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{{Short description|Expected reproductive success}}
{{evolutionary biology}}
{{merge from|Organismal performance|date=July 2024}}
'''Fitness''' (often denoted '''<math>w</math>''' or '''ω''' in [[population genetics]] models) is
With [[asexual reproduction]], it is sufficient to assign fitnesses to genotypes. With [[sexual reproduction]],
The term "Darwinian fitness" can be 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> Fitness does not include a measure of survival or life-span; [[Herbert Spencer]]'s well-known phrase "[[survival of the fittest]]" should be interpreted as: "Survival of the form (phenotypic or genotypic) that will leave the most copies of itself in successive generations."
[[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. To avoid double counting, inclusive fitness excludes the contribution of other individuals to the survival and reproduction of the focal individual. One mechanism of inclusive fitness is [[kin selection]].
==Fitness
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
Alternatively, "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|978-0-87893-271-9}}</ref>
== Models of fitness
===Absolute fitness===
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Absolute fitnesses can be used to calculate relative fitness, since <math>p(t+1)=n(t+1)/N(t+1)=(W/\overline{W})p(t)</math> (we have used the fact that <math>N(t+1)=\overline{W} N(t) </math>, where <math>\overline{W}</math> is the mean absolute fitness in the population). This implies that <math>w/\overline{w}=W/\overline{W}</math>, or in other words, relative fitness is proportional to <math>W/\overline{W}</math>. It is not possible to calculate absolute fitnesses from relative fitnesses alone, since relative fitnesses contain no information about changes in overall population abundance <math>N(t)</math>.
Assigning relative fitness values to genotypes is mathematically appropriate when two conditions are met: first, the population is at demographic equilibrium, and second, individuals vary in their birth rate, contest ability, or death rate, but not a combination of these traits.<ref>{{cite journal |last1=Bertram |first1=Jason |last2=Masel |first2=Joanna |title=Density-dependent selection and the limits of relative fitness |journal=Theoretical Population Biology |date=January 2019 |volume=129 |pages=81–92 |doi=10.1016/j.tpb.2018.11.006|pmid=30664884 |doi-access=free |bibcode=2019TPBio.129...81B }}</ref>
=== Change in genotype frequencies due to selection ===
[[File:Selective sweep, frequency vs time.jpg|thumb|Increase in frequency over time of genotype <math>A</math>, which has a 1% greater relative fitness than the other genotype present, <math>B</math>
The change in genotype frequencies due to selection follows immediately from the definition of relative fitness,
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== History ==
[[File:Herbert Spencer.jpg|thumb|[[Herbert Spencer]]
The [[United Kingdom|British]] [[sociologist]] [[Herbert Spencer]] coined the phrase "[[survival of the fittest]]" in his 1864 work ''Principles of Biology'' to characterise what [[Charles Darwin]] had called [[natural selection]].<ref name="sotf">{{cite web | url=http://www.darwinproject.ac.uk/entry-5140#back-mark-5140.f5 | title=Letter 5140 – Wallace, A. R. to Darwin, C. R., 2 July 1866 | publisher=Darwin Correspondence Project | access-date=12 January 2010}}<br />{{cite web | url=http://www.darwinproject.ac.uk/entry-5145#mark-5145.f3 | title=Letter 5145 – Darwin, C. R. to Wallace, A. R., 5 July (1866) | publisher=Darwin Correspondence Project | access-date=12 January 2010}}<br />
^ "Herbert Spencer in his ''Principles of Biology'' of 1864, vol. 1, p. 444, wrote: 'This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called "natural selection", or the preservation of favoured races in the struggle for life.'" {{Citation | url=http://works.bepress.com/cgi/viewcontent.cgi?article=1000&context=maurice_stucke | title=Better Competition Advocacy | access-date=29 August 2007 | author=Maurice E. Stucke}}, citing HERBERT SPENCER, THE PRINCIPLES OF BIOLOGY 444 (Univ. Press of the Pac. 2002.)</ref>
The British-Indian biologist [[J.B.S. Haldane]] was the first to quantify fitness, in terms of the [[Modern synthesis (20th century)|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 Genetical Evolution of Social Behaviour]]''.
== Genetic load ==
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* [http://plato.stanford.edu/entries/fitness/ Stanford Encyclopedia of Philosophy entry]
{{Evolution}}
{{Authority control}}▼
{{Evolutionary psychology}}
{{Population genetics}}
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[[Category:Evolutionary biology concepts]]
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[[Category:Population genetics]]
[[Category:Sexual selection]]
[[Category:Natural selection]]
[[Category:Darwinism]]
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