A physical barrier separates a single species, causing two separate populations to form. Over time these two populations adapt to their environments. Eventually, these two populations are no longer able to successfully reproduce with each other. Such species are called
When members of a species are separated from each other geographically by a waterbody or a mountain range, preventing them from interbreeding, they develop new adaptations to the demands of their unique habitats. Such species are called allopatric species. Here a species separates into two groups
When a small group of individuals of a species breaks off from a larger group of species to form new species by developing new characteristics, The species are called peripatric speciation.
When members of a species spread over a large geographical area, develop new characteristics and form a species, they are called parapatric species. Here both the new and old group are present in the same geographical area but are exposed to different environments so have different habitats. Pollution in an area could cause the development of parapatric species.
Two species that exist in the same habitat and arose from the same common ancestors are called sympatric species.