Adaptive radiation



An adaptive radiation is a rapid evolutionary radiation characterised by an increase in the morphological and ecological diversity of a single, rapidly diversifying lineage. Phenotypes adapt in response to the environment, with new and useful traits arising. This is an evolutionary process driven by natural selection.

Innovation
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 utilised, with species able to specialise on feeding on a range of foodstuffs. 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 provide another example; the evolution of flight opened new avenues for evolution to explore, initiating an adaptive radiation.

Opportunity
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 colonising population may diversify rapidly to take advantage of all possible niches.

In Lake Victoria, an isolated lake which formed recently in the African rift valley, over 300 species of cichlid fish adaptively radiated from one parent species in just 15,000 years.

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..