The term adaptation is used in two distinct, but related, ways in evolutionary biology. An adaptation is any behavioral, physiological, or anatomical feature of an organism that has been shaped by the evolutionary process to perform a specific function. Eyes, for example, are adaptations for seeing—and eyes have evolved independently in several different lineages of animals, a reflection of the importance of vision in the life of most animals.
The second sense of the term adaptation is the process whereby the features of organisms are modified in evolutionary history to perform new functions, or to perform functions more efficiently. For example, increase in brain size in human evolutionary history came about over time through adaptation that involved increase in cognitive capacity (thinking ability). The term adaptation, referring to a process, is sometimes also used in biology in a nonevolutionary, purely physiological sense. For example, when a person walks into a room and notices a distinct odor, that odor usually becomes less noticeable within a few minutes. This is an example of an internal adjustment in the person's olfactory sensory apparatus through the process of "physiological adaptation," rather than the actual disappearance of the odor itself. The remainder of this article treats adaptation only in the evolutionary sense, and it will be clear from the context in which sense—the general evolutionary process of adaptation, or the specific organismic feature—the term adaptation is being used.
Adaptation can be considered the central concept of evolutionary biology. Although (as discussed below) not all evolutionary change involves adaptation (that is, not all evolutionary change is adaptive), the original questions that prompted Charles Darwin, Alfred Russell Wallace, and others to develop a theory of evolution are these: Why are there so many kinds of animals, plants, fungi, and microbes? Why, in other words, is there so much diversity (see Evolutionary Biodiversity) in the world? And why are organisms that are basically very similar nonetheless different in some consistent ways—for example, why are eastern, western, and mountain bluebirds in the United States differently colored?
Darwin and other early evolutionary biologists observed the obvious functions played by anatomical and behavioral features of organisms and saw that features which differ between organisms (for example, the number, size, and shape of the digits of the front feet of vertebrate animals) correlate directly with the different functions played by those features. The human hand has five fingers including an opposable thumb, which makes it ideal for grasping and manipulating objects. The front foot of a horse, in contrast, has one digit, is covered by a hoof, and is used exclusively for support of the animal as it stands, walks, and runs. There must be, these early biologists reasoned, some process in biological history that can modify organis-mic features into the diverse array we see in the biological world.
That process is natural selection; Darwin and Wallace saw that not all organisms born each generation can survive and reproduce (or else the world would be quickly overrun by a single species). They also knew that organisms resemble their parents—though they were ignorant of the basic mechanisms of heredity (the modern science of genetics was not founded until around 1900, and Darwin and Wallace jointly announced their theory of evolution by natural selection nearly a halfcentury earlier). Finally, both men recognized that there is much variation within a population of organisms.
Both Darwin and Wallace reasoned that only those organisms best suited ("adapted") to surviving (finding food, avoiding being eaten by predators, or succumbing to disease) on average would be the ones that would reproduce—and thus pass on to their offspring what we would now call the genetic information that made them (the parents) more successful than other organisms in their population.
Should the environment change, perhaps other variants in the population would then be the more successful ones, and succeeding generations would inherit a different mixture of features. Finally, through a process now known as mutation, new forms of genetic information arise and may ultimately prove beneficial to organisms in a species by paving the way for new structures or behaviors to be selected in the general process of adaptive change.
Darwin also saw that though many adaptations influence the ways organisms survive (for example, different beak shapes and sizes in birds for capturing and eating different kinds of food—seeds, insects, rodents), some features seem to be more about the process of reproduction itself. The elaborate fantail of the male peacock, for example, is used in conjunction with his mating display in the attempt to attract females. The differences in bluebird coloration, likewise, are thought not to reflect differences in the ecology of the eastern, mountain, and western bluebird species, but rather the need for females to recognize males; the divergence in coloration between the different species arose, presumably, during periods when populations were isolated, and natural selection (actually, what Darwin called sexual selection) acted to maintain breeding recognition between local males and females. The populations drifted apart, developing different color patterns (and mating songs) in the process known as speciation—such that eastern, western, and mountain bluebirds, when they do occasionally run across one another, do not "recognize" each other as suitable mates, though eastern and mountain bluebirds are known to occasionally hybridize (interbreed).
Although reproductive adaptations are of course important—and are increasingly the focus of research by sociobiologists and others—it is the economic adaptations of organisms
(those concerned specifically with surviving, with acquiring food) that are the most spectacular. For example:
• The golden bamboo lemur (Hapalemur aureus) in Madagascar eats only the tender shoots of young bamboo, which are loaded with cyanide—a substance that, for most organisms, is a deadly poison. These little animals (adults weigh only 1.35 kg) consume enough bamboo each day to kill six adult men. They are "adapted" to eating a species of bamboo that would prove fatal to anything else that ate it—thereby ensuring themselves exclusive grazing rights on this particular food source. The physiological mechanism by which these lemurs detoxify the cyanide is still unknown.
