Geographic isolation is when a group of living things gets physically separated from others of the same species by a barrier like a mountain range, river, ocean, desert, or simply long distance, so they can no longer interbreed or mix their genes easily. Over many generations, the separated groups adapt to their own environments, their genes and traits gradually diverge, and they can eventually become so different that they form new species.

A simple way to picture it: imagine one population of animals split in two when a river changes course and cuts through their habitat. Each side now faces slightly different food, predators, and climate, so natural selection pushes them in different directions until they are no longer alike enough to successfully mate.