Natural selection will stabilise the population if it favours an average phenotype by selecting against extreme variation. Natural selection will favour individuals who blend in best with the forest floor and are less likely to be detected by predators in a population of mice living in the woods, for example. Assuming the ground is a fairly consistent shade of brown, mice with fur that is the most nearly matched to that colour will most likely survive and breed, passing on their brown coat genes. Mice with alleles that make them somewhat lighter or darker stand out against the ground and are more likely to be eaten by predators. The genetic variation of the population will diminish as a result of this stabilising selection.

When the environment changes, populations frequently go through directional selection, which favours phenotypes on one extreme of the current variation spectrum. The evolution of the peppered moth in eighteenth- and nineteenth-century England is a typical example of this form of selection. Prior to the Industrial Revolution, moths were mostly light in colour, allowing them to blend in with nearby light-colored trees and lichens. As industries began to spew soot, the trees darkened, making light-colored moths easier to spot for predatory birds.

Natural selection can sometimes favour two or more unique phenotypes, each with its own set of advantages. The intermediate genotypes are generally less suited than their extreme counterparts in these situations. Diversifying or disruptive selection is evident in many populations of animals, such as lobsters, that have numerous male mating techniques. Large, dominant alpha males gain mates by brutal force, whereas little males can slip in for furtive copulations with the females in an alpha male’s domain.Both dominant males and “sneaking” men will be picked in this circumstance, but medium-sized males, who cannot surpass alpha males and are too large to sneak copulations, will be rejected.