An adaptation is any developmental, behavioral, physiological, or anatomical change in an organism that gives that organism a better chance to survive and reproduce. The word "adaptation" also refers to the fitting of a whole species, over time, to function in its particular environment, and to those specific features of a species that make it better-adapted. Adaptations acquired by individuals during their lifetime, such as muscles strengthened by exercise or behaviors honed by experience, make an individual organism better-adapted; species as a whole, however, generally become better adapted to their environments only by the process of natural selection. Except in the form of learned behavior, adaptations achieved by individual organisms cannot be passed on to offspring.
Less-adapted species are less perfectly attuned to a particular environment but may be better-suited to survive changes in that environment or to colonize new areas. Highly adapted species are well-suited to their particular environment, but being more specialized, are less likely to survive changes to that environment or to spread to other environments. An example of a highly adapted species would be a flower that depends on a specific insect that exists only or primarily in its present environment for pollination. The plant may achieve highly reliable pollination by these means, but if its target species of insect becomes extinct, the plant will also become extinct—unless the species can adapt to make use of another pollinator.
At the level of the individual organism, an adaptation is a change in response to conditions. This is a short-term change with a short-term benefit. An example of an adaptation of this type is the production of sweat to increase cooling on a hot day.
Another type of adaptation is sensory adaptation. If a receptor or sense organ is over-stimulated, its excitability is reduced. For example, continually applied pressure to an area of skin eventually causes the area to become numb to feeling and a considerably larger pressure has to be applied to the area subsequently to elicit a similar response. This form of adaptation enables animals to ignore most of their skin most of the time, freeing their attention for more pressing concerns.
Whether occurring within a span of minutes, over an organism's lifetime, or over thousands or millions of years, adaptation serves to increase the efficiency of organisms and thus, ultimately, their chances of survival.
Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.