Real Dictionary

adaptive radiation in American English
the dispersal of, and adaptation to new environments by, a line of animals or plants, resulting in the evolution of divergent forms, often new species, specialized for surviving in the new habitats
noun: the diversification of an ancestral group of organisms into a variety of related forms specialized to fit different environments or ways of life, each often further diversifying into more specialized types

adaptive radiation in British English
noun: evolution of a number of divergent species from a common ancestor, each species becoming adapted to occupy a different environment. This type of evolution occurred in the Tertiary manuals and the Mesozoic reptiles