What is the term for the rapid evolution of many new species from a single ancestor?

The term for the rapid evolution of many new species from a single ancestor is adaptive radiation. It occurs when descendants diversify into different ecological niches, often after an environmental change creates new opportunities or reduces competition.

A classic example is Darwin’s finches in the Galápagos Islands. A common ancestral finch population gave rise to multiple species with differently shaped beaks, each suited to particular foods. Similar radiations have occurred among Hawaiian plants, African cichlid fishes, and mammals after the extinction of competing groups.

Adaptive radiation is more specific than ordinary speciation: it combines common ancestry, ecological differences, useful inherited traits, and relatively rapid diversification. It is also different from convergent evolution, in which unrelated organisms independently evolve similar traits. The word “adaptive” refers to the relationship between evolved features and environmental opportunities, not merely to any increase in species number.