Adaptive radiation is a term used in evolutionary biology. It is a process in which organisms diversify rapidly in to new forms. This happens mostly when a change in the environment make new resources available.

An apatemyid is a member of the Apatemyidae family of animals, which took part in the first placental evolutionary radiation together with other early mammals.

Extinction events, such as the Permian extinction and the KT extinction event. You have adaptive radiation driving evolutionary change after such events.

Google " the rise of the mammals. "

Adaptive radiation is one good explanation for rapid evolutionary change. When a dominant taxa goes extinct, such as the dinosaurs, the surviving species/taxa, such as the mammals have many niches that they can now fill, so any variation in individuals against immediate environments can lead to survival and reproductive advantages that translate into rapid allelic changes in populations; rapid evolutionary change.