“ADAPTATION” can be a confusing term because of the many ways it is used. For example, “adaptation” can refer to both processes and products:
(1) a “developmental” change such as that of a sense organ that make it less responsive to repetitive but irrelevant stimuli, or
(2) an evolutionary change such as those caused by selection pressures on some attribute of a trait that helps the organism cope with a changing environment over the generations, or
(3) a biological TRAIT that exists because it confers or is linked to a trait that now (or in the past) has conferred a biological advantage enhancing an organism’s FITNESS.
Organisms can be viewed as complex, interconnected ensembles of anatomical, physiological, or behavioral TRAITS that contribute to FITNESS —an individual’s ability to survive, thrive, and reproduce. LIFE depends on how well animals COPE with challenges to their biological NEEDS, and arguably, the supreme need is 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. It might even be an ancestral trait that is no longer of use (vestigial) but was (again arguably) of use at some point in the organism’s developmental or evolutionary history. The context in which such traits first appeared is studied as the “environment of evolutionary adaptedness” (EEA)[i].
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.
ADAPTATION is a complex term because of the many ways it is used. It can refer to a TRAIT that confers some FITNESS on an animal, BUT it also represents the PROCESS by which that trait has come about.
“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 .
DEFINITIONS (you would expect that such a key word has come to have many subtle nuances of meaning –BUT there IS an irreducible core of meaning which you must understand.)
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).
Below is a definition by an anthropologist writing in the Annual Review of Ecology and Systematics. I like it because it touches every important base without becoming too diffuse:
Adaptation is . . .
“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).
The several senses of the term “adaptation” have something in common –the idea of compensation for change:
short-term adaptations are behavioral or physiological, longer- term adaptations may be developmental (environmentally induced changes in anatomy, physiology, or behavior), and the longest-term adaptations are genetic (more-or-less “programmed” changes in anatomy, physiology, or behavior; more- programmed=relatively “closed,” i.e. not susceptible to environmental influences; less programmed=relatively “open,” i.e. susceptible to environmental influences.)
ADAPTATIONISM — is a term that is sometimes used in a negative sense to refer to an explanation for a trait that uncritically applies evolutionary ideas. Others use the term in a positive sense to refer to speculative hypotheses about how a trait may have come about. [more on adaptationism]
[i] The Environment of Evolutionary Adaptedness (EEA) is a prominent part of Evolutionary Psychology; Commented by Edward Hagen: http://www.anth.ucsb.edu/projects/human/epfaq/eea.html