Speciation describes the evolutionary process in which a population splits into reproductively isolated groups. Once gene flow between the groups is sufficiently limited, genetic differences can accumulate until they form separate species.
Speciation can occur through geographic separation, ecological differences, changes in mating behavior, or chromosome changes. Reproductive isolation may prevent mating, fertilization, or the production of fertile offspring.
Allopatric speciation involves geographic separation, while sympatric speciation occurs without a physical barrier. Speciation is not instantaneous in every case; some lineages remain partly compatible for long periods while their differences develop.