What is meant by "adaptation" in the context of biology?

In biology, "adaptation" refers specifically to evolutionary changes that enhance an organism's ability to survive and reproduce in its environment. Adaptations can occur at various levels, including physiological, morphological, and behavioral changes that arise over generations as a response to environmental pressures. These changes improve an organism's fitness—a measure of its ability to leave viable offspring in the population.

Adaptation is a key mechanism of evolution, as it enables species to optimize their traits to better suit their habitats, whether it be through developing physical characteristics like camouflage, behavioral changes like migration patterns, or physiological adjustments like changes in metabolism. This process occurs over long timescales and is driven by natural selection, where beneficial traits become more common in a population.

In contrast, the other options do not encapsulate the definition of adaptation accurately: the ability to reproduce relates to overall fitness, changes in behavior may or may not be permanent evolutionary changes, and cellular organization pertains more to the structure of organisms rather than evolutionary changes driven by environmental adaptation.