Editing Directional selection (section)

{{Short description|Type of genetic selection favoring one extreme phenotype}}
{{redirect|Positive selection|positive selection of thymocytes during maturation |Thymocyte#Event during maturation{{!}}Thymocyte}}
{{for|theories of goal-directed evolution|Orthogenesis}}

[[File:Genetic Distribution.svg|thumb|Three different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the amount of organisms. Group A is the original population and Group B is the population after selection. Top (Graph 1) represents directional selection with one extreme favored. Middle (Graph 2) represents stabilizing selection with the moderate trait favored. Bottom (Graph 3) represents disruptive selection with both extremes being favored.]]

In [[population genetics]], '''directional selection''' is a type of '''[[natural selection]]''' in which one extreme [[phenotype]] is favored over both the other extreme and moderate phenotypes. This genetic selection causes the [[allele frequency]] to shift toward the chosen extreme over time as allele ratios change from generation to generation. The advantageous extreme allele will increase in frequency among the population as a consequence of survival and reproduction differences among the different present phenotypes in the population. The allele fluctuations as a result of directional selection can be independent of the dominance of the allele, and in some cases if the allele is [[recessive]], it can eventually become [[Fixation (population genetics)|fixed]] in the population.<ref>{{cite book |last=Molles |first=MC |year=2010 |title=Ecology Concepts and Applications |publisher=McGraw-Hill Higher Learning}}</ref><ref>{{Cite journal |last1=Teshima |first1=Kosuke M. |last2=Przeworski |first2=Molly |date=January 2006 |title=Directional Positive Selection on an Allele of Arbitrary Dominance |journal=Genetics |volume=172 |issue=1 |pages=713–718 |doi=10.1534/genetics.105.044065 |pmid=16219788 |pmc=1456198 }}</ref>

Directional selection was first identified and described by naturalist [[Charles Darwin]] in his book ''On the Origin of Species'' published in 1859.<ref>{{Cite web |last=Kaiser |first=Margaret |date=November 2014 |title=First editions of Darwin's 'Origin of Species' |url=https://circulatingnow.nlm.nih.gov/2014/11/24/first-editions-of-darwins-origin-of-species/#:~:text=On%20November%2024%2C%201859%2C%20Charles,out%20on%20the%20first%20day. |website=National Library of Medicine}}</ref> He identified it as a type of natural selection along with [[stabilizing selection]] and [[disruptive selection]].<ref>{{cite book |last=Darwin |first=C |year=1859 |title=On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life |url=https://archive.org/details/onoriginofspec00darw |publisher=John Murray |location=London}}</ref> These types of selection also operate by favoring a specific allele and influencing the population's future phenotypic ratio. Disruptive selection favors both extreme phenotypes while the moderate phenotype will be selected against. The frequency of both extreme alleles will increase while the frequency of the moderate allele will decrease, differing from the trend in directional selection in which only one extreme allele is favored. Stabilizing selection favors the moderate phenotype and will select against both extreme phenotypes.<ref name=":0">{{cite journal |last1=Mitchell-Olds |first1=Thomas |last2=Willis |first2=John H. |last3=Goldstein |first3=David B. |year=2007 |title=Which evolutionary processes influence natural genetic variation for phenotypic traits? |journal=Nature Reviews Genetics |publisher=Springer Nature |volume=8 |issue=11 |pages=845–856 |doi=10.1038/nrg2207 |issn=1471-0056 |pmid=17943192 |s2cid=14914998}}</ref> Directional selection can be observed in [[finch]] beak size, peppered moth color, African cichlid mouth types, and sockeye salmon migration periods.

If there is continuous allele frequency change as a result of directional selection generation from generation, there will be observable changes in the phenotypes of the entire population over time. Directional selection can change the genotypic and phenotypic variation of a population and cause a trend toward one specific phenotype.<ref>{{Cite journal |last1=Melo |first1=Diogo |last2=Marroig |first2=Gabriel |date=January 2015 |title=Directional selection can drive the evolution of modularity in complex traits |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=2 |pages=470–475 |doi=10.1073/pnas.1322632112 |doi-access=free |pmid=25548154 |pmc=4299217 |bibcode=2015PNAS..112..470M }}</ref> This selection is an important mechanism in the selection of complex and diversifying traits, and is also a primary force of speciation.<ref name=":1">{{Cite journal |last1=Rieseberg |first1=Loren H. |last2=Widmer |first2=Alex |last3=Arntz |first3=A. Michele |last4=Burke |first4=John M. |date=September 2002 |title=Directional selection is the primary cause of phenotypic diversification |journal=Proceedings of the National Academy of Sciences |volume=99 |issue=19 |pages=12242–12245 |doi=10.1073/pnas.192360899 |doi-access=free |pmid=12221290 |pmc=129429 |bibcode=2002PNAS...9912242R }}</ref> Changes in a genotype and consequently a phenotype can either be advantageous, harmful, or neutral and depend on the environment in which the phenotypic shift is happening.<ref>{{Cite journal |last1=Thiltgen |first1=Grant |last2=dos Reis |first2=Mario |last3=Goldstein |first3=Richard A. |date=December 2016 |title=Finding Direction in the Search for Selection |journal=Journal of Molecular Evolution |volume=84 |issue=1 |pages=39–50 |doi=10.1007/s00239-016-9765-5 |pmid=27913840 |pmc=5253163 }}</ref>