Natural selection can change the distribution of a trait in different ways depending on how fitness varies with phenotype.

Consider a measurable trait such as body size.

Directional selection

If one extreme has higher fitness than the rest, the population mean can shift toward that extreme.

fitness
  ^          /
  |        /
  |      /
  +------------> trait value

A sustained environmental change can produce directional selection if larger or smaller values consistently improve reproductive success.

Stabilizing selection

If intermediate phenotypes have the highest fitness, both extremes are selected against. The mean can remain similar while variation decreases.

fitness
  ^       /\
  |      /  \
  |_____/    \_____
  +----------------> trait value

Disruptive selection

If both extremes outperform intermediates, variation can increase and the distribution can become bimodal.

fitness
  ^    /\      /\
  |   /  \____/  \
  +----------------> trait value

These patterns describe the relationship between phenotype and fitness; they do not identify the ecological cause by themselves.

Selection on a trait also does not imply that the trait is controlled by a single gene. Quantitative traits often reflect many loci plus environmental effects.

The three patterns are therefore best understood as different fitness landscapes across phenotype values, not as three unrelated evolutionary mechanisms.