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| {{evolutionary biology}}
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| '''Fitness''' (often denoted '''<math>w</math>''' in [[population genetics]] models) is a central idea in [[evolution|evolutionary theory]]. It can be defined either with respect to a [[genotype]] or to a [[phenotype]] in a given environment. In either case, it describes the ability to both survive and reproduce, and is equal to the average contribution to the [[gene pool]] of the next generation that is made by an average individual of the specified genotype or phenotype. The term "Darwinian fitness" is often used to make clear the distinction with [[physical fitness]].<ref>Wassersug, J. D., and R. J. Wassersug, 1986. Fitness fallacies. Natural History 3:34-37.</ref> If differences between [[alleles]] of a given [[gene]] affect Darwinian fitness, then the frequencies of the alleles will change across generations; the alleles with higher fitness become more common. This process is called [[natural selection]]. | |
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| An individual's fitness is manifested through its [[phenotype]]. The phenotype is affected by the developmental environment as well as by genes, and the fitness of a given phenotype can be different in different environments. The fitnesses of different individuals with the same genotype are therefore not necessarily equal. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype in a given environment or set of environments.
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| [[Inclusive fitness]] differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele. One mechanism of inclusive fitness is [[kin selection]].
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| ==Fitness is a propensity== | | '''MathML''' |
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| Fitness is often defined as a [[propensity]] or probability, rather than the actual number of offspring. For example, according to Maynard Smith, "Fitness is a property, not of an individual, but of a class of individuals — for example homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky." <ref>Maynard-Smith, J. (1989) ''Evolutionary Genetics'' ISBN 0-19-854215-1</ref> Equivalently, "the fitness of the individual - having an array x of [[phenotypes]] — is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."<ref>Hartl, D. L. (1981) ''A Primer of Population Genetics'' ISBN 0-87893-271-2</ref>
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| == Measures of fitness == | | '''source''' |
| There are two commonly used measures of fitness; absolute fitness and relative fitness.
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| ===Absolute fitness=== | | <span style="color: red">Follow this [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering link] to change your Math rendering settings.</span> You can also add a [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering-skin Custom CSS] to force the MathML/SVG rendering or select different font families. See [https://www.mediawiki.org/wiki/Extension:Math#CSS_for_the_MathML_with_SVG_fallback_mode these examples]. |
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| '''''Absolute fitness''''' (<math>w_{\mathrm{abs}}</math>) of a genotype is defined as the [[ratio]] between the number of individuals with that genotype after selection to those before selection. It is calculated for a single [[generation]] and must be calculated from absolute numbers. When the absolute fitness is larger than 1, the number of individuals bearing that genotype increases; an absolute fitness smaller than 1 indicates an absolute fall in the number of individuals bearing the genotype. If the number of individuals in a population stays constant, then the average absolute fitness must be equal to 1.
| | ==Demos== |
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| :<math>{w_{\mathrm{abs}}} = {{N_{\mathrm{after}}} \over {N_{\mathrm{before}}}}</math> | | Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]: |
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| Absolute fitness for a genotype can also be calculated as the product of the probability of [[wiktionary:survival|survival]] multiplied by the average [[fecundity]]. Absolute fitness is used in [[Fisher's fundamental theorem]].
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| ===Relative fitness===
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| '''''Relative fitness''''' is quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation, i.e. one genotype is normalized at <math>w=1</math> and the fitnesses of other genotypes are measured with respect to that genotype. Relative fitness can therefore take any nonnegative value, including 0. Relative fitness is used in the standard [[Genetic drift#Wright.E2.80.93Fisher_model|Wright-Fisher]] and [[Moran process|Moran model]]s of population genetics.
| | ==Test pages == |
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| The two concepts are related, as can be seen by dividing each by the [[mean]] fitness, which is weighted by [[genotype frequencies]].
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| :<math>{\frac{w_{abs}}{\overline{w}_{abs}} = \frac{w_{rel}}{\overline{w}_{rel}}}</math>
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| == History ==
| | ==Bug reporting== |
| [[Image:Herbert Spencer.jpg|thumb|150px|[[Herbert Spencer]] (27 April 1820 - 8 December 1903) was an English philosopher.]] | | If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de . |
| The [[United Kingdom|British]] [[sociologist]] [[Herbert Spencer]] coined the phrase "[[survival of the fittest]]" (though originally, and perhaps more accurately, "survival of the best fitted") in his 1864 work ''Principles of Biology'' to characterise what [[Charles Darwin]] had called [[natural selection]].