• Plants and many microorganisms have the ability to photosynthesize—in other words, to utilize sunlight to produce sugar from carbon dioxide and water. Photosynthesis is itself a spectacular adaptation, and without it life would never have diversified beyond the bacterial stage of existence. Sugars are a form of stored (solar) energy that lie at the base of the food chain in all of the world's terrestrial and aquatic ecosystems except one: the deep-sea vent faunas. In the deepest oceanic trenches, where sunlight cannot penetrate, there are nonetheless many species of crustaceans, worms, and other forms of marine life that thrive, forming complex, diverse ecosystems—all because some bacteria are able to convert the thermal (heat) energy flowing from cracks in the earth's crust (the heat is derived from radioactive decay deep within the earth). By this unique biochemical pathway, life has adapted to the sunless depths of the deepest ocean floor.
• Termites are famous for being able to digest cellulose—the stiff material that forms much of plant tissue, indigestible to humans, cows, and virtually all other herbivores (which obtain nutrients and sugars from the plants they eat—eliminating the residue cellulose). And though termites pose a threat to the owners of wooden houses, without them, in many ecosystems (especially in drier tropical regions), there would be little or no cellular breakdown of plant material after death—and such ecosystems would quickly become clogged with dead plant life. But it turns out that it is not the termites themselves that actually perform the task of cellulose digestion. Rather, fungi and certain microbes with which they lead a commensal (mutually beneficial) existence do the work. Some termite species maintain great fungal "gardens" below ground, where the fungi are put to work breaking down the cellulose. Other termite species house the fungi (and certain microbes) in their hindgut, providing food and shelter to the fungi and microbes which, in breaking down cellulose in the termite's gut, provide the termite with nourishment. The fungi and microbes are adapted to life in the termite gut, and the termite is adapted to housing an internal "flora" that takes care of much of its nutritional needs. (The presence of the bacterium Escherichia coli in the human gut is an adaptation of both humans and E. coli along similar, digestive lines).
• The human eye was presented to Charles Darwin as an example of an anatomical structure so incredibly complex that it could not possibly have evolved by a series of adaptive stages through the action of natural selection. Darwin and later biologists were able to show, in response, that there is an entire spectrum of complexity among eyes in the animal world—with some "eyes" being simple cups lined by photoreceptor cells with a thin translucent covering that is the simplest imaginable "lens." But even though the complexity of the vertebrate eye (human and otherwise) no longer seems a credible argument against adaptation through natural selection—and hence against the very notion of evolution— nonetheless the intricate workings of all the parts that make up such eyes remain impressive. That the eyes of cephalopod mollusks, such as octopi and squids, very closely resemble the structure of vertebrate eyes, yet were evolved independently, shows how the process of adaptation frequently results in similar-looking structures—there being but a few ways that an eye can be constructed out of organismic tissues (see also Convergence and Parallelism).
Evolutionary biologists have recently become aware that the fact that a feature of an organism performs some function is not proof that the feature is an evolutionary adaptation. Biologists have come to avoid what they call "Just So Stories" (after the famous stories by Rudyard Kipling, such as "How the Elephant Got Its Trunk"). We see feathers on a bird, for example, and we perform experiments which show that birds cannot fly without feathers on their wings (indeed, most of a bird's wing consists of feathers). But that does not mean that feathers were evolved for flight. More likely, feathers served as thermoregula-tory (body heat) devices, and it is now becoming clear that many Mesozoic groups of dinosaurs also had feathers—yet were not adapted to fly.
Thus the process of adaptation may well go through many phases, wherein a structure that is developed for one function is then put to use for still other functions—for which it did not originally evolve. Paleontologists Elisabeth S. Vrba and Stephen Jay Gould coined the term exaptation for such instances in which a struc-
ture evolved as an adaptation for one use and is then used for other functions, often with little or no further anatomical modification. The example they gave is the African black heron, a bird that hunts for frogs and fish in shallow freshwater environments, as do countless other species of herons and egrets the world over. But with the African black heron there is a difference: it folds its wings over in front of itself, forming an "umbrella" that casts a much bigger shadow than the one formed by just its body alone. Fish love the shade, and they congregate into these heron-made shadows—only to be speared by the heron as it patiently sits with its wings folded over in a way not done by any other bird. Obviously, this species of heron's wings were not evolved to cast shadows; rather, they are wings much like those in all other herons and egrets—and, in a general way, like those of all other birds.
The wings were evolved for flight (in contrast to the feathers per se—see above); the new adaptation here is the use of these wings to hunt fish, a behavioral adaptation that is genetically based and so definitely an "adaptation." But the process of adaptation in this instance involved simple transfer of function of a preexisting structure: in terms of casting shadows to catch fish, the wings of the African black heron are an example of an exaptation.
See also: Convergence and Parallelism; Darwin, Charles; Ecosystems; Evolution; Evolutionary Biodiversity; Human Evolution; Natural Selection; Spe-ciation; Wallace, Alfred Russel
Darwin, Charles R. 1859. On the Origin off Species. London: John Murray; Eldredge, Niles. 1989. Macroevo-lutionary 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; Maynard Smith, John. 1993. The Theory of Evolution. Cambridge: Cambridge University Press; Mayr, Ernst. 2001. What Evolution Is. New York: Basic Books; Williams, George C. 1966. Adaptation and Natural Selection. Princeton: Princeton University Press.
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