Directional or progressive selection is the mode of selection associated with environmental change. We said earlier that in an unchanging environment stabilising selection removes "unusual" forms of traits or extreme measurements every generation. But in an altered environment, the previous mean may no longer be the optimal measurement of the trait. Under these circumstances, selection would weed out the individuals from the two ends of the normal curve unequally.
This in turn shifts the mean of the trait to a new favoured extreme, You may understand that directional selection is a modified form of stabilising selection in that it does not work evenly.at both ends of the distribution curve. A revealing example of directional selection in operation is the ability of pest populations to develop resistance to pesticides. Man has been largely responsible for changes in the environment of many organisms. In order to adapt thcmsclvcs to changes in environment, organisms respond rapidly through directional selection. Ever since the first report on the resistance of houseflies to DDT was made in 1947, more and more reports of insects becoming resistant to a variety of pesticides have been coming in over the past four decades. In all cases the story.is same: Initially a l small concentration of a new pesticide is sufficient to control thc pests. Slowly the concentration is increased even as the insects develop resistance to higher concentrations. Finally the pesticide becomes totally ineffective or the use of it is economically impractical.