The situation of populations being separated by barriers like rivers, mountains, or bodies of water is called isolation.

This isolation can lead to:

* Geographic isolation: When populations are physically separated by a barrier, preventing gene flow between them.

* Reproductive isolation: When populations evolve to the point where they can no longer interbreed, even if they are no longer physically separated.

Isolation is a key factor in speciation, the process by which new species evolve.