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If there is continuous allele frequencies changes as a result of directional selection generation to 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 web |last=Melo |first=Diogo |last2=Marroig |first2=Gabriel |date=January 2015 |title=Directional selection can drive the evolution of modularity in complex traits |url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4299217/ |website=National Library of Medicine}}</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 web |last=Rieseberg |first=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 |url=https://www.pnas.org/doi/full/10.1073/pnas.192360899 |website=Proceedings of National Academy of Sciences}}</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 web |last=Thiltgen |first=Grant |last2=dos Reis |first2=Mario |last3=Goldstein |first3=Richard A. |date=December 2016 |title=Finding Direction in the Search for Selection |url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5253163/ |website=National Library of Medicine}}</ref>
If there is continuous allele frequencies changes as a result of directional selection generation to 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 web |last=Melo |first=Diogo |last2=Marroig |first2=Gabriel |date=January 2015 |title=Directional selection can drive the evolution of modularity in complex traits |url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4299217/ |website=National Library of Medicine}}</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 web |last=Rieseberg |first=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 |url=https://www.pnas.org/doi/full/10.1073/pnas.192360899 |website=Proceedings of National Academy of Sciences}}</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 web |last=Thiltgen |first=Grant |last2=dos Reis |first2=Mario |last3=Goldstein |first3=Richard A. |date=December 2016 |title=Finding Direction in the Search for Selection |url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5253163/ |website=National Library of Medicine}}</ref>
==Evidence==
==Evidence==
Directional selection most often occurs during environmental changes or population migrations to new areas with different environmental pressures. Directional selection allows for swift changes in allele frequency that can accompany rapidly changing environmental factors and plays a major role in speciation.<ref name=":1" /> Analysis on quantitative trait locus ([[QTL]]) effects has been used to examine the impact of directional selection in phenotypic diversification. This analysis showed that directional changes in QTLs affecting various traits were more common than expected by chance among diverse species (QTL sign test); meaning directional selection is a primary cause of the phenotypic diversification that can result in speciation.<ref>{{cite journal | last1=Rieseberg | first1=Loren H. | last2=Widmer | first2=Alex | last3=Arntz | first3=A. Michele | last4=Burke | first4=John M. | title=Directional selection is the primary cause of phenotypic diversification | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=99 | issue=19 | pages=12242–5 | date=2002-09-17 | pmid=12221290 | doi=10.1073/pnas.192360899 |pmc=129429 | bibcode=2002PNAS...9912242R | doi-access=free }}</ref>
Directional selection most often occurs during environmental changes or population migrations to new areas with different environmental pressures. Directional selection allows for swift changes in allele frequency that can accompany rapidly changing environmental factors and plays a major role in speciation.<ref name=":1" /> Analysis on quantitative trait locus ([[QTL]]) effects has been used to examine the impact of directional selection in phenotypic diversification. QTL is a region of a gene that corresponds to a specific phenotypic trait, and the measuring the statistical frequencies of the traits can be helpful in analyzing phenotypic trends.<ref>{{Cite web |last=Powder |first=Kara E. |date=March 2024 |title=Quantitative Trait Loci (QTL) Mapping |url=https://pubmed.ncbi.nlm.nih.gov/31849018/ |website=National Library of Medicine}}</ref> This analysis showed that directional changes in QTLs affecting various traits were more common than expected by chance among diverse species (QTL sign test); meaning directional selection is a primary cause of the phenotypic diversification that can result in speciation.<ref>{{cite journal | last1=Rieseberg | first1=Loren H. | last2=Widmer | first2=Alex | last3=Arntz | first3=A. Michele | last4=Burke | first4=John M. | title=Directional selection is the primary cause of phenotypic diversification | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=99 | issue=19 | pages=12242–5 | date=2002-09-17 | pmid=12221290 | doi=10.1073/pnas.192360899 |pmc=129429 | bibcode=2002PNAS...9912242R | doi-access=free }}</ref>


===Detection methods===
===Detection methods===

Revision as of 19:08, 24 March 2024

"Positive selection" redirects here. For positive selection of thymocytes during maturation, see Thymocyte.
For theories of goal-directed evolution, see Orthogenesis.
The 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. Graph 1 shows directional selection, in which a single extreme phenotype is favored. Graph 2 depicts stabilizing selection, where the intermediate phenotype is favored over the extreme traits. Graph 3 shows disruptive selection, in which the extreme phenotypes are favored over the intermediate.

