Trait Selection Explorer

Directional selection favors individuals at one extreme of the trait distribution. Over time the population mean shifts toward the favored phenotype.

A classic example is beak depth in Darwin's finches on the Galapagos Islands. During droughts, deeper beaks are favored because they can crack larger, tougher seeds.

After many generations of directional selection, the trait distribution shifts entirely in the favored direction while maintaining a roughly normal shape.

Stabilizing selection favors individuals near the population mean and selects against both extremes. The mean stays the same but variance decreases over time.

Human birth weight is a well-studied example. Babies that are too small have lower survival, and very large babies face complications during delivery. The optimal weight is near the population average.

This is the most common type of selection in nature, maintaining well-adapted phenotypes and reducing phenotypic variation.

Disruptive selection favors both extremes of the trait distribution and selects against intermediate phenotypes. Variance increases and the distribution can become bimodal.

African seed-cracker finches (Pyrenestes ostrinus) show this pattern. Large-billed birds crack hard seeds efficiently, and small-billed birds handle soft seeds well. Intermediate bills are poor at both.

Disruptive selection can eventually lead to speciation if gene flow between the two phenotypic groups is reduced.

The breeder's equation describes how a population's mean trait value changes in response to selection.

R (response to selection) is the change in mean trait value between generations. S (selection differential) is the difference between the mean of selected parents and the overall population mean. h² (heritability) is the fraction of phenotypic variation due to additive genetic variation.

Higher heritability means selection is more effective at shifting the population mean. With h² = 0, no matter how strong the selection, the offspring mean is unchanged.