As shown, directional selection results in a change in the mean value of the trait toward the form that has highest fitness. At this point, any phenotypic variation would depend on direct environmental effects rather than on genetic differences among individuals, and the heritability of the trait would be zero, or at least very low. Disruptive selection results in an increase in both extremes and a loss of intermediate forms. Over a long period of time directional selection will result in a shift in the frequency of individuals with different traits until the average form has highest fitness. At this point, the situation becomes one of stabilizing selection, and the extreme forms of the trait will be lost.
Disruptive selection is somewhat more complicated.
We will consider under what circumstances this might happen when we study speciation later in the semester. Look over these forms of selection again and note that the typical ultimate response is a loss of genetic variation as all individuals end up with the alleles for the highest fitness form of the trait. This is a typical result of natural selection. Compare this to what you have discovered about single gene traits from your computer modeling assignment and you will see that in most single gene situations natural selection also results in loss, or near loss, of genetic variation. Superficially, heterosis seems similar to stabilizing selection in that in both heterosis and stabilizing selection the intermediate form has higher fitness than does either extreme.