Teoría neutralista da evolución molecular: Diferenzas entre revisións
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== Introdución ==
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While some scientists, such as Freese (1962) <ref>Freese, E. (1962). On the evolution of base composition of DNA. J THeor Biol, 3:82-101.</ref> and Freese and Yoshida (1965),<ref>Freese, E. and Yoshida, A. (1965). The role of mutations in evolution. In V Bryson, and H J Vogel, eds. Evolving Genes and Proteins, pp. 341-55. Academic, New York.</ref> had suggested that neutral [[mutation]]s were probably widespread, a coherent theory of neutral evolution was proposed by [[Motoo Kimura]] in 1968,<ref>Kimura M. (1968). Evolutionary Rate at the Molecular Level. ''Nature'' 217:624-6.</ref> and by King and Jukes independently in 1969.<ref>King JL, Jukes TH. (1969). Non-Darwinian Evolution. ''Science'' 164:788-97.</ref>▼
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Kimura, King, e Jukes suxeriron que cando se comparan os [[xenoma]]s das especies existentes, a gran maioría das diferenzas moleculares son "neutras" en canto á selección natural, é dicir, os cambios moleculares representados por estas diferenzas non inflúen na eficiencia biolóxica ou ''[[fitness (bioloxía)|fitness]]'' dos organismos. Como resultado, a teoría considera que estas características xenómicas non están suxeitas nin se poden explicar por selección natural. Esta idea está baseada en parte en que o [[código xenético]] está [[dexeneración do codón|dexenerado]], que quere dicir que pode haber varias secuencias distintas de tres [[nucleótido]]s ([[codón]]s) que codifican o mesmo [[aminoácido]] (por exemplo, os codóns ''GCC'' e ''GCA'' codifican ambos a [[alanina]]). Consecuentemente, moitos cambios potenciais dun só nucleótido son "silenciosos" ou "non expresados", se ocurre que o codón que codificaba un aminoácido cambia a outro codó que tamén codificaba ese mesmo aminoácido. Tales cambios considérase que teñen pouco ou ningún efecto biolóxico.
Unha segunda hipótese da teoría neutralista é que a maioría dos cambios evolutivos son o resultado da [[deriva xenética]] que actúa sobre os [àlelo]]s neutros, en vez de, por exemplo, do [[autostop xenético]] dun alelo neutro debido a que ten un [[ligamento xenético]] con alelos non neutros. Un alelo neutro, unha vez que aparece por mutación, pode facerse máis común na poboación por causa da deriva xenética. Usualmente, o alelo vaise perder, pero en raros casos pode [[Fixación (xenética de poboacións)|fixarse]] na poboación, o que significa que o novo alelo faise o alelo estándar na poboación. Este proceso estocástico considérase que obedece a ecuacións que describen a deriva xenética aleatoria por causa de accidentes de mostraxe.
According to the neutral theory, mutations appear at rate μ in each of the 2N copies of a gene, and fix with probability 1/(2N). This means that if all mutations were neutral, the rate at which fixed differences accumulate between divergent populations is predicted to be equal to the per-individual mutation rate, e.g. during errors in [[DNA replication]]; both are equal to μ. When the proportion of mutations that are neutral is constant, so is the divergence rate between populations. This provides a rationale for the [[molecular clock]], although the discovery of a molecular clock predated neutral theory.<ref name=Zuckerkand62>{{cite book | author = [[Emile Zuckerkandl|Zuckerkandl, E.]] and [[Linus Pauling|Pauling, L.B.]] | year = 1962 | title = Horizons in Biochemistry | chapter = Molecular disease, evolution, and genetic heterogeneity | editor = Kasha, M. and Pullman, B (editors) | pages = 189–225 | publisher = Academic Press, New York}}</ref>▼
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Many [[molecular biology|molecular biologists]] and [[population genetics|population geneticists]] also contributed to the development of the neutral theory, which is different from the neo-Darwinian theory.<ref name=Kimura83/><ref name=Nei2005>Nei, M. (2005). Selectionism and neutralism in molecular evolution. ''Mol Biol Evol'', 22: 2318-42</ref><ref name=Nei2013>Nei, M. (2013). Mutation-driven evolution. Oxford University Press, Oxford</ref>▼
