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Encyclopedia > Directional selection

In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. It is in contradistinction to balancing selection where selection may favor multiple alleles, or background selection which removes deleterious mutations from a population. Directional selection is a particular mode or mechanism of natural selection. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the five evolutionary forces: natural selection, genetic drift, mutation, migration and nonrandom mating. ... Natural selection is the process by which biological individuals that are endowed with favorable or deleterious traits end up reproducing more or less than other individuals that do not possess such traits. ... An allele is any one of a number of viable DNA codings of the same gene (sometimes the term refers to a non-gene sequence) occupying a given locus (position) on a chromosome. ... Allele frequency is a term of population genetics that is used in characterizing the genetic diversity of a species population, or equivalently the richness of its gene pool. ... Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. ...


Example

A common example is the peppered moth (Biston betularia). Before the industrial revolution in England (1740?), the peppered moth was mostly found in a light gray form with little black speckled spots. The allele for dark-bodied moths is dominant, while the allele for light-bodied moths is recessive. The light-bodied moths were able to blend in with the light colored lichens and tree bark. The less common black peppered moth was more likely to be eaten by birds. Therefore, the frequency of the dark allele was about 0.01%. During the industrial revolution in England, many of the light-bodied lichens died from sulphur dioxide emissons. The trees became covered with soot from the new coal-burning factories. This led to an increase in bird predation for the light-colored moths (they no longer blended in as well). The dark-bodied moths, however, blended in very well with the trees. Binomial name Biston (moth) betularia Linnaeus, 1758 Subspecies betularia cognataria parva The peppered moth (Biston betularia) is a temperate species of night-flying moth often used by educators as an example of natural selection (see theory of evolution, industrial melanism). ... For other things named Lichen, see: Lichen (disambiguation). ...


As a result, during reproduction, a lot of light-bodied moths were produced, and a few dark-bodied moths. Most of the light-bodied moths didn't survive, while the black-bodied moths continued to survive. Gradually, the allele frequency shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce.


Sources

  • http://www.globalchange.umich.edu/globalchange1/current/labs/peppered_moth/pepperedmoth.html
  • http://www.nslc.wustl.edu/courses/bio100a/templeton/evolution2.pdf (link is dead)

  Results from FactBites:
 
Directional selection - Wikipedia, the free encyclopedia (281 words)
In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shift in one direction.
It is in contradistinction to balancing selection where selection may favor multiple alleles, or background selection which removes deleterious mutations from a population.
Directional selection is a particular mode or mechanism of natural selection.
Darwin was right, IU scientists show (513 words)
Directional natural selection, in which certain traits such as a giraffe's long neck are favored over others, causes most of the differences between species.
Directional selection is a specific kind of natural selection that favors less or more of a particular trait, such as smaller fingers or darker fur giving an individual advantages over other members of its species.
Article citation: "Directional selection is the primary cause of phenotypic diversification," Proceedings of the National Academy of Sciences of the United States of America, Vol.
  More results at FactBites »

 
 

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