A specific kind of population evolution
that takes place according to the more general theory of natural selection
In directional selection, "sudden" environmental changes select for individuals of one phenotype over those of another phenotype within a species. This means that those individuals are more likely to survive long enough to bear offspring, and so their genotypes will predominate in the population.
A good example of this is the peppered moth population of pre-industrial England. They hide from predators by blending into their surroundings--particularly the lichen-covered trees in forests that bordered factory towns. Before these towns were industrialized, the trees were light in color. Most of the peppered moths at this time were light grey. However, as soot began to accumulate and darken the tree trunks, the light-colored moths were at a competitive disadvantage, and dark peppered moths (once a rare mutation in the species) began to occur more and more frequently. Eventually, the light moths became the rarer form.