Directional selection

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A chart showing three types of selection

Directional selection is a particular mode or mechanism of natural selection.

In population genetics, directional selection occurs when natural selection favors a single phenotype and therefore allele frequency continuously shifts in one direction. Under directional selection, the advantageous allele will increase in frequency independently of its dominance relative to other alleles (i.e. even if the advantageous allele is recessive, it will eventually become fixed). Directional selection stands in contrast to balancing selection where selection may favor multiple alleles, and is the same as purifying selection which removes deleterious mutations from a population, in other words it is directional selection in favor of the advantageous heterozygote.

See also

• Balancing selection
• Disruptive selection
• Stabilizing selection

References

• Sabeti PC, et. al. (2006). "Positive Natural Selection in the Human Lineage". Science 312. 
• Pickrell JK, Coop G, Novembre J et. al. (May 2009). "Signals of recent positive selection in a worldwide sample of human populations". Genome Research.