What is the definition of a geographical barrier?

A geographical barrier can be defined as a naturally occurring formation or body of water that segregates and isolates an area. Common geographic barriers are mountains, glaciers, islands, lakes, oceans, rivers and canyons.

A geographical barrier can cause allopatric speciation. This occurs when a geographical barrier isolates a species and causes it to evolve into a new species. The population of a species becomes separated by the geographical barrier, forming two groups. The group that is isolated by the barrier can not mate with the other group, which causes the gene pool of the isolated species to become significantly smaller, leading to the formation of a brand new species.