Speciation is the splitting of one species into two distinct species. The biological species concept defines a species as a group that can interbreed and produce fertile offspring under natural conditions, while being reproductively isolated from other groups. Reproductive isolation occurs when two populations diverge genetically enough that their offspring are infertile or inviable. Allopatric speciation, where populations are isolated geographically, is the most common speciation mechanism. Over time, separated populations accumulate genetic differences due to random genetic drift and independent evolution, eventually becoming reproductively isolated.

3. Genetic Divergence