Natural selection is the mechanism by which beneficial alleles rise in frequency in the population over time.
The peppered moth is an example of natural selection acting to change the frequency of a certain trait in the population over time. During the Industrial Revolution, there was a directional change in the color of the peppered moth population around Manchester. That is, there was an increase in the frequency of dark colored peppered moths within the population. This was because the increased air pollution due to the industrial revolution created a situation where the dark colored version of the peppered moths had a higher relative fitness than the light colored peppered moths (which were the traditional variety). After pollution was reduced later on, there was a increase in frequency of light colored moths, as the dark colored moths no longer received a relative fitness benefit from their dark coloration.
