How does a geographic barrier contribute to evolutionary change?

A geographic barrier eg mountain range, river, ocean, desert, can split a species into two populations which can no longer mix with each other.

By splitting up a species into two separate populations a geographic barrier can lead to the formation of a new species.

The two separate populations start to develop in isolation from each other. Different mutations will occur in the two populations and natural selection will adapt them to the slightly different conditions in the two areas.

Given enough time the two populations will become so different that if they are brought together again they will no longer be able to interbreed ie they will have become two different species.

This is called allopatric speciation.