The term for the evolution of new species from a common ancestor is speciation. It occurs when populations become genetically distinct enough that they form separate evolutionary lineages, usually through the development of reproductive isolation.
Speciation can begin when populations are separated geographically. This is called allopatric speciation and may occur when a mountain range, river, island, or other barrier interrupts gene flow. Other forms include sympatric speciation, which occurs in the same geographic area, and parapatric or peripatric speciation, which involve more limited forms of geographic separation.
Natural selection, genetic drift, sexual selection, mutation, and changes in chromosome number can all contribute. In plants, polyploidy can sometimes create reproductive isolation in a single generation. Hybridization can also produce a new lineage when the hybrid population remains reproductively separate and is favored by its environment.
Mutation is not synonymous with speciation: mutations supply genetic variation, while speciation describes the broader population-level process that results in distinct species. Adaptation can occur without producing a new species, and migration generally increases gene flow rather than dividing lineages.