The lava released by a volcanic eruption separates two populations of a species of monkey. what does this represent?

Geographical Isolation

Explanation:

Geographic isolation consists of separating a population by a geographical barrier, thus forming subpopulations. The geographical barrier can be a river, a mountain or a volcanic eruption, as shown in the question above.

As a result of geographic isolation, subpopulations begin to suffer different pressures and consequently the genes selected in one subpopulation will be different from the other subpopulation. Due to the geographical barrier, the two subpopulations will be prevented from crossing and the differences between them will become increasingly pronounced. Thus arose the subspecies.