Speciation

Speciation is the splitting of one evolutionary lineage into two or more separate species. Under the biological species concept, a species is a group whose members can interbreed and produce fertile offspring, so speciation is complete when populations become reproductively isolated and genes no longer flow between them. Each branch point on a phylogenetic tree marks a speciation event.

Isolating barriers act before or after fertilization. Prezygotic barriers prevent mating or fertilization, for example when two species breed at different times, live in different habitats, ignore each other's courtship signals or have incompatible gametes. Postzygotic barriers act afterward, when hybrids die early or, like mules, survive but are sterile. In allopatric speciation, a geographic barrier such as a river, mountain range or stretch of ocean divides a population, and the separated groups diverge through natural selection, genetic drift and mutation. In sympatric speciation, new species arise in the same area without a physical barrier; in plants this often happens through polyploidy, an error in cell division that gives offspring extra sets of chromosomes.

When one ancestor colonizes many different habitats, it may split into many species in a burst called adaptive radiation, as in Darwin's finches and Hawaiian honeycreepers.