17.2.2—Stabilising, disruptive and directional selection

Selection patterns describe which phenotype values are favoured or selected against in an environment. Stabilising selection favours the intermediate phenotype, directional selection favours one extreme, and disruptive selection favours both extremes over the intermediate.

• Stabilising: both extremes are selected against, so intermediate phenotypes have the greatest survival or reproductive success; the distribution becomes narrower around a similar mean.
• Directional: one extreme is favoured, usually after a change in the environment or selection pressure; the population distribution shifts towards that extreme and the mean changes.
• Disruptive: both extremes are favoured while intermediate phenotypes are selected against; the distribution separates towards two peaks and intermediate values become less common.

To identify the pattern, compare the phenotype distribution before and after selection, locate the values with the greatest reproductive success, and ask which values the environmental pressure removes or favours. The graph shape is evidence for the pattern; it is not a complete explanation of the underlying genetic architecture.

“Favoured” means better suited to that particular environment, not universally better. A selection label describes the outcome for phenotype values; it does not by itself prove that the trait is genetic, calculate a test statistic, or identify a fixed outcome when the environment changes.