Directional selection is a type of natural selection that occurs when conditions favor a specific characteristic of a species over another. This type of selection increases the frequency distribution of these characteristics, while the other characteristics drop away.
Directional selection occurs when the favored characteristic keeps those organisms that have it alive over ones that do not, or the characteristics make them more favorable for mating. These scenarios mean that only those organisms with the traits will be able to pass them onto the next generations. As factors change, these traits may become less desirable and shift back to the other direction. Traits can shift from generation to generation to ensure the species continues.