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. Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... In biology, a species is one of the basic units of biodiversity. ... Allopatric speciation (also known as Allopatry) is speciation by geographical isolation. ... Sympatry is one of three theoretical models for the phenomenon of speciation. ...

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. An abstract model (or conceptual model) is a theoretical construct that represents physical, biological or social processes, with a set of variables and a set of logical and quantitative relationships between them. ... An environmental gradient is a gradual and continuous change in communities and environmental condition. ...