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 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 fixed in the population.[1][2]

Directional selection was first identified and described by naturalist Charles Darwin in his book On the Origin of Species published in 1859.[3] He identified it as a type of natural selection along with stabilizing selection and disruptive selection.[4] 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 trait 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 when only one extreme allele is favored. Stabilizing selection favors the moderate phenotype and will select against both extreme phenotypes.[5]

If there is continuous allele frequencies changes as a result of directional selection generation to 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. [6] This selection is an important mechanism in the selection of complex and diversifying traits, and is also a primary force of speciation. [7] 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. [8]

Evidence

Directional selection most often occurs during environmental changes or population migrations to new areas with different environmental pressures. Directional selection allows for swift changes in allele frequency that can accompany rapidly changing environmental factors and plays a major role in speciation.[7] Analysis on quantitative trait locus (QTL) effects has been used to examine the impact of directional selection in phenotypic diversification. QTL is a region of a gene that corresponds to a specific phenotypic trait, and the measuring the statistical frequencies of the traits can be helpful in analyzing phenotypic trends.[9] This analysis showed that directional changes in QTLs affecting various traits were more common than expected by chance among diverse species (QTL sign test); meaning directional selection is a primary cause of the phenotypic diversification that can result in speciation.[10]

Detection methods

There are different statistical tests that can be run to test for the presence of directional selection in a population. A few of the tests include the QTL sign test, Ka/Ks ratio test and the relative rate test. The QTL sign test compares the number of antagonistic QTL to a neutral model, and allows for testing of directional selection against genetic drift.[11] The Ka/Ks ratio test compares the number of non-synonymous to synonymous substitutions, and a ratio that is greater than 1 indicates directional selection.[12] The relative ratio test looks at the accumulation of advantageous against a neutral model, but needs a phylogenetic tree for comparison.[13]

Examples

An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches. Throughout the wet years, small seeds were more common and there was such a large supply of the small seeds that the finches rarely ate large seeds. During the dry years, none of the seeds were in great abundance, but the birds usually ate more large seeds. The change in diet of the finches affected the depth of the birds’ beaks in future generations. Their beaks range from large and tough to small and smooth.[14]

African cichlids

African cichlids are known to be some of the most diverse fish and evolved extremely quickly. These fish evolved within the same habitat, but have a variety of morphologies, especially pertaining to the mouth and jaw. Albertson et al. 2003 tested this by crossing two species of African cichlids with very different mouth morphologies. The cross between Labeotropheus fuelleborni (subterminal mouth for biting algae off rocks) and Metriaclima zebra (terminal mouth for suction feeding) allowed for mapping of QTLs affecting feeding morphology. Using the QTL sign test definitive evidence was shown to support the existence of directional selection in the oral jaw apparatus. However, this was not the case for the suspensorium or skull (suggesting genetic drift or stabilizing selection).[15]

Sockeye salmon

Sockeye salmon are one of the many species of fish that are anadromous. Individuals migrate to the same rivers in which they were born to reproduce. These migrations happen around the same time every year, but Quinn et al. 2007 shows that sockeye salmon found in the waters of the Bristol Bay in Alaska have recently undergone directional selection on the timing of migration. In this study two populations of sockeye salmon were observed (Egegik and Ugashik). Data from 1969–2003 provided by the Alaska Department of Fish and Game were divided into five sets of seven years and plotted for average arrival to the fishery. After analyzing the data it was determined that in both populations average migration date was earlier and was undergoing directional selection. The Egegik population experienced stronger selection and shifted 4 days. Water temperature is thought to cause earlier migration date, but in this study there was no statistically significant correlation. The paper suggests that fisheries can be a factor driving this selection because fishing occurs more in the later periods of migration (especially in the Egegik district), preventing those fish from reproducing.[16]

Ecological impact

Directional selection can quickly lead to vast changes in allele frequencies in a population. Because the main cause for directional selection is different and changing environmental pressures, rapidly changing environments, such as climate change, can cause drastic changes within populations.

Timescale

Typically directional selection acts strongly for short bursts and is not sustained over long periods of time.[17] If it did, a population might hit biological constraints such that it no longer responds to selection. However, it is possible for directional selection to take a very long time to find even a local optimum on a fitness landscape.[18] A possible example of long-term directional selection is the tendency of proteins to become more hydrophobic over time,[19] and to have their hydrophobic amino acids more interspersed along the sequence.[20]

