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In biogeography, parapatry is the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they only occur together in a narrow contact zone. This geographical distribution is opposed to sympatry (same area) and allopatry or peripatry (two cases of distinct areas).
 Parapatric speciation model
In the parapatric speciation model, the population of a species constitutes one or more biogeographically distinct subpopulations with a small, continuous overlap or minimal contact zone between populations. This minimal contact zone may be the result of unequal dispersal or distribution, incomplete geographical barriers, or divergent expressions of animal behavior, among other things. A parapatric population distribution may result in nonrandom mating and unequal gene flow, which can then produce an increase in the dimorphism between populations. Parapatric speciation is distinct from allopatric speciation (in which an absolute physical barrier is formed between two populations of a species), peripatric speciation (in which a subpopulation of a species enters a new niche that becomes geographically isolated), and sympatric speciation (in which a genetic polymorphism occurs within a continuous population with equal gene flow). In parapatric speciation, there is an intrinsic barrier of nonrandom mating and distinct selection pressures that create unequal gene flow. Parapatric speciation is the culmination of this unequal gene flow effect, in which genotypic dimorphism between populations results in speciation of the population and redefines the population as two or more sister species.
 Environmental gradients
Because of the continuous nature of parapatric population distribution, population niches will often overlap and produce a continuum in the species’ ecological role across an environmental gradient. Whereas in allopatric or peripatric speciation - in which geographically isolated subpopulations may produce discretely separate niches - the reduced gene flow of parapatric speciation will often produce a cline in which a variation in evolutionary pressures causes a change to occur in allele frequencies within the gene pool between populations. This environmental gradient ultimately results in genetically distinct sister species.
 Ring species
Parapatric speciation can occur across a set of neighboring populations in which each can interbreed with closer populations but end populations are too distantly related to interbreed. This population distribution of a species is referred to as a ring species. When an ancestral population extends its range around a geographical barrier and differentiates (despite low-level gene flow), reproductive isolation occurs between terminal populations. This results in technically continuous populations that undergo parapatric speciation; fertility is negatively correlated with distance between populations, and terminal populations between ring species become distinct sister species.
In the Martinique's Anole, quantitative trait variation in dewlap and dorsum color, color pattern, body proportions and scalation demonstrated nascent parapatric speciation between adjacent sets of populations. Large quantitative trait divergences were observed with association differences in habitat rather than geographic allopatry.
In the grass Anthoxanthum, mine contamination in an area has caused the species to undergo parapatric speciation by creating a selection pressure for tolerance of metal contaminants. This changes the flowering time of the grass and creates strong selection against interbreeding, causing many plants in the population to become self-pollinating.
In the Gyrinophilus Tennessee Cave Salamander, qualitative timing of migration was used to infer the differences in gene flow 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.
In Ephedra gymnosperms of North American, Bayesian inference of DNA molecular markers is used to estimate divergences of sister species. Ecological niche models are generated for parapatric and sympatric sister species, resulting in evidence for parapatric niche divergence for the sister species pairs E. californica and E. trifurca.
 See also
- allopatry, allopatric speciation
- peripatry, peripatric speciation
- sympatry, sympatric speciation
- Adaptive radiation
- Evolutionary radiation
- The University of California Museum of Paleontology, Berkeley (2006) - Parapatric speciation
- Bank, C., Burger, R., & Hermisson, J. (2012). The limits to parapatric speciation: dobzhansky-muller incompatibilities in a continent-island model. Genetics, 191(3), 845-863.
- Doebeli, M., & Dieckmann, U. (2003). Speciation along environmental gradients. Nature 421, 259-264.
- Monahan, W., Pereira, R., & Wake, D. (2012) Ring distributions leading to species formation: a global topographic analysis of geographic barriers associated with ring species. BMC Biology, 10, 1-22
- Thorpe, R., Surget-Groba, Y., & Johansson, H. (2012). Quantitative traits and mode of speciation in Martinique anoles. Molecular Ecology, 21(21), 5299-5308.
- Antonovics, J. (2006). Evolution in closely adjacent plant populations X: long term persistence of prereproductive isolation at a mine boundary. Heredity, 97(1), 33-37..
- Niemiller, M., Fitzpatrick, B., & Miller, B. (2008). Recent divergence with gene flow in Tennessee cave salamanders (plethodontidae: gyrinophilus) inferred from gene geneaologies. Molecular Ecology, 17(9), 2258-2275.
- Loera, I., Victoria, S., & Ickert-Bond, S. (2012). Diversification in north american arid lands: niche conservatism, divergence and expansion of habitat explain speciation in the genus ephedra. Molecular Phylogenetics and Evolution, 65(2), 437-450.