Adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available or alters biotic interactions. Here are some key features of adaptive radiation:
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Common ancestry of component species: Specifically, a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.
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Phenotype-environment correlation: A significant association between environments and the morphological and physiological traits used to exploit those environments.
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Triggering factors: Adaptive radiations are thought to be triggered by an ecological opportunity or a new adaptive zone. Sources of ecological opportunity can be the loss of antagonists (competitors or predators), the evolution of a key innovation, or dispersal to a new environment. Any one of these ecological opportunities has the potential to result in an increase in population size and relaxed selection, which can lead to the evolution of new traits and the formation of new species.
Adaptive radiations are best exemplified in closely related groups that have evolved in a relatively short time. Examples of adaptive radiation include the radiation of basal mammalian stock into forms adapted to running, leaping, climbing, swimming, and flying, the diversification of Australian marsupials from a single ancestral stock in the Australian subcontinent, and the development of different finch species on the Galapagos Islands.
