variation

Variation is the difference or deviation among individuals, items, or measurements within a group. In biology, variation among individuals in a population provides the material on which natural selection can act.

Genetic variation arises from mutations, genetic recombination during meiosis, gene flow, and sexual reproduction. Environmental factors and genotype‑environment interactions also influence observed traits, creating phenotypic variation even when genotypes are similar. Heritability quantifies how much of the observed variation is due to genetic differences.

Variation can be continuous, with a spectrum of values (for example height), or discrete, falling into separate categories (blood type). It can be spatial, such as geographic variation across populations, or temporal, changing during development or over generations. Phenotypic plasticity describes how the same genotype may produce different phenotypes in different environments.

In statistics and data analysis, variation measures dispersion in a data set. Common metrics include range, variance, and standard deviation. Variation affects estimates, confidence, risk assessment, and decision making.

Outside biology, variation also appears in music (alternating variations on a theme), language (regional or social variation in speech or writing), and culture. Across disciplines, studying variation helps explain diversity, adaptation, and the limits of generalization.