adaptive radiation

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Four of the 14 finch species found on the Galápagos Archipelago, are thought to have evolved by an adaptive radiation that diversified their beak shapes to adapt them to different food sources.

An adaptive radiation is a rapid evolutionary radiation characterized 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.^[1] This is an evolutionary process driven by natural selection.

Causes: 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 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.^[2] Birds find other ways to provide for each other, ie. the evolution of flight opened new avenues for evolution to explore, initiating an adaptive radiation.^[3]

Causes: 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 colonizing 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..

1. Species A migrates from the mainland to the first island. 2. Isolated from the mainland, species A evolves to species B. 3. Species B migrates to the second island. 4. Species B evolves in species C. 5. Species C recolonizes the first islands, but is now unable to reproduce with species B. 6. Species C migrates to the third island. 7. Species C evolves into species D. 8. Species D migrates to the first and second island. 9. Species D evolves to species E.

This process could go on indefinitely until a large diversity is reached.

See also

• Evolutionary radiation – a more general term to describe any radiation
• Cambrian explosion – the most famous evolutionary radiation

Further reading

• Wilson, E. et al. Life on Earth, by Wilson,E.; Eisner,T.; Briggs,W.; Dickerson,R.; Metzenberg,R.; O'brien,R.; Susman,M.; Boggs,W.; (Sinauer Associates, Inc., Publishers, Stamford, Connecticut), c 1974. Chapters: The Multiplication of Species; Biogeography, pp 824–877. 40 Graphs, w species pictures, also Tables, Photos, etc. Includes Galápagos Islands, Hawaii, and Australia subcontinent, (plus St. Helena Island, etc.).
• Leakey, Richard. The Origin of Humankind – on adaptive radiation in biology and human evolution, pp. 28–32, 1994, Orion Publishing.
• Grant, P.R. 1999. The ecology and evolution of Darwin's Finches. Princeton University Press, Princeton, NJ.
• Mayer, Ernst. 2001. What evolution is. Basic Books, New York, NY.
• Kemp, A.C. 1978. A review of the hornbills: biology and radiation. The Living Bird 17: 105–136.

References