The organisms separated by geographical barriers are termed

Allopatric speciation is a type of speciation that occurs when population of a species become geographically isolated. When a population become separated by the formation of a new branch of river or soil erosion or mountain range, rockslide gene flows between them ceases. Over time, the populations becomes genetically different in response to the natural selection imposed by their different environments. Hence, organisms of same ancestral species evolve into two different species after a period of physical separation.