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.
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