Adaptation
An adaptation is an inherited trait, structural or behavioral, that increases an organism's chance of surviving and reproducing in its environment.
Definition
An adaptation is a feature, a body part or a behavior, that helps a living thing survive where it lives, such as a duck's webbed feet or birds migrating south for winter, and it is inherited rather than learned, since a giraffe is born with a long neck rather than stretching it longer over its lifetime. More precisely, an adaptation is a heritable trait shaped by natural selection because it increases an organism's fitness, its ability to survive and reproduce, in a particular environment, and it can be structural, physiological, or behavioral; because environments differ, a trait adaptive in one place can be a disadvantage in another, and adaptation happens to populations across generations rather than to an individual within its lifetime. At the deepest level, an adaptation is a trait whose current function was shaped by selection for that function, distinct from an exaptation co-opted for a new role or a non-adaptive byproduct, and adaptations are compromises among competing demands, constrained by developmental history, genetics, and physics, so they are locally rather than globally optimal; convergent evolution, in which unrelated lineages evolve similar solutions to similar challenges, is strong evidence that selection shapes form to function.
Example
A cactus stores water in thick stems and loses less of it through spines instead of leaves, while a polar bear keeps warm with thick fur and a layer of fat. Camels can lose up to $25$ percent of their body water without harm and close their nostrils against blowing sand, and Arctic foxes reduce heat loss with small ears and short legs while desert foxes radiate heat through large ones. Streamlined bodies evolved independently in sharks, ichthyosaurs, and dolphins because drag in water follows the same physics for all three, and high-altitude Tibetan populations carry EPAS1 gene variants that blunt overproduction of red blood cells, an adaptation that arose within the last several thousand years.
Key Insight
Adaptations reveal both the power and the limits of selection: the vertebrate eye is remarkable, yet its retina is wired backward with a blind spot, a trace of the developmental path evolution had to work with rather than a sign of poor engineering.