The major effect of a species' adaptive radiation is a dramatic increase in the diversity of the population gene pool. This increased diversity can lead, in time, to a dramatic increase in the number of distinct species in an environment, explains Understanding Evolution.
The concept of adaptive radiation is not precisely defined, and biologists disagree over the length of time required to designate an "official" radiation event, but the central idea is that of a rapid diversification of a single clade into numerous diverse forms, according to Understanding Evolution. Such adaptive radiations usually happen when a relatively homogeneous population is introduced to a new environment with unique pressures, such as new predators or a novel food supply that the organisms have to adapt to fully exploit.
Adaptive radiation can also occur when the species' present environment is cleared of competitors for the available resources, notes Understanding Evolution. Such an event is believed to have transpired at the end of the Mesozoic Era, when non-avian dinosaurs went extinct and left behind a large variety of empty ecological niches. The effect was to create an environment in which mammals were free to diversify and greatly expand their range. Within just 20 million years, all of the modern classes of mammal were clearly distinguishable in their new environments.