The separation of a group of individuals from the rest of the population is referred to as

Speciation refers to the genesis of a new species from an ancestral species. There are two basic ways this can happen. Anagenesis involves one species evolving into a different species. Cladogenesis occurs when one species splits into two or more species. Cladogenesis is of greater interest in terms of biodiversity and is the type of speciation discussed here. Speciation has two primary components: diversification and genetic isolation. The two principal types of cladogenesis are allopatric speciation and sympatric speciation.

Answer is Speciation!