How does speciation occur when populations become genetically isolated?

Speciation is the process by which a single evolutionary lineage splits into two or more genetically independent species. This process requires the interruption of gene flow between populations, which prevents them from sharing a common gene pool and allows them to diverge.12

When populations become genetically isolated, speciation typically follows two stages:

• First Stage (Genetic Divergence): As populations adapt to different environments or experience random genetic drift, their gene pools become increasingly distinct. This divergence often results in "incidental" reproductive isolation as a by-product; because the populations are no longer adapting in concert, any resulting hybrids may suffer from reduced viability or fertility (postzygotic reproductive isolating mechanisms).23
• Second Stage (Natural Selection): If the two populations come into contact and produce hybrids with low fitness, natural selection may directly promote the evolution of prezygotic mechanisms—such as behavioral or temporal differences—that prevent hybridization entirely.2

Isolation can be initiated by geographic barriers (allopatric speciation) or by genetic or ecological shifts within the same territory (sympatric speciation).12