Parapatry
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Parapatry is a type of speciation where the ranges of two species abut but do not overlap. The two species often have contact at the common boundary of their zones, and speciation occurs in partial, but not complete, isolation. It contrasts with allopatry where different species are geographically isolated from each other, and sympatry where different species evolve in the same area.
One common model of parapatric speciation is the clinal model. In clinal models, populations diverge along an environmental gradient. The populations at the ends of the gradient can become locally adapted to their environments, and this can lead to speciation. Although parapatric speciation is theoretically plausible, it is thought to be much less common than allopatric speciation in nature.