The term adaptation in biology refers to the ability of living organisms to change their metabolic, physiological and behavioral processes, allowing them to adapt to the conditions of the environment in which they live. Specifically, adaptation refers to changes in an anatomical structure, physiological process, or behavioral trait of an organism that has evolved over a period of time as an effect of natural selection in a manner that increases the reproductive success of that organism (under the given environmental conditions in which the organism is found). Adaptation can increase efficiency in obtaining or utilizing basic resources such as air, light, water, and nutrients; allow the organism to withstand certain harsh physical conditions, such as low or high temperatures, and the absence of light; or increase its ability to defend itself against a predator.

The term in biology, has two meanings: the process of adaptation and the state of adaptation. The process of adaptation is identified with the evolutionary process and the second represents the result. The process of adaptation can occur due to the existence of a variability of genetic nature, and therefore heritable, of populations of living organisms. This variability is produced by the random mutation events that occur in the genetic makeup of organisms. On this basic variability acts the natural selection, favoring the most suitable variants (more suitable) in that environment at that time.

Consequence of natural selection is that the most suitable organisms will transmit their characteristics to a greater number of descendants, so that in one or, normally, more generations, the characters of the most suitable individuals will prevail in the population and the total variability will be modified; the population, that is, will be adapted to that environment. Since the environment is changeable and constantly influences organisms, adaptation is a process by which the characteristics of populations of living organisms continually adjust to those of the environment. If we accepts this mechanism, there is no need to postulate unknown intrinsic forces that drive organisms to modify themselves to survive better (see Lamarckism).

Generic adaptations usually allow ready responses to environmental changes. In contrast, specialized adaptations, e.g., those of internal parasites, result in reduced adaptive response capabilities and increased vulnerability for the species that possess them. On the other hand, during rapid and profound changes in the environment (e.g., floods, volcanic eruptions, forest destruction, etc.), the respective populations may not contain genetic variants compatible with the new situation and would face extinction.

Adaptive status is the condition of fitness acquired by an individual or a population as a result of a process of adaptation. Adaptation is therefore also the adaptive or adapted character, which has survival value, and the complex of these characters. Adaptation is generally a result of compromise between different selective forces and therefore is not a perfect solution, but rather optimal, of the problems related to survival and is always relative to a particular environment.

The process of adaptation in fact guarantees at most the short-term survival of a population, but the possibility for it to survive indefinitely lies, rather than in an episodic and global process of adaptation, in its ability to adapt continuously. The state of adaptation represents, at the same time, the result of an evolutionary process and the basis for a subsequent evolution. In this context it should be noted that some hereditary characters, not necessarily advantageous, may represent pre-adaptations, which can acquire survival value in changed environmental conditions or allow the expression of new ways of life. For example, the webbed legs of many wading birds, adapted to muddy soils, may represent a preadaptation to swimming.

Adaptation in psychology is defined as a process through which an individual adapts to the environment (physical and social), modifying their behavior patterns (passive adaptation) or operating on the environment itself to transform it according to their needs (active adaptation, some authors prefer in this case to speak of adjustment). However, it should be noted that these are not two alternative modes of adaptation, but two mutually integrated and interdependent processes.

Moreover, the term refers both to general conditions and to specific situations; in this case, we speak of school, work, family, etc. adaptation. According to a model proposed by the Swiss psychologist Jean Piaget and widely accepted, adaptation is based on the two processes of assimilation and accommodation, through which the mental schemes are enriched by the incorporation of contributions from the environment and continuously modified to meet new needs that arise.

Adaptation is thus understood in terms of the balance between the individual and the environment (see homeostasis). Inadequate adaptation is referred to as maladaptation and can lead to behavioral disorders. An adaptation that follows a previous maladaptation is more specifically designated as readaptation. Adaptation can be measured by specific psychological tests (questionnaires, inventories, and others).

Sensory adaptation

In psychophysiology, sensory adaptation is the functional change that occurs in a sensory receptor under particular conditions of stimulation. We speak of positive adaptation when the receptor increases its sensitivity as a result of reduced stimulation, negative adaptation when conversely prolonged and monotonous stimulation causes a decrease in sensitivity. In this last case we also speak of habituation.

Adaptation level

The level of adaptation is a concept developed by the American psychologist H. Helson, with which is understood the level of balance of the organism (behavioral homeostasis). The level of adaptation (or AL) corresponds approximately to the weighted logarithmic average of all stimuli that reach an organism, current and past. A stimulus whose intensity equals the adaptation level is perceived as neutral; above and below this value, opposite responses will be elicited (e.g., for temperature, “hot” and “cold”).

The existence of this level of equilibrium implies, however, in general a bipolarity of behavior; examples similar to the one above can be made, among others, for what concerns attitudes. However, the theory of the level of adaptation stands as a generally valid model for interpreting all behavior. As Helson observes, the fact that the level of adaptation is the result of a process of integration which leads to establish an average (averaging), and the fact that this leads to an equilibrium induces one to interpret in the same theoretical context all the psychological processes which take place when the individual is faced with problems posed by the environment; in the first place, therefore, learning and acquisition in general. In addition to this, it should be noted that the model also has its validity in social psychology, as groups also tend to express their own level of balance. It should be noted, however, that studies on the level of adaptation have so far been directed, with extremely interesting results, mainly to the field of perception.