Speciation

Speciation is the evolutionary process by which populations diverge into distinct species, a term the biologist Orator F. Cook coined in 1906 for the splitting of one evolutionary lineage into two, as distinct from anagenesis, in which a single lineage simply changes over time. Its main geographic modes are allopatric speciation, in which a population splits into geographically separated groups that evolve independently until they can no longer interbreed if they meet again; peripatric speciation, a form of allopatric speciation in a small, isolated population in which genetic drift plays a large role; parapatric speciation, in which diverging populations stay partly in contact but hybrids are less fit, driving the populations apart; and sympatric speciation, in which new species arise from a single ancestral species within the same location, as in the cichlid fishes of East Africa. In every mode the decisive factor is reproductive isolation, the point at which formerly interbreeding populations can no longer merge back into one.