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Neutral theory does not deny the occurrence of natural selection. Hughes writes: "Evolutionary biologists typically distinguish two main types of natural selection: ''[[purifying selection]]'', which acts to eliminate deleterious mutations; and ''positive (Darwinian) selection'', which favors advantageous mutations. Positive selection can, in turn, be further subdivided into [[directional selection]], which tends toward fixation of an advantageous allele, and [[balancing selection]], which maintains a [[Polymorphism (biology)|polymorphism]]. The neutral theory of molecular evolution predicts that purifying selection is ubiquitous, but that both forms of positive selection are rare, whereas not denying the importance of positive selection in the origin of adaptations."<ref name="hughes2">{{cite journal|first=Austin L.|last=Hughes|title=Looking for Darwin in all the wrong places: the misguided quest for positive selection at the nucleotide sequence level|journal=Heredity|volume=99|year=2007|pages=364–373|doi=10.1038/sj.hdy.6801031|pmid=17622265|issue=4}}</ref> In another essay, Hughes writes: "Purifying selection is the norm in the evolution of protein coding genes. Positive selection is a relative rarity — but of great interest, precisely because it represents a departure from the norm."<ref name="hughes1">{{cite book|first=Austin L.|last=Hughes|year=2000|title=Adaptive Evolution of Genes and Genomes|publisher=Oxford University Press|page=53|isbn=0-19-511626-7}}</ref> A more general and more recent view of molecular evolution is presented by [[Masatoshi Nei|Nei]].<ref name=Nei2013 />▼
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== O debagte "neutralista–selectionista" ==▼
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{{see also|History of evolutionary thought|History of molecular evolution}}
A heated debate arose when Kimura's theory was published, largely revolving around the relative percentages of alleles that are "neutral" versus "non-neutral" in any given [[genome]]. Contrary to the perception of many onlookers, the debate was not about whether natural selection does occur. Kimura argued that [[molecular evolution]] is dominated by selectively neutral evolution but at the [[phenotypic]] level, changes in characters were probably dominated by [[natural selection]] rather than [[genetic drift]].<ref>''Provine (1991)</ref>
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There are a large number of statistical methods for testing whether neutral theory is a good description of evolution (e.g., [[McDonald-Kreitman test]] <ref>{{cite journal|last=Kreitman|first=Martin|title=M D S P A H|journal=Annual Review of Genomics and Human Genetics|year= 2000|volume=1|issue=1|pages=539–559|doi=10.1146/annurev.genom.1.1.539|pmid=11701640}}</ref>), and many authors claimed detection of selection (Fay et al. 2002,<ref>Fay, J. C., Wyckoff, G. J., and Wu, C. I. (2002). Testing the neutral theory of molecular evolution with genomic data from Drosophila. ''Nature'', 415:1024-6</ref> Begun et al. 2007,<ref>Begun, D. J., Holloway, A. K., Stevens, K., Hillier, L. W., Poh, Y. P. et al. (2007). Population genomics: whole-genome analysis of polymorphism and divergence in Drosophila simulans. ''PLoS Biol'', 5:e310.</ref> Shapiro et al. 2007,<ref>Shapiro J. A., Huang W., Zhang C., Hubisz M. J., Lu J. et al. 2007. Adaptive genic evolution in the Drosophila genomes. ''Proc Natl Acad Sci USA'' 104:2271–76.</ref> Hahn 2008,<ref name="Hahn">{{cite journal | author = Hahn, M.W. | year = 2008 | title=Toward a selection theory of molecular evolution |journal=Evolution |pages=255–265 |volume=62 |doi=10.1111/j.1558-5646.2007.00308.x | pmid=18302709}}</ref> Akey 2009.<ref>Akey J. M. (2009). Constructing genomic maps of positive selection in humans: where do we go from here? ''Genome Res'' 19:711–22.</ref>). However, Nei et al. (2010).<ref>Nei, M., Suzuki, Y., and M. Nozawa. (2010). The neutral theory of molecular evolution in the genomic era. Annu Rev Genom Hum Genet. 11:265-89.</ref> have argued that their methods for claiming so depend on many assumptions which are not biologically justified.
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== Notas ==
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