Adaptation is one of the basic phenomena of biology. It is the process whereby an organism becomes better suited to its habitat. Also, the term adaptation may refer to a characteristic which is especially important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or their ability to run fast and escape predators. Such adaptations are produced in a variable population by the better suited forms reproducing more successfully, that is, by natural selection.

General principles

The significance of an adaptation can only be understood in relation to the total biology of the species. Julian Huxley

Adaptation is, first of all, a process, rather than a physical part of a body. The distinction may be seen in an internal parasite (such as a fluke), where the bodily structure is greatly simplified, but nevertheless the organism is highly adapted to its unusual environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life-cycle, which is often quite complex. However, as a practical term, adaptation is often used for the product: those features of a species which result from the process. Many aspects of an animal or plant can be correctly called adaptations, though there are always some features whose function is in doubt. By using the term adaptation for the evolutionary process, and adaptive trait for the bodily part or function (the product), the two senses of the word may be distinguished.

Adaptation may be seen as one aspect of a two-stage process. First, there is speciation (species-splitting or cladogenesis), caused by geographical isolation or some other mechanism. Second, there follows adaptation, driven by natural selection. Something like this must have happened with Darwin's finches, and there are many other examples. The present favourite is the evolution of cichlid fish in African lakes, where the question of reproductive isolation is much more complex.

Another great principle is that an organism must be viable at all stages of its development and at all stages of its evolution. This is obviously true, and it follows that there are constraints on the evolution of development, behaviour and structure of organisms. The main constraint, over which there has been much debate, is the requirement that changes in the system during evolution should be relatively small changes, because the body systems are so complex and interlinked. This is a sound principle, though there may be rare exceptions: polyploidy in plants is common, and the symbiosis of micro-organisms that formed the eukaryota is a more exotic example.

All adaptations help organisms survive in their ecological niches. These adaptative traits may be structural, behavioral or physiological. Structural adaptations are physical features of an organism (shape, body covering, defensive or offensive armament); and also the internal organization). Behavioural adaptations are composed of inherited behaviour chains and/or the ability to learn: behaviours may be inherited in detail (instincts), or a tendency for learning may be inherited (see neuropsychology). Examples: searching for food, mating, vocalizations. Physiological adaptations permit the organism to perform special functions (for instance, making venom, secreting slime, phototropism); but also more general functions such as growth and development, temperature regulation, ionic balance and other aspects of homeostasis. Adaptation, then, affects all aspects of the life of an organism.