Directional selection

Directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. This process is a key mechanism of evolution, leading to changes in the genetic makeup of a population over generations.

Directional selection occurs when individuals at one end of a phenotype range are more successful at surviving and reproducing in a particular environment. This can result in a shift in the population's phenotype distribution toward one end of the spectrum. Common examples include the development of resistance to pesticides in insects and the development of antibiotic resistance in bacteria.

Mechanism

The mechanism of directional selection begins with the variation in a trait within a population. This variation is often the result of mutations, genetic recombination, and other sources of genetic diversity. When an environmental pressure, such as a change in climate, predation, or availability of resources, favors one extreme of this variation, individuals with the favored trait are more likely to survive and reproduce. Over time, these traits become more common in the population, leading to evolutionary change.

Examples

One classic example of directional selection is the case of the peppered moth (Biston betularia) in England during the Industrial Revolution. The moths come in two color morphs: light and dark (melanic). Before the Industrial Revolution, the light-colored moths were more common because they could camouflage against the light-colored trees and lichens. However, as pollution killed the lichens and darkened the trees with soot, the dark-colored moths became less visible to predators and thus more likely to survive and reproduce. Over time, the population shifted towards the dark-colored phenotype.

Another example is the development of resistance to pesticides in agricultural pests. Pesticides typically kill a large percentage of the pest population, but individuals with mutations that confer resistance to the pesticide are more likely to survive and reproduce. As these resistant individuals become more common, the population as a whole becomes more resistant to the pesticide, necessitating the development of new pest control methods.

Impact on Evolution

Directional selection plays a significant role in shaping the evolutionary path of species. By favoring individuals with certain traits, it can lead to significant changes in the physical characteristics, behaviors, and ecological niches of species. This can result in the emergence of new species and the extinction of others, thereby altering the course of biodiversity on Earth.

See also

• Stabilizing selection
• Disruptive selection
• Artificial selection
• Sexual selection