User:Cara.begley/Directional selection
outline
"Positive selection" redirects here. [should negative selection direct here also?] For positive selection of thymocytes during maturation, see Thymocyte. For theories of goal-directed evolution, see Orthogenesis.
In population genetics, directional selection is a type of natural selection in which an extreme phenotype is favored the other extreme phenotype and the moderate phenotype, causing the allele frequency to shift toward the chosen extreme over time as allele ratios change generation to generation. Under directional selection, the advantageous extreme allele increases as a consequence of selection differences in survival and reproduction among the different present phenotypes. The increases are independent of the dominance of the allele, and if the allele is recessive, it will eventually become fixed. [source?]
Directional selection was first identified and described by naturalist Charles Darwin in the book On the Origin of Species as a form of natural selection. Other types of natural selection include stabilizing and disruptive selection. These types of selection also operate by acting on either an extreme or moderate allele and influencing the eventual phenotypic ratio. Disruptive selection favors both extreme phenotypes and will select against the moderate trait. Stabilizing selection favors the moderate phenotype and against extreme phenotypes, causing a decline in the population variation over time.
can restructure variation , pro or con (1)
positive selection and fixation in the recessive pool isn't necessarily a given, should include that in the last sentence of the first paragraph (2)
add information on pros and cons at the end of second intro paragraph
The examples section could do well with some restructuring.
Directional selection often occurs under environmental changes and population migrations 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.
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