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. This means that the populations of species live independently of each other long enough that they can no longer produce offspring that can survive. 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 (see parapatric speciation). The genetic differences among divergent populations can involve silent mutations (that have no effect on the phenotype) or give rise to significant morphological and/or physiological changes.
On a molecular genetics level, genetic divergence is due to changes in a small number of genes in a species, resulting in speciation. However, researchers argue that it is unlikely that divergence is a result of a significant, single, dominant mutation in a genetic locus because if that were so, the individual with that mutation would have zero fitness. Consequently, they could not reproduce and pass the mutation on to further generations. Hence, it is more likely that divergence, and subsequently reproductive isolation, are the outcome of multiple small mutations over evolutionary time.