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Types of natural selection
Occurrence of a range of traits is represented by a bell-shaped curve. Natural selection acts on this curve at different points (arrow) to produce different outcomes. a) Directional selection. Selection acts against one extreme, here the light colored morph, so that in subsequent generations it becomes less and less infrequent moving the moths in the directions of the now more frequent dark morph. b) Disruptive selection. Individuals with intermediate features are selected against, eventually producing over the subsequent generations a divided result with two color morphs at the extremes with their own bell-shaped distributions. c) Stabilizing selection. The extremes are selected against producing over subsequent generations a population with less variation, reflected in the tighter bell-shaped curve. The example shown here is birth weights in human infants, which tend to predominate around the middle of the extremes.