Adaptive Radiation

An adaptive radiation is a series of events in evolutionary history in which one ancestral species gives rise to many descendant species that are adapted to different ecological settings. Thus many descendant species in an adaptive radiation appear to be very different from the ancestral species. Adaptive radiations generally occur in specific geographic regions and usually occur over a relatively short span of geological time.

Adaptive radiations are informative case histories in evolution. Biologists believe that the normal evolutionary processes of adapta tion through natural selection and speciation, acting on isolated species with little or no competition—and often upon arrival of the ancestor in an isolated region, such as an oceanic island chain—is sufficient to generate the richly diverse arrays of descendant species.

There is a wide spectrum of events that falls under the category of adaptive radiation. At the lower end, small bursts of speciation in a relatively constricted area commonly produce an array of adaptively divergent species. Indeed, the best example of such a "mini" adaptive radiation is provided by the famous "Darwin's finches" of the Galapagos Islands. The thirteen species currently alive have diverged into an array of small, medium, and large bill sizes, adapted for eating seeds of different sizes and toughness. Some species forage on the ground for seeds, while others hunt for insects in shrubs and trees. But others of these finch species have diverged further from the ancestral seed-cracking finches. One species, the woodpecker finch, is adapted to eating insects that it finds by using its modified beak to pry up pieces of bark. Some woodpecker finches have developed the ability to use twigs held in their beaks to pry insects out of cavities in the wood. Another highly divergent Galapagos finch species is the warbler finch; with its thin, pointed bill, this finch forages for insects on bushes, living very much like members of the New World warblers (Family Parulidae).

The Galapagos finches have diverged mainly in terms of the size and shape of their beaks, although also in their overall body size. Otherwise, they all continue to resemble each other—often to a confusing degree. In larger-scale adaptive radiations, far greater anatomical divergences commonly evolve. For example, the Hawaiian Islands provide the scene for a number of adaptive radiations among its native birds, insects, and plants. The Hawaiian honeycreepers (a subfamily of finches, as are the Galapagos finches) are considered by some to be perhaps the most famous example of an adaptive radiation. The full extent of the radiation of the Hawaiian honeycreepers will never be known, because many species have become extinct as the result of human disruption of their habitat—most notably including importation of mosquitoes. Most honey-creeper species are currently severely threatened, able to survive only in the ever-shrinking forests of native plant species in the higher elevations in remoter regions of the islands where agriculture has not yet taken over.

Ornithologists first thought that the Hawaiian honeycreepers belonged to several different families, so great has been their divergence from the common ancestor (see, for example, Pratt, Bruner, and Berrett, 1987, p. 295). Although there is a group of them that, like the Galapagos finches, remain confus-ingly similar (these are the "small green birds"), a group of nectar-feeding honeycreepers has evolved not only a great array of bill types but also different body sizes and, most noticeably, striking plumage. The adult iiwi, for example, has a bright scarlet body with black wings (similar to the scarlet tanager seen in summertime in North America); its bill is thin and pointed, curved downward as an adaptation for feeding on nectar from flowers of a particular size. The larger black mamo, in contrast, is all black, with a beak similar to that of the iiwi although correspondingly longer. The akohekohe, in contrast, is a large bird with a rather small beak that it uses in nectar feeding at the tops of trees. Still other honey-creepers look more like the ancestral finch and have remained seed eaters.

Yet adaptive radiations can occur on a far grander scale. Until the arrival of mankind (Homo sapiens) in Australia some 40,000 years ago (and perhaps earlier), the only mammals there were marsupials—mammals whose young develop in an external pouch on the mother's body. The most famous of these, of course, are the kangaroos and wallabies, which, though adapted to a form of hopping on the hind feet, nonetheless are ecologically very much like deer. Koala "bears" are also marsupials, as are the extinct thylacine "wolves" and even an extinct species of saber-toothed carnivore that looked very similar to the true saber-toothed cats of the Northern Hemisphere. Such close similarities between some of these species and their nonmarsupial counterparts are examples of convergent evolution. Thus marsupials evolved to play most of the roles typically played by placental mammals in North America and Eurasia.

—Niles Eldredge

See also: Convergence and Parallelism; Evolution; Galapagos Islands and Darwin's Finches; Natural Selection; Speciation

Bibliography

Eldredge, Niles. 1989. Macroevolutionary Dynamics: Species, Niches and Adaptive Peaks. New York: McGraw-Hill; Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Futuyma, Douglas J. 1997. Evolutionary Biology. Sunderland, MA: Sinauer; Pratt, H. Douglas, Phillip L. Bruner, and Delwyn G. Berrett. 1987. The Birds of Hawaii and the Tropical Pacific. Princeton: Princeton University Press; Weiner, Jonathan. 1994. The Beak of the Finch: A Story of Evolution in Our Time. New York: Alfred A. Knopf.

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