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directional selection
In population genetics, 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. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and even if the allele is recessive, it will eventually become fixed.〔Molles, MC (2010). ''Ecology Concepts and Applications. ''McGraw-Hill Higher Learning.〕
Directional selection was first described by Charles Darwin in the book ''Origin of Species'' as a form of natural selection.〔Darwin, C. (1859) On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. John Murray, London.〕 Other types of natural selection include stabilizing and disruptive selection. Each type of selection contains the same principles, but is slightly different. Disruptive selection favors both extreme phenotypes, different from one extreme in directional selection. Stabilizing selection favors the middle phenotype, causing the decline in variation in a population over time.
==Evidence==
Directional selection occurs most often under environmental changes and when populations migrate to new areas with different environmental pressures. Directional selection allows for fast changes in allele frequency, and plays a major role in speciation. Analysis on QTL effects has been used to examine the impact of directional selection in phenotypic diversification. This analysis showed that the genetic loci correlating to directional selection was higher than expected; meaning directional selection is a primary cause of phenotypic diversification, which leads to speciation.〔Rieseberg LH, A Widmer , AM Arntz, & JM Burke. 2002. Directional selection is the primary cause of phenotypic diversification''. ''PNAS 99: 12242-12245.〕
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