stabilizing selection

Meaning: Stabilizing selection is natural selection in which individuals near the population average for a trait have higher fitness than individuals at either extreme. Extreme phenotypes are removed more often, so the distribution narrows (variance falls) while the mean stays roughly the same. Over generations this tends to reduce variation in the trait. It is typical when the environment is stable and the intermediate phenotype suits it. A classic example is human birth weight, where very small and very large babies have lower survival.

On the MCAT: A passage gives a bell curve of a trait before and after selection, or a plot of fitness against phenotype. For stabilizing selection, the fitness plot peaks in the middle, and the population curve becomes taller and narrower with the peak in the same place. Compare it with directional selection, where the mean shifts toward one extreme, and disruptive selection, where both extremes are favored and the curve can split into two peaks.

Common confusion: Stabilizing does not mean no selection is occurring, nor that the population is in Hardy-Weinberg equilibrium. Selection is acting, which violates the Hardy-Weinberg assumption of no selection. What stays stable is the mean phenotype.