Geographical isolation occurs when members of one species living in a particular area become physically separated from each other. The immediate result is the formation of two separate populations of the species. The long-term result can be speciation, the evolution of two separate species.

Organisms are geographically isolated by physical barriers, such as mountains, rivers, oceans, islands, forests or deserts. These physical barriers prevent organisms from intermingling and interbreeding with one another. For example, part of a population of field mice might be separated from the rest of the population during a period of heavy rain, when a creek forms in the middle of the field from rainwater runoff. If the creek becomes permanent and turns into a river, the mice on one side would be geographically isolated from those on the other side. Organisms with greater mobility, such as birds and some types of fish...