Buettner-Janusch has defined adaptive radiation as the rapid increase in numbers, and kinds of any evolving group of organisms into several distinctive econiches to which each group is particularly adapted.
Thus, adaptive radiation is the spread of a species by way of evolving into progressively distinct types of organisms. From a single ancestral form different new forms may be evolved to adapt to the rapidly changing environmental conditions.
Reptiles are cold-blooded animals, while the mammals are warm-blooded. During the Paleocene period when the climate grew colder the dinosaurs could not adapt and therefore become extinct. But the mammals evolved into distinct types to adjust to the new environments. Of the mammals, the primates took to the trees and the bats to the air, whales, etc. became adapted to survive in water, the carnivores became specialized for hunting, the rodents evolved to fit for gnawing, the Ungulata order developed organs for grazing and so forth. Each of the mammalian orders, again developed into different forms, each form acquiring a new mode of life to adjust itself to the environment. In this way, the mammals radiated into the different parts of the earth.
The morphological characters of the fossils and comparative studies of the living forms help us to understand adaptive radiation.