The accumulation of different mutations in genetically separated populations is known as and is the key to speciation.

Genetic Divergence

Explanation:

Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes (mutations) through time, often after the populations have become reproductively isolated for some period of time. In some cases, subpopulations living in ecologically distinct peripheral environments can exhibit genetic divergence from the remainder of a population, especially where the range of a population is very large

(I got this from reading through some articles online, but it's expressly explained in Wikipedia and POSTING LINKS is beyond my jurisdiction as a law abiding tutor but I am sure that helps as well)