Directional selection.

This pattern of natural selection selects for a phenotype at one end of the spectrum that confers the highest fitness in the population's environment. (a) In this example, a mutation for darker fur arises in a population of mice. This new genotype confers higher Darwinian fitness, because mice with dark fur can evade predators and are more likely to survive and reproduce. Over many generations, directional selection will favour the prevalence of darker individuals. (b) These graphs show the change in fur colour phenotypes in this mouse population before and after directional selection.