In general, geographical isolation occurs when

A) populations migrate to new environments.
B) gene flow is reduced by more than 75 percent due to climate change.
C) mutations overcome genetic drift across physical barriers.
D) populations are separated by a distance that is great enough to limit gene flow.

Explanation:

Geographical isolation is the physical separation of two populations by geographic barriers (like by mountains, rivers, hills etc). In other words, Geographical isolation is when, organisms that are separated from exchanging their genetic material with other organisms of the same species ,

Geographical isolation occurs by adaptive radiation and allopatric speciation

As generally, geographical isolation occurs when populations are separated from each other by a distance that is great enough to limit gene flow.

layer 3 is younger than layer 6.