Adaptive radiation

Adaptive radiation is a burst of evolution in which a single ancestral lineage splits into many descendant species that spread into different niches and evolve different adaptations to fill them. The word radiation describes the pattern on a family tree, with many branches fanning out from one point. The best-known examples come from islands: the Galapagos finches, whose beaks became suited to seeds, insects or cactus flowers, and the Hawaiian honeycreepers, which evolved from one founding bird species into dozens with very different bills.

Radiations are usually set off by opportunity. Colonists that reach remote islands or new lakes find empty niches and few competitors. A key adaptation, such as the ability to feed on flowering plants, can open many new ways of life at once. Mass extinctions also clear space: after the non-bird dinosaurs died out about 66 million years ago, mammals diversified into forms as different as bats, whales and horses. Natural selection in each new setting, often combined with geographic isolation, drives the populations apart until they become separate species.

The cichlid fishes of the great lakes of East Africa, with hundreds of species that arose in a geologically short time, are a modern textbook case.