Natural selection can sometimes favor extremes over the intermediate types. This is called disruptive selection.
In nature, sexual dimorphism is probably a common example; but here we shall use an experiment by Thoday and Gibson on fruitflies as an example. Thoday and Gibson bred from fruitflies with high, or low, numbers of bristles on a certain region of the body; individuals with intermediate numbers of bristles were prevented from breeding. After 12 generations of this disruptive selection, the population had noticeably diverged (see graph). Disruptive selection is of particular theoretical interest, both because it can increase the genetic diversity of a population (by frequency-dependent selection) and promote speciation.
Natural selection may also be directional or stabilizing.
Figure: experimental disruptive selectionon sternopleural bristle number in the fruitfly Drosophila melanogaster. Individuals with many or few bristles were allowed to breed, those with intermediate numbers were not; and the population rapidly diverged. After Thoday and Gibson (1962).