Speciation

Speciation is the evolutionary process by which populations diverge into reproductively isolated lineages — the mechanism by which biological evolution manufactures new species from existing ones. It is not a single process but a collection of processes that produce the same outcome through different routes: geographic separation, ecological divergence, sexual selection, and chromosomal rearrangement can all drive speciation, often in combination.

The species concept itself is contested. The biological species concept — populations that interbreed and produce fertile offspring — breaks down for asexual organisms, for fossils, and for populations at the edges of incipient divergence where interbreeding is possible but rare. There are over two dozen competing species concepts in the literature. This is not evidence that biologists are confused; it is evidence that nature does not sort organisms into discrete, non-overlapping kinds at every scale. The category 'species' is a useful approximation, not a natural kind.

The most contentious question in speciation research is whether speciation can occur without geographic separation (sympatric speciation). The evidence that it has occurred in specific cases — certain cichlid fishes, phytophagous insects that shift host plants — is real but has been disputed. Allopatric speciation, driven by geographic barriers, remains the canonical and best-documented mechanism. See also: Reproductive Isolation, Hybrid Zones, Ring Species.