Diversifying selection can also be called Disruptive selection. It involves changes in a population that leads to extreme values for a trait. This means that, instead of having several small differencies in a characteristic amongst the organisms, the organisms tends to have only two "options" of how this characteristic is available. A simple example is that, instead of several different colors of flowers in different nuances of yellow and green, including colors that are intermediate between yellow and green, we will find only two colors: a strong yellow and a strong green. Individuals can develop a tendency of developing one of the two extreme ways of expressing a trait, against the intermediate ones. It is part of natural selection, it is not artificial selection, and it favours two variants, instead of many variants. So you will have two variants of the same trait appearing, and they tend to be extremely opposed to each other.