Adaptation

I -INTRODUCTIONAdaptation word used by biologists in two different senses, both of which imply the accommodation of a living organism to its environment. One form of adaptation, called physiological adaptation, involves the acclimatization of an individual organism to a sudden change in environment. The other kind of adaptation, discussed here, occurs during the slow course of evolution and hence is called evolutionary adaptation.

II -MECHANISMS OF ADAPTATION
Evolutionary adaptations are the result of the competition among individuals of a particular species over many generations in response to an ever-changing environment, including other animals and plants. Certain traits are culled by natural selection (see Evolution), favoring those individual organisms that produce the most offspring. This is such a broad concept that, theoretically, all the features of any animal or plant could be considered adaptive. For example, the leaves, trunk, and roots of a tree all arose by selection and help the individual tree in its competition for space, soil, and sunlight.

Biologists have been accused of assuming adaptive ness for all such features of a species, but few cases have actually been demonstrated. Indeed, biologists find it difficult to be certain whether any particular structure of an organism arose by selection and hence can be called adaptive or whether it arose by chance and is selectively neutral.

The best example of an evolutionary development with evidence for adaptation is mimicry. Biologists can show experimentally that some organisms escape predators by trying to be inconspicuous and blend into their environment and that other organisms imitate the coloration of species distasteful to predators. These tested cases are only a handful, however, and many supposed cases of adaptation are simply assumed.

On the contrary, it is possible that some features of an organism may be retained because they are adaptive for special, limited reasons, even though they may be maladaptive on the whole. The large antlers of an elk or moose, for example, may be effective in sexual selection for mating but could well be maladaptive at all other times of the year. In addition, a species feature that now has one adaptive significance may have been produced as an adaptation to quite different circumstances. For example, lungs probably evolved in adaptation to life in water that sometimes ran low on oxygen. Fish with lungs were then “preadapted” in a way that accidentally allowed their descendants to become terrestrial.

III -ADAPTIVE RADIATION
Because the environment exerts such control over the adaptations that arise by natural selection—including the coadaptations of different species evolving together, such as flowers and pollinators—the kind of organism that would fill a particular environmental niche ought to be predictable in general terms. An example of this process of adaptative radiation, or filling out of environmental niches by the development of new species, is provided by Australia.

When Australia became a separate continent some 60 million years ago, only monotremes and marsupials lived there, with no competition from the placental mammals that were emerging on other continents. Although only two living monotremes are found in Australia today, the marsupials have filled most of the niches open to terrestrial mammals on that continent. Because Australian habitats resemble those in other parts of the world, marsupial equivalents can be found to the major placental herbivores, carnivores, and even rodents and moles.

This pattern can be observed on a restricted scale as well. In some sparsely populated islands, for example, one species of bird might enter the region, find little or no competition, and evolve rapidly into a number of species adapted to the available niches. A well-known instance of such adaptive radiation was discovered by Charles Darwin in the Galápagos Islands. He presumed, probably correctly, that one species of finch colonized the islands thousands of years ago and gave rise to the 14 species of finchlike birds that exist there now. Thus, one finch behaves like a warbler, another like a woodpecker, and so on. The greatest differences in their appearance lie in the shapes of the bills, adapted to the types of food each species eats.

IV -ANALOGY AND HOMOLOGY
When different species are compared, some adaptive features can be described as analogous or homologous. For example, flight requires certain rigid aeronautical principles of design; yet birds, bats, and insects have all conquered the air. In this case the flight structures are said to be analogous—that is, they have different embryological origins but perform the same function. By contrast, structures that arise from the same structures in the embryo but are used in entirely different kinds of functions, such as the forelimb of a rat and the wing of a bat, are said to be homologous.

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