Adaptive radiation
Adaptive radiation is an evolutionary process whereby a single ancestral species or form diversifies or speciates into several or many related forms or species. The basic form of the different descendant taxa are similar, but each is adapted for a particular environmental niche.
The presence of over 250,000 species of beetles, the 14 different species of Darwin's finches on the Galápagos Islands, the over 25,000 types of teleost fishes, and the different marsupials in Australia are considered to be examples of adaptive radiation (Luria et al. 1981).
Causes of adaptive radiation
The vigorous phase of diversification characteristic of adaptive radiation is linked to a new design, particularly when it is tied to movement in to a new ecological space, such as an unoccupied territory or a new mode of life, such as the development of flight by insects more than 300 million years ago (Luria et al. 1981).
Opportunity
Isolated ecosystems, such as archipelagos and mountain areas, can be colonized by a species which, upon establishing itself, undergoes rapid divergent evolution. Monotremes and marsupials are examples of geographic isolation. Monotremes evolved before the evolution of placental mammals, and they are found today only in Australia, an island. 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.
isolaed for 50 mililion years marsupials in Austrail other contentines (partial exception of Soputh America), mostly placentals.
Richard Leakey (see below) wrote, "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."
Extinction
Adaptive radiation can also occur after mass extinctions. The best 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.
Darwin's finches
Darwin's finches—13 speceis of finches that occup the two dozen or so volcanic islads in the Galapago, are often cited as examples of adaptive radition. These finches difer mainly int eh size nd shape of their beaks, with the beaks adapated to the different good they eat (Wells 2000). Although commonly cited as instrumental in helping Darwin formulate his theory, Wells (2000) notes that they had almost nothing to do with his theory, not bieng mentioned in the Origin of Species and having only a passing reference in his diary of the voyage of the beagle. He did collect 9 of the 13 speices (identifying only six of them as species), , but failed to correlate beak shape with diet and made no effort ot separate them by island. They do not have seemed to have made much of a impression on Darwin as evidence of evolution. But they do seem to be a textbook case of natural selection, and Dawin did go back years later and reinterprett tehm in the list of this theory, thus are now known as "Darwins' finches." According to Sulloway (1982), "Darwin was increasingly given credit after 1947 for finches he never saw and for observations and insights about them he never made." Wells (2000) does not even find them convincing evidence for speciation (there clearly was natural selection), finding little direct or genetic evidence and field studies, that originally showed some directional changes in beak depth, were reversed in subsequent studies. and a suggestion of some merging of the species, rather than furhter diversification.
ReferencesISBN links support NWE through referral fees
- 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.
Sulloway, F. J. 1982. Darwin and his finches: The evolution of a legend. Journal of the History of Biology 15: 1-53).
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