family of weakly electric freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 2 ...
fish ('' Pollimyrus castelnaui''); whereas Rocha and Bowen contend that parapatric speciation is the primary mode among coral-reef fish. Evidence for a clinal model of parapatric speciation was found to occur in Salpidae. Nancy Knowlton found numerous examples of parapatry in a large survey of marine organisms.