evolution of an animal or plant group into a wide variety of types adapted to specialized modes of life. Adaptive radiations are best exemplified in closely related groups that have evolved in a relatively short time. A striking example is the radiation, beginning in the Tertiary Period (beginning 65.5 million years ago), of basal mammalian stock into forms adapted to running, leaping, climbing, swimming, and flying. Other examples include Australian marsupials, cichlid fish, and Darwin's finches (also known as Galapagos finches).