An adaptation is a trait that has been shaped by natural selection to perform a function that enhances the organism’s fitness — its survival and reproductive success — in a particular environment. The term refers both to the trait itself and to the evolutionary process that produced it.
Examples span every level of biological organization. The hemoglobin molecule’s affinity for oxygen is a biochemical adaptation. The woodpecker’s skull structure — reinforced bone, spongy tissue, a hyoid bone that wraps around the skull — is an anatomical adaptation for impact absorption. The arctic fox’s seasonal coat color change is a behavioral and physiological adaptation. The Venus flytrap’s snap-closing leaves are a morphological adaptation for nutrient acquisition in nitrogen-poor soils.
Not every trait is an adaptation. Some traits are byproducts of other adaptations (Stephen Jay Gould and Richard Lewontin called these “spandrels”). Some are relics of past environments no longer relevant. Some reflect genetic drift or developmental constraint rather than selection. Determining whether a trait is an adaptation requires evidence that it was shaped by selection for its current function — not merely that it is useful.
Adaptation is always relative to an environment. A trait adaptive in one context may be neutral or harmful in another. The sickle cell allele is harmful in homozygous form (causing sickle cell disease) but adaptive in heterozygous form in malaria-endemic regions (conferring resistance to the malaria parasite). This context-dependence means adaptation is not a permanent achievement but a contingent fit between organism and environment — one that can be undone if conditions change.
Related terms
- Natural selection — the process that produces adaptations
- Evolution — the broader process of which adaptation is one outcome
- Phenotype — the observable traits on which selection acts
- Species — the population within which adaptive evolution occurs