continuous variation

Continuous variation describes a trait whose values shade smoothly from one extreme to another with no natural gaps — picture a paint store's gradient strip going from palest to deepest, with every intermediate present. Height, blood pressure, and milk yield all behave this way: you can always imagine a value between any two individuals.

This pattern is the visible signature of many genes of small effect acting together, their additive contributions smeared further by environmental differences. The result, when you plot how common each value is, is typically the familiar bell-shaped curve, where most individuals sit near the average and fewer lie at the extremes. Continuous variation is therefore the population-level appearance of polygenic, quantitative inheritance.

It contrasts with discontinuous (or discrete) variation, where a trait falls into separate categories such as blood type or the presence versus absence of a feature. The line between the two is not always sharp: a trait with only a few underlying genes can show distinct classes that begin to overlap and merge into continuity once enough loci and enough environmental noise are added.