See also

References

  1. ^ Molles, MC (2010). Ecology Concepts and Applications. McGraw-Hill Higher Learning.
  2. ^ Teshima, Kosuke M.; Przeworski, Molly (January 2006). "Directional Positive Selection on an Allele of Arbitrary Dominance".
  3. ^ Kaiser, Margaret (November 2014). "First editions of Darwin's 'Origin of Species'". National Library of Medicine.
  4. ^ Darwin, C (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray.
  5. ^ Mitchell-Olds, Thomas; Willis, John H.; Goldstein, David B. (2007). "Which evolutionary processes influence natural genetic variation for phenotypic traits?". Nature Reviews Genetics. 8 (11). Springer Nature: 845–856. doi:10.1038/nrg2207. ISSN 1471-0056. PMID 17943192. S2CID 14914998.
  6. ^ Melo, Diogo; Marroig, Gabriel (January 2015). "Directional selection can drive the evolution of modularity in complex traits". National Library of Medicine.
  7. ^ a b Rieseberg, Loren H.; Widmer, Alex; Arntz, A. Michele; Burke, John M. (September 2002). "Directional selection is the primary cause of phenotypic diversification". Proceedings of National Academy of Sciences.
  8. ^ Thiltgen, Grant; dos Reis, Mario; Goldstein, Richard A. (December 2016). "Finding Direction in the Search for Selection". National Library of Medicine.
  9. ^ Powder, Kara E. (March 2024). "Quantitative Trait Loci (QTL) Mapping". National Library of Medicine.
  10. ^ Rieseberg, Loren H.; Widmer, Alex; Arntz, A. Michele; Burke, John M. (2002-09-17). "Directional selection is the primary cause of phenotypic diversification". Proceedings of the National Academy of Sciences of the United States of America. 99 (19): 12242–5. Bibcode:2002PNAS...9912242R. doi:10.1073/pnas.192360899. PMC 129429. PMID 12221290.
  11. ^ Orr, H.A. (1998). "Testing Natural Selection vs. Genetic Drift in Phenotypic Evolution Using Quantitative Trait Locus Data". Genetics. 149 (4): 2099–2104. doi:10.1093/genetics/149.4.2099. PMC 1460271. PMID 9691061.
  12. ^ Hurst, Laurence D (2002). "The Ka/Ks ratio: diagnosing the form of sequence evolution". Trends in Genetics. 18 (9). Elsevier BV: 486–487. doi:10.1016/s0168-9525(02)02722-1. ISSN 0168-9525. PMID 12175810.
  13. ^ Creevey, Christopher J.; McInerney, James O. (2002). "An algorithm for detecting directional and non-directional positive selection, neutrality and negative selection in protein coding DNA sequences". Gene. 300 (1–2). Elsevier BV: 43–51. doi:10.1016/s0378-1119(02)01039-9. ISSN 0378-1119. PMID 12468084.
  14. ^ Campbell, Neil A.; Reece, Jane B. (2002). Biology (6th ed.). Benjamin Cummings. pp. 450–451. ISBN 978-0-8053-6624-2.
  15. ^ Albertson, R. C.; Streelman, J. T.; Kocher, T. D. (2003-04-18). "Directional selection has shaped the oral jaws of Lake Malawi cichlid fishes". Proceedings of the National Academy of Sciences. 100 (9): 5252–5257. Bibcode:2003PNAS..100.5252A. doi:10.1073/pnas.0930235100. ISSN 0027-8424. PMC 154331. PMID 12704237.
  16. ^ Quinn, Thomas P.; Hodgson, Sayre; Flynn, Lucy; Hilborn, Ray; Rogers, Donald E. (2007). "Directional Selection by Fisheries and the Timing of Sockeye Salmon (Oncorhynchus Nerka) Migrations". Ecological Applications. 17 (3). Wiley: 731–739. doi:10.1890/06-0771. ISSN 1051-0761. PMID 17494392.
  17. ^ Hoekstra, H. E.; Hoekstra, J. M.; Berrigan, D.; Vignieri, S. N.; Hoang, A.; Hill, C. E.; Beerli, P.; Kingsolver, J. G. (2001-07-24). "Strength and tempo of directional selection in the wild". Proceedings of the National Academy of Sciences. 98 (16): 9157–9160. Bibcode:2001PNAS...98.9157H. doi:10.1073/pnas.161281098. ISSN 0027-8424. PMC 55389. PMID 11470913.
  18. ^ Kaznatcheev, Artem (May 2019). "Computational Complexity as an Ultimate Constraint on Evolution". Genetics. 212 (1): 245–265. doi:10.1534/genetics.119.302000. PMC 6499524. PMID 30833289.
  19. ^ Wilson, Benjamin A.; Foy, Scott G.; Neme, Rafik; Masel, Joanna (24 April 2017). "Young genes are highly disordered as predicted by the preadaptation hypothesis of de novo gene birth" (PDF). Nature Ecology & Evolution. 1 (6): 0146–146. doi:10.1038/s41559-017-0146. hdl:10150/627822. PMC 5476217. PMID 28642936.
  20. ^ Foy, Scott G.; Wilson, Benjamin A.; Bertram, Jason; Cordes, Matthew H. J.; Masel, Joanna (April 2019). "A Shift in Aggregation Avoidance Strategy Marks a Long-Term Direction to Protein Evolution". Genetics. 211 (4): 1345–1355. doi:10.1534/genetics.118.301719. PMC 6456324. PMID 30692195.

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