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In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges and opens environmental niches. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits with which they can exploit a range of divergent environments.
Adaptive radiation, a characteristic example of cladogenesis, can be graphically illustrated as a "bush", or clade, of coexisting species (on the tree of life).  Caribbean anoline lizards are a particularly interesting example of an adaptive radiation.
Four features can be used to identify an adaptive radiation:
- A 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.
- A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.
- Trait utility: the performance or fitness advantages of trait values in their corresponding environments.
- Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.
The evolution of a novel feature may permit a clade to diversify by making new areas of morphospace accessible. A classic example is the evolution of a fourth cusp in the mammalian tooth. This trait permits a vast increase in the range of foodstuffs which can be fed on. Evolution of this character has thus increased the number of ecological niches available to mammals. The trait arose a number of times in different groups during the Cenozoic, and in each instance was immediately followed by an adaptive radiation. Birds find other ways to provide for each other, i.e. the evolution of flight opened new avenues for evolution to explore, initiating an adaptive radiation. Other examples include placental gestation (for eutherian mammals), or bipedal locomotion (in hominins).
Adaptive radiations often occur as a result of an organism arising in an environment with unoccupied niches, such as a newly formed lake or isolated island chain. The colonizing population may diversify rapidly to take advantage of all possible niches.
Adaptive radiations commonly follow mass extinctions: following an extinction, many niches are left vacant. A classic example of this is the replacement of the non-avian dinosaurs with mammals at the end of the Cretaceous, and of brachiopods by bivalves at the Permo-Triassic boundary.
- Cambrian explosion—the most famous evolutionary radiation
- Evolutionary radiation—a more general term to describe any radiation
- List of adaptive radiated Hawaiian honeycreepers by form
- List of adaptive radiated marsupials by form
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