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{{short description|Results of a hypothetical replicated artificial selection experiment with three treatments}}
[[File:Selection_Limits_1.svg|thumb|right|Results of a hypothetical replicated artificial selection experiment with three treatments. At generation 0, the base population of organisms had been randomly sampled to create six lines, two of which would be selectively bred for high values of the trait, two for low values of the trait, and two of which would not experience any intentional selection. The control lines diverged somewhat by random [[genetic drift]] and possibly unique [[mutations]], but, overall, did not change in their average phenotype from the beginning of the experiment. Both of the high-selected lines reached apparent selection limits around generation 20. Both of the low-selected reached absolute limits near zero around generation 25.]]

A '''selection limit''' is a term from [[animal breeding]] and [[quantitative genetics]] that refers to a cessation of progress even when continued directional selection is being applied to a trait, such as body [[size]]. In other words, a breeder or scientist is using [[selective breeding]] (artificial selection) and choosing individuals as breeders within a population based on some [[phenotypic trait]] or traits. If this is done, then the [[average]] value of the population typically evolves across generations in the direction being favored by selection (i.e., for higher or lower values of the trait), but then at some point the population stops evolving. The trait under selection is then said to have reached a limit or plateau at that value.

== Details ==
The existence of limits in artificial selection experiments was discussed in the [[scientific literature]] in the 1940s or earlier.<ref name="Lerner&Dempster1951">{{cite journal
| last = Lerner | first = I. M.
| author2 = E. R. Dempster
| title = Attenuation of genetic progress under continued selection in poultry
| journal = Heredity
| volume = 275
| issue = 1
| pages = 75–94
| date = 1951| doi = 10.1038/hdy.1951.4
| pmid = 14840759
}}</ref> The most obvious possible cause of reaching a limit (or plateau) when a population is under continued [[directional selection]] is that all of the additive-genetic variation (see [[additive genetic effects]]) related to that trait gets "used up" or fixed.<ref name="Falconer&Mackay1996">{{cite book
| last = Falconer | first = D. S.
| author2 = T. F. C. Mackay
| title = Introduction to quantitative genetics. 4th edition
| publisher = Pearson Education Limited
| place = Harlow, Essex, England
| date = 1996}}</ref> For example, if a trait, such as body mass, is under selection to increase, then, over time (i.e., across generations), the [[alleles]] (genetic variants) at all [[Locus (genetics)|loci]] (most simply, positions on chromosomes) that tend to make individuals larger than average will increase in frequency, while those that tend to make an individual smaller than average will decrease in frequency. Eventually, in principle, the favored alleles at all relevant loci will become the only ones remaining at those loci. In reality, [[mutation]], random [[genetic drift]] (especially in small populations), and [[gene flow]] from immigrants may stop some loci from becoming [[Fixation (population genetics)|fixed]] for the "good" alleles.

However, other factors may interfere with the realization of genetic gains before loss of genetic variation causes a selection limit. As noted by Lerner and Dempster,<ref name="Lerner&Dempster1951" /> these factors are generally one of two types: 1) negative relations with Darwinian fitness; 2) non-additive gene action and/or [[Gene–environment interaction|genotype-environment interaction]] (although others are possible<ref name="Al-Murrani1974">{{cite journal
| last = Al-Murrani | first = W. K.
| title = The limits to artificial selection
| journal = Animal Breeding Abstracts
| volume = 42
| pages = 587–592
| date = 1974}}</ref>
<ref name="Falconer1992">{{cite journal
|last=Falconer|first=D. S.
|year=1992
|title=Early selection experiments
|journal=Annual Review of Genetics
|volume=26
|pages=1–14|doi=10.1146/annurev.ge.26.120192.000245
|pmid=1482107
}}</ref>
<ref name="Falconer&Mackay1996" />
<ref name="Hill&Kirkpatrick2010">{{cite journal
|last=Hill|first=W. G.
|author2=M. Kirkpatrick
|year=2010
|title=What animal breeding has taught us about evolution
|journal=Annual Review of Ecology, Evolution, and Systematics
|volume=41
|pages=1–19|doi=10.1146/annurev-ecolsys-102209-144728
}}</ref>).

A negative relation with [[Fitness (biology)|Darwinian fitness]] is a situation in which an allele that is "good" for the trait under directional selection is "bad" with respect to lifetime reproductive success. For example, an allele that tends to confer larger body size might also lead to [[infertility]], thus reducing the ability of individuals with that allele to produce [[offspring]], limiting further response to selection, and sometimes even leading to extinction of the selected line.<ref name="Mather&Harrison1949">{{cite journal
|last1=Mather |first1=K.
|last2=Harrison |first2=B. J.
|year=1949
|title=The manifold effect of selection. Part I
|journal=Heredity
|volume=3
|pages=1–52|doi=10.1038/hdy.1949.1
}}</ref>

Non-additive gene action refers to such situations as [[heterozygote advantage]], where heterozygous individuals have higher (or lower) values for a trait (such as body size) than do either of the two homozygotes. In such a case, selection will tend to maintain more than one allele in the population, and a selection limit may be reached while additive-genetic variation (narrow-sense [[heritability]]) remains for the trait under directional selection.

