Directional selection
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In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shift in one direction. It is in contradistinction to balancing selection where selection may favor multiple alleles, or background selection which removes deleterious mutations from a population. Directional selection is a particular mode or mechanism of natural selection.
Example
A common example is the peppered moth (Biston betularia). Before the industrial revolution in England (1740?), the peppered moth was mostly found in a light gray form with little black speckled spots. The allele for dark-bodied moths is dominant, while the allele for light-bodied moths is recessive. The light-bodied moths were able to blend in with the light colored lichens and tree bark. The less common black peppered moth was more likely to be eaten by birds. Therefore, the frequency of the dark allele was about 0.01%. During the industrial revolution in England, many of the light-bodied lichens died from sulphur dioxide emissons. The trees became covered with soot from the new coal-burning factories. This led to an increase in bird predation for the light-colored moths (they no longer blended in as well). The dark-bodied moths, however, blended in very well with the trees.
As a result, during reproduction, a lot of light-bodied moths were produced, and a few dark-bodied moths. Most of the light-bodied moths didn't survive, while the black-bodied moths continued to survive. Gradually, the allele frequency shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce.