In population genetics, directional selection is a mode of natural selection in which individuals with a trait (for example, beak size) at one extreme of a phenotypic distribution have better fitness than individuals with intermediate or opposite extreme phenotypes. Over time, the allele frequencies, and consequently the population mean for the trait, shift consistently in the direction of the extreme phenotype with greater fitness. An example is the evolution of antibiotic resistance in bacteria – the introduction of a strong selective pressure (the antibiotic) selects resistant strains of bacteria, thereby shifting allele frequencies toward phenotypes with strong resistance to the antibiotic.

This type of selection plays an important role in the emergence of complex and diversifying traits and is also a primary force in speciation. Natural phenomena that might promote strong directional selection include: 1) Sudden environmental changes (biotic or abiotic) favour one phenotype over a previously dominant phenotype; 2) Colonization of a new habitat with novel selection pressures (as was the case with Darwin’s finches migrating to the Galápagos Islands two million years ago); 3) The genetic context offers sufficient heritable variation and involves relatively minor interactions or correlations among genes (pleiotropy or epistasis) and trade-offs (antagonistic pleiotropy). These would not necessarily preclude directional selection, but would make it more complicated.

Directional selection was first identified and described by naturalist Charles Darwin in his book On the Origin of Species published in 1859. He identified it as a type of natural selection along with stabilizing selection and disruptive selection. These types of selection also operate by favoring specific alleles and influencing the population's future phenotypic frequency distribution.

Stabilizing selection favors the intermediate phenotypes and selects against extreme phenotypes. It tends to reduce variance around the mean. Disruptive selection favors extreme phenotypes while, at the same time, selecting against the moderate phenotypes. The frequency of extreme alleles increases while the frequency of the moderate alleles decreases, possibly leading to a bimodal distribution.

At the genetic level, directional selection corresponds to the increase in frequency of one allele (or combination of alleles) that confers higher fitness, possibly ultimately causing it to reach fixation (that is, the relative frequency in the population is one or close to one). Directional selection can act on genetic mutations or on existing gene variation:

The limits of directional selection are apparent when, even under continued selection pressures, selection slows down or stops as available genetic variation is exhausted or genetic/correlated constraints are reached (so-called “selection limits”). A selection limit is a term from animal breeding and quantitative genetics that refers to a cessation of phenotypic change even when continued directional selection is being applied to a trait, such as body size. For example, an allele that is ‘good’ for the trait under directional selection may be ‘bad’ with respect to lifetime reproductive success. Under the body size scenario (which, for example, might enhance the ability of large predators to successfully hunt large prey), this might mean that an allele that confers larger body size might also reduce fertility, possibly eliminating the fitness benefits of the trait that directional selection was originally acting on.

A much-studied example of directional selection is the fluctuation of light and dark phenotypes in peppered moths in the 1800s. During the industrial revolution, environmental conditions were rapidly changing with the growing emissions of black smoke from coal-powered factories. The soot changed the color of trees, rocks, and other niches of moths. Before the industrial revolution, the most prominent phenotype in the peppered moth population was the lighter, speckled moths. They thrived on the light birch trees and their phenotype provided them with better camouflage from predators. After the Industrial Revolution as the trees become darker with soot, the moths with the darker phenotype were better able to blend in and avoid predators better than their white counterparts. As time went on, the darker moths were positively directionally selected for and the allele frequency began to shift due to the increase in the number of darker moths.

African cichlids are known to be a diverse fish species, with evidence indicating that they evolved extremely quickly. These fish evolved within the same habitat, but have a variety of morphologies, especially pertaining to the mouth and jaw. Experiments pertaining the cichlid jaw phenotypes was done by Albertson and others in 2003 by crossing two species of African cichlids with very different mouth morphologies. The cross between Labeotropheus fuelleborni (subterminal mouth for biting algae off rocks) and Metriaclima zebra (terminal mouth for suction feeding) allowed for mapping of QTLs affecting feeding morphology. Using the QTL sign test, definitive evidence was used to support the existence of directional selection in the oral jaw apparatus in African cichlids. However, this was not the case for the suspensorium or skull QTLs, suggesting genetic drift or stabilizing selection as mechanisms for the speciation.