adaptation, in biology, the process by which a species becomes fitted to its environment; it is the result of natural selection’s acting upon heritable variation over several generations. Organisms are adapted to their environments in a great variety of ways: in their structure, physiology, and genetics, in their locomotion or dispersal, in their means of defense and attack, in their reproduction and development, and in other respects.

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The word adaptation does not stem from its current usage in evolutionary biology but rather dates back to the early 17th century, when it indicated a relation between design and function or how something fits into something else. In biology this general idea has been coopted so that adaptation has three meanings. First, in a physiological sense, an animal or plant can adapt by adjusting to its immediate environment—for instance, by changing its temperature or metabolism with an increase in altitude. Second, and more commonly, the word adaptation refers either to the process of becoming adapted or to the features of organisms that promote reproductive success relative to other possible features. Here the process of adaptation is driven by genetic variations among individuals that become adapted to—that is, have greater success in—a specific environmental context. A classic example is shown by the melanistic (dark) phenotype of the peppered moth (Biston betularia), which increased in numbers in Britain following the Industrial Revolution as dark-coloured moths appeared cryptic against soot-darkened trees and escaped predation by birds. The process of adaptation occurs through an eventual change in the gene frequency relative to advantages conferred by a particular characteristic, as with the coloration of wings in the moths.

The third and more popular view of adaptation is in regard to the form of a feature that has evolved by natural selection for a specific function. Examples include the long necks of giraffes for feeding in the tops of trees, the streamlined bodies of aquatic fish and mammals, the light bones of flying birds and mammals, and the long daggerlike canine teeth of carnivores.

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All biologists agree that organismal traits commonly reflect adaptations. However, much disagreement has arisen over the role of history and constraint in the appearance of traits as well as the best methodology for showing that a trait is truly an adaptation. A trait may be a function of history rather than adaptation. The so-called panda’s thumb, or radial sesamoid bone, is a wrist bone that now functions as an opposable thumb, allowing giant pandas to grasp and manipulate bamboo stems with dexterity. The ancestors of giant pandas and all closely related species, such as black bears, raccoons, and red pandas, also have sesamoid bones, though the latter species do not feed on bamboo or use the bone for feeding behaviour. Therefore, this bone is not an adaptation for bamboo feeding.

The English naturalist Charles Darwin, in On the Origin of Species by Means of Natural Selection (1859), recognized the problem of determining whether a feature evolved for the function it currently serves:

The sutures of the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition [birth], and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which only have to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.

Thus, before explaining that a trait is an adaptation, it is necessary to identify whether it is also shown in ancestors and therefore may have evolved historically for different functions from those that it now serves.

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Another problem in designating a trait as an adaptation is that the trait may be a necessary consequence, or constraint, of physics or chemistry. One of the most common forms of constraint involves the function of anatomical traits that differ in size. For example, canine teeth are larger in carnivores than in herbivores. This difference in size is often explained as an adaptation for predation. However, the size of canine teeth is also related to overall body size (such scaling is known as allometry), as shown by large carnivores such as leopards that have bigger canines than do small carnivores such as weasels. Thus, differences in many animal and plant characteristics, such as the sizes of young, duration of developmental periods (e.g., gestation, longevity), or patterns and sizes of tree leaves, are related to physical size constraints.

Adaptive explanations in biology are difficult to test because they include many traits and require different methodologies. Experimental approaches are important for showing that any small variability, as in many physiological or behavioral differences, is an adaptation. The most rigorous methods are those that combine experimental approaches with information from natural settings—for example, in showing that the beaks of different species of Galapagos finch are shaped differently because they are adapted to feed on seeds of different sizes.

The comparative method, using comparisons across species that have evolved independently, is an effective means for studying historical and physical constraints. This approach involves using statistical methods to account for differences in size (allometry) and evolutionary trees (phylogenies) for tracing trait evolution among lineages.