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| The British biologist [[J.B.S. Haldane]] was the first to quantify fitness, in terms of the [[modern evolutionary synthesis]] of Darwinism and [[Mendelian genetics]] starting with his 1924 paper ''[[A Mathematical Theory of Natural and Artificial Selection]]''. The next further advance was the introduction of the concept of [[inclusive fitness]] by the British biologist [[W.D. Hamilton]] in 1964 in his paper on ''[[The Evolution of Social Behavior]]''.
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| == Fitness landscape == | |
| {{Main|Fitness landscape}}
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| [[File:Fitness-landscape-cartoon.png|thumb|right|Natural selection pushes fitness towards nearby peaks, but lacks the foresight to select the highest peak.]]
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| A [[fitness landscape]], first conceptualized by [[Sewall Wright]], is a way of visualising fitness in terms of a high-dimensional surface. Height indicates fitness, while each of the other dimensions may represent allele identity at one gene, allele frequency at one gene, or one phenotypic trait. These options represent three different ways in which the term fitness landscape is used.<ref>{{cite book|last=Provine|first=William B.|title=Sewall Wright and Evolutionary Biology|year=1986|publisher=University of Chicago Press}}</ref> Peaks correspond to local fitness maxima; it is often said that natural selection always progresses uphill but can only do so locally. This can result in suboptimal local maxima becoming stable, because natural selection cannot return to the less-fit "valleys" of the landscape on the way to reach higher peaks.
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| == Genetic load ==
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| [[Genetic load]] measures the average fitness of a population of individuals, relative to a hypothetical population in which the most fit genotype has become [[fixation (population genetics)|fixed]].
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| Genetic load is the probability that an average individual will die or fail to reproduce because of its harmful genes. It is a number between 0 and 1 that measures the extent to which the average individual is inferior to the best individual.<ref name=Ridley>{{Cite web | last = Ridley | first = Mark | title = Evolution A-Z | work = Genetic load | publisher = Blackwell Publishing | url = http://www.blackwellpublishing.com/ridley/a-z/Genetic_load.asp | accessdate = April 17, 2011}}</ref>
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| If there are a number of genotypes in a population, each with its characteristic fitness; the genotype with the highest fitness is called W<sub>opt</sub>. The average fitness of the whole population is the fitness of each genotype multiplied by its frequency: this is called mean fitness. V symbolizes mean fitness. The formula for genetic load (L) is as follows:
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| L = (W<sub>opt</sub>-V)/(W<sub>opt</sub>)
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| If all the individuals in the population have the optimal genotype, then v = W<sub>opt</sub> and the load is zero. If all but one have a genotype of zero fitness then v = 0 and L = 1.<ref name=Ridley/>
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| == See also ==
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| * [[Gene-centered view of evolution]]
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| * [[Inclusive fitness]]
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| * [[Natural selection]]
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| * [[Reproductive success]]
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| * [[Selection coefficient]]
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| * [[Universal Darwinism]]
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| ==Notes==
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| {{Reflist}}
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| ==Further reading==
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| * [[Elliott Sober|Sober, E.]] (2001). The Two Faces of Fitness. In R. Singh, D. Paul, C. Krimbas, and J. Beatty (Eds.), ''Thinking about Evolution: Historical, Philosophical, and Political Perspectives''. Cambridge University Press, pp. 309–321. [http://philosophy.wisc.edu/sober/tff.pdf Full text]
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| * {{cite journal |author=Orr HA |title=Fitness and its role in evolutionary genetics |journal=Nat. Rev. Genet. |volume=10 |issue=8 |pages=531–9 |date=August 2009 |pmid=19546856 |doi=10.1038/nrg2603 |pmc=2753274}}
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| == External links ==
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| * [http://www.blackwellpublishing.com/ridley/a-z/Fitness.asp Evolution A-Z: Fitness]
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| * [http://plato.stanford.edu/entries/fitness/ Stanford Encyclopedia of Philosophy entry]
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| [[Category:Evolutionary biology]]
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| [[Category:Population genetics]]
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