Directional selection is a type of natural selection that favors one extreme phenotype over the mean or other extreme. This phenomena is usually seen in environments that have changed over time. Changes in weather, climate, or food availability lead to directional selection.
The population bell curve shifts either farther left or farther right due to directional selection. However, unlike stabilizing selection, the height of the bell curve does not change. There are far fewer "average" individuals in this population.
Human interaction can also speed up directional selection. Hunters most often kill the bigger individuals of the population for their meat or other large ornamental or useful parts. Therefore, the population tends to skew toward the smaller individuals. This causes the directional selection bell curve to shift to the left. Similarly, many slower individuals in a prey population are killed and eaten, meaning directional selection in this case would skew toward the faster individuals.
Charles Darwin studied what was to become known as directional selection while he was in the Galapagos Islands. The beak length of the Galapagos finches changed over time due to available food sources. When there was a lack of insects to eat, finches with larger and deeper beaks survived because they could crack seeds. Over time, as insects became more plentiful, directional selection favored finches with smaller and longer beaks.