[[Gene–environment interaction|Genotype-environment interaction]] occurs when the [[phenotype]] produced by a particular set of alleles (at one or more loci) confers relatively higher or lower values of a trait depending on the environmental circumstances in which an individual is born or raised, or under which the trait is measured. For instance, somewhat different [[gene]]s (a term that can refer to alleles or loci) tend to give the highest value of a trait depending on the [[season]]. If this occurs, then directional selection will act to favor some genes in winter and others in summer, for example. Again, the result may be that a selection plateau is attained while the population retains some additive-genetic variance for the trait under directional selection.{{citation needed|date= June 2024}}

Some traits have a natural physical limit beyond which a trait cannot possibly go.<ref name="Falconer&Mackay1996" /> For example, replicated selection for the building of small thermoregulatory [[nest]]s in mice reached a limit at or near zero (i.e., none of the provided cotton was being used to make nests).<ref name="Lynch1994">{{cite book
|last1 = Lynch
|first1 = C. B.
|editor-last1 = Boake
|editor-first1 = C. R. B.
|year = 1994
|chapter = Evolutionary inferences from genetic analyses of cold adaptation in laboratory and wild populations of the house mouse
|title = Quantitative genetic studies of behavioral evolution
|url =
|volume =
|edition =
|location = Chicago
|publisher = University of Chicago Press
|pages = 278–301
|doi =
|isbn =
|issn =
}}</ref> Similarly, lines of [[maize]] selected for low oil or protein content in the [[Corn kernel|kernels]] reached limits near to zero percent.<ref name="Falconer1992" />

Aside from absolute physical limits, and whatever their cause, limits or plateaus have often been observed in artificial selection experiments with animals, including: bristle number in fruit flies (''[[Drosophila]]'');<ref name="Mather1941">{{cite journal
| last = Mather | first = K.
| title = Variation and selection of polygenic characters
| journal = Journal of Genetics
| volume = 41
| pages = 159–193
| date = 1941| issue = 2–3
| doi = 10.1007/BF02983019
}}</ref> avoidance behavior in [[laboratory rats]];<ref name="Brushetal1979">{{cite journal
|last=Brush|first=F. R.
|author2=J.C. Froehlich
|author3=P.C. Sakellaris
|year=1979
|title=Genetic selection for avoidance behavior in the rat
|journal=Behavior Genetics
|volume=9
|issue=4
|pages=309–316|doi=10.1007/BF01068209
|pmid=518469
}}</ref> and large body size,<ref name="Falconer1992" /> large [[Litter (zoology)|litter size]],<ref name="Eklund&Bradford1977">{{cite journal
|last=Eklund|first=J.
|author2=G. E. Bradford
|year=1977
|title=Genetic analysis of a strain of mice plateaued for litter size
|journal=Genetics
|volume=85
|issue=3
|pages=529–542|doi=10.1093/genetics/85.3.529
|pmid=558934
|pmc=1224586
}}</ref> large nest size,<ref name="Lynch1994" /> and high voluntary [[Hamster wheel|wheel]]-running behavior in laboratory house mice.<ref name="Careauetal2013">{{cite journal
|last=Careau|first=V. C.
|author2=M. E. Wolak
|author3=P. A. Carter
|author4=T. Garland, Jr.
|year=2013
|title=Limits to behavioral evolution: the quantitative genetics of a complex trait under directional selection
|journal=Evolution
|volume=67
|issue=11
|pages=3102–3119|doi=10.1111/evo.12200
|pmid=24151996
}}</ref>

==Experiments to identify causes of selection limits==
Experimental approaches to probe the causes of selection are of two general types, quantitative genetic and functional. The former asks general questions about the [[genetic architecture]] of the trait when a limit has been attained (e.g., has narrow-sense heritability gone to zero?), whereas the latter attempts to determine what aspect of physiological or other function might have reached a limit or constraint. Experimental studies may involve attempts to break an apparent selection limit. As an example of a genetic approach, two replicate lines of mice at a limit for large nest size were crossed and selection was continued on this new population, resulting in further increase in nest size.<ref name="Bult&Lynch2000">{{cite journal
|last=Bult|first=A.
|author2=C. B. Lynch
|year=2000
|title=Breaking through artificial selection limits of an adaptive behavior in mice and the consequences for correlated responses
|journal=Behavior Genetics
|volume=30
|issue=3
|pages=193–206|doi=10.1023/A:1001962124005
|pmid=11105393
}}</ref> From a functional perspective, in lines of mice at a selection limit for high wheel running, administration of an [[erythropoietin]] analog increased the [[VO2 max|maximal rate of oxygen consumption during forced exercise]], but did not increase wheel running, a result suggesting that [[motivation]] for [[exercise]] may be limiting the behavior, rather than inherent ability to run on wheels.<ref name="Kolbetal2010">{{cite journal
|last=Kolb|first=E. M.
|author2=S. A. Kelly; K. M. Middleton; L. S. Sermsakdi; M. A. Chappell; and T. Garland, Jr.
|year=2010
|title=Erythropoietin elevates VO2,max but not voluntary wheel running in mice
|journal=Journal of Experimental Biology
|volume=213
|issue=3
|pages=510–519|doi=10.1242/jeb.029074
|pmid=20086137
}}</ref>

==See also==
* [[Bill Hill (geneticist)]]
* [[I. Michael Lerner]]
* [[Kenneth Mather]]
* [[Douglas Scott Falconer]]
* [[Experimental evolution]]
* [[Plant breeding]]

==References==
{{Reflist|2}}

{{DEFAULTSORT:Selection limits}}
[[Category:Evolutionary biology]]
[[Category:Genetics]]

Selection limits - История изменений

Admin: 1 версия импортирована

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