Adaptive radiation
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Four of the 13 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.

Adaptive radiation is the occurrence of the diversification of a single or small groups of species into a large number of descendants that occupy various ecological niches. This is an evolutionary process driven by natural selection.

Causes of adaptive radiation

Opportunity

Isolated ecosystems, by isolating mechanisms, geographic, strerol-hybridization, and genetic disjucture, such as archipelagos, mountain areas, and newly formed lakes can be colonized by a species which, upon establishing itself, undergoes rapid divergent evolution. Monotremes, marsupials, and cichlids are examples of geographic isolation. Monotremes evolved before the evolution of placental mammals, and they are found today only in Australia, a 'continent country'. Marsupials, which also evolved before the appearance of placental mammals are also common in Australia. In Australia, marsupials evolved to fill many ecological niches that placental mammals fill on other continents. In Lake Victoria in Africa, it is thought that more than 300 species of cichlids adaptively radiated from one parent species in only 15,000 years. Biologists who have studied the fossil record know that when a new species evolves with a novel adaptation, there is often a burgeoning of descendent species over the next few million years expressing various themes on that initial adaptation - a burgeoning known as adaptive radiation. The Cambridge University anthropologist Robert Foley has calculated that if the evolutionary history of the bipedal apes followed the usual pattern of adaptive radiation, at least sixteen species should have existed between the group's origin 7 million years ago and today." Travis Was here

Extinction

Adaptive radiation can also occur after mass extinctions. An example of this is after the Permian-Triassic extinction event, where biodiversity increased massively in the Triassic. The end of the Ediacaran and the beginnings of multicellular life lead to adaptive radiations and the genesis of new phyla in the Cambrian period.

References

• 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.

Phylogeny of hornbills, work in progress, 2008, A.C. Kemp.