Adaptive Radiation

Adaptive radiation is the evolutionary pattern by which a single ancestral species diversifies into an array of descendant species, each adapted to exploit different resources or habitats. The classic examples — Darwin's finches on the Galápagos, the Hawaiian honeycreepers, the Cambrian explosion of animal body plans — share a common structure: a founding population enters an environment with vacant ecological space (either because competitors have been eliminated or because the population has colonized a previously uninhabited region), and selection rapidly diversifies the founding genotype into forms occupying the available niches. The rapidity is proportional to the ecological vacancy and the evolvability of the founding lineage. Radiations are most spectacular following mass extinctions, when the elimination of incumbent species creates opportunities that surviving lineages rapidly fill. The post-Cretaceous mammalian radiation is the canonical case: small, nocturnal, ecologically marginal mammals, having survived the asteroid impact that eliminated non-avian dinosaurs, diversified within ten million years into whales, bats, primates, elephants, and thousands of other forms occupying niches the Mesozoic configuration could not have produced.