"Adaptation" --the good fit of organisms to their environment--can occur at three hierarchical levels with different causes."
1. Physiological adaptation [homeostatic; but animals can be conformers/regulators]
2. Cultural adaptations [a decorporealized or transindividual developmental change?]
3. Evolutionary adaptation (Darwinian mechanism of selection upon genetic variation)
Organisms can be viewed as ensembles of anatomical, physiological, or behavioral TRAITS that contribute to FITNESS --an individual's ability to survive, thrive, and reproduce. the major quality animals must manifest is a capacity to COPE with challenges to its fitness. Ordinarily, there is no attribute of an organism that can be defined (including the relationships between attributes and the timing of their expression) that is not subject to natural selection
SO, when we speak of an organism's adaptations we are referring to traits subject to natural selection which persist because they contribute to fitness (or at least, contribute more than they cost). Adaptations are the means by which organisms cope with environmental changes and stresses.
An Adaptation can be manifest at any level of organization from subcellular through the ecosystem in which any level of organism -- environment conformity can be discerned. Within a single organism, "adaptation" can encompass morphology, physiology, development (through organizational effects or through differential timing of developmental events), and behavior.
"adaptations are traits (or characters) that have been subjected to natural selection" This means that the trait has "evolved" (been modified during its evolutionary history) in ways that have contributed to the FITNESS of the organism manifesting it .
An adaptation is an anatomical, physiological, or behavioral trait that contributes to an individual's ability to survive and reproduce ("fitness") in competition with conspecifics in the environment in which it evolved (Williams, G. 1966. Adaptation and Natural Selection Princeton).
Adaptation: "According to strict usage in evolutionary biology, it is correct to consider a character an "adaptation" for a particular task only if there is some evidence that it has evolved (been modified during its evolutionary history) in specific ways to make it effective in the performance of that task, and that the change has occurred due to the increased fitness that results" (p.13). . . . (West-Eberhard, Mary Jane. 1992. Adaptation: Current Usages, in Keywords in Evolutionary Biology Harvard University Press)
"The processes by which organisms or groups of organisms maintain homeostasis in and among themselves in the face of both short-term environmental fluctuations and long-term changes in the composition and structure of their environments." (Rappaport, 1971).
Patterns in biology are both adaptive and constrained
Functionally, an adaptation is "a change in a phenotype that occurs in response to a specific environmental signal and has a clear functional relationship to that signal that results in an improvement in growth, survival, or reproduction" (Strearns, S.S. 1992 (The evolution of Life Histories, Oxford) p 16; citing Williams 1966a and Curio 1973)).
In the biomechanical sense, constraint refers to the necessity that organisms obey the laws of physics and chemistry. The systems definition holds that "each stage of development must proceed from where the last one left off" (Oster & Alberch 1982:450; cited by Strearns 1992); or, more convincingly, "Developmental constraints [are] biases on the production of variant phenotypes or limitations on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system" (Maynard Smith et al. 1985:265; cited by Strearns 1992:18). In concert with this definition one can interpret constraint as the progressive irreversible integration or fixation of traits.