Examples of directional selection include giraffes that have long necks and the darkening of London's peppered moths after the Industrial Revolution. Directional selection is the shift of the population of one species towards a certain phenotype.
Directional selection causes one phenotype of a species to have an advantage over another phenotype of the same species. Phenotypes are the physical features of animals and humans. Selective pressures are environmental factors that lead to directional selection.
In an environment where a disease wipes out all the shrubbery that is low to the ground, directional selection favors giraffes with long necks and puts giraffes with short necks at a disadvantage. Giraffes with longer necks survive and reproduce due to their ability to reach taller trees to eat the greenery. As a result, the majority of the giraffe species evolve to have longer necks.
London's peppered moths are another example of directional selection. The majority of these moths were white prior to the Industrial Revolution. This allowed them to blend into the bark of trees and hide themselves in order to escape predators. The emergence of factories released soot into the air and darkened the trees. Over time, the white moth population decreased and the population of darker moths, who were once the minority phenotype, greatly increased.