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| {{lowercase|title=r/K selection theory}}
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| [[File:Eubalaena glacialis with calf.jpg|thumb|250px|A [[North Atlantic right whale]] with solitary calf. Whale reproduction follows a K-selection strategy, with few offspring, long gestation, long parental care, and a long period until sexual maturity.]]
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| In [[ecology]], '''r/K selection theory''' relates to the [[natural selection|selection]] of combinations of [[Trait (biological)|traits]] in a species that inversely relate [[parental investment]] and the quantity and quality of offspring. Each selection seems to promote success in different environments.
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| r-selection species spread parental investment across many offspring whereas K-selected species focus theirs on a few. Neither mode of propagation is intrinsically superior, and they can coexist in the same habitat; e.g., rodents and elephants. r/K selection theory aids studying the progression of ecological and historical differences between subspecies; e.g., the African honey bee, [[African bee|''A. m. scutellata'']], and the Italian bee, [[Italian bee|''A. m. ligustica'']].<ref name="Fewell and Bertram, 2002">{{cite journal|last=Fewell|first=Jennifer H.|coauthors=Susan M. Bertram|title=Evidence for genetic variation in worker task performance by African and European honeybees|journal=Behavioral Ecology and Sociobiology|year=2002|volume=52|pages=318-25}}</ref>
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| The theory was popular in the 1970s and 1980s, when it was used as a [[heuristic]] device, but lost importance in the early 1990s, when several empirical studies criticized it.<ref>{{cite book |first=Derek A. |last=Roff |title=Evolution Of Life Histories: Theory and Analysis |url=http://books.google.com/books?id=_pv37gw8CIoC |year=1993 |publisher=Springer |isbn=978-0-412-02391-0}}</ref><ref>{{cite book |first=Stephen C. |last=Stearns |title=The Evolution of Life Histories |year=1992 |publisher=Oxford University Press |isbn=978-0-19-857741-6 |url=http://www.amazon.com/dp/0198577419 }}</ref> A [[Life history theory|life-history]] paradigm has replaced the r/K selection paradigm and incorporates many of its important themes.<ref name=ReznickEA2002/>
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| The terminology of r/K-selection was coined by the ecologists [[Robert MacArthur]] and [[E. O. Wilson]]<ref name="Pianka, E.R 1970">{{cite journal | last1 = Pianka | first1 = E.R. | authorlink = Eric Pianka | year = 1970 | title = On r and K selection | journal = American Naturalist | volume = 104 | issue = 940| pages = 592–597 | url=http://www.zoology.siu.edu/sears/Pianka1970.pdf | doi = 10.1086/282697 }}</ref> based on their work on [[island biogeography]],<ref>{{cite book |authorlink1=Robert MacArthur |last1=MacArthur |first1=R. |authorlink2=E. O. Wilson |last2=Wilson |first2=E.O. |title=[[The Theory of Island Biogeography]] |publisher=Princeton University Press |year=1967 |isbn=0-691-08836-5M |edition=2001 reprint}}</ref> although the concept of the evolution of life history strategies has a longer history.<ref>For example: {{cite journal | first1= R. |last1=Margalef |authorlink=Ramon Margalef |title=Mode of evolution of species in relation to their places in ecological succession |year=1959 |journal=XVth International Congress of Zoology}}</ref>
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| ==Overview==
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| [[File:Mouse litter.jpg|thumb|250px|A litter of [[rat]]s with their mother. The reproduction of rats follows an r-selection strategy, with many offspring, short gestation, less parental care, and a short time until sexual maturity.]]
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| In r/K selection theory, selective pressures are [[hypothesis]]ed to drive [[evolution]] in one of two generalized directions: ''r''- or ''K''-selection.<ref name="Pianka, E.R 1970"/> These terms, r and K, are so drawn from standard ecological [[algebra]] as illustrated in the simplified [[Logistic function#In ecology: modeling population growth|Verhulst model]] of [[population dynamics]]:<ref>{{cite journal | last1 = Verhulst | first1 = P.F. | authorlink = Pierre François Verhulst | year = 1838 | title = Notice sur la loi que la population pursuit dans son accroissement | journal = Corresp. Math. Phys. | volume = 10 | pages = 113–121 |url=http://books.google.com.au/books?id=eRNbAAAAYAAJ&pg=PA113
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| }}</ref>
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| :<math> \frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right) </math>
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| wherein ''r'' is the maximum [[population growth rate|growth rate]] of the [[population]] (''N''), and ''K'' is the [[carrying capacity]] of its local environmental setting, and the notation ''dN/dt'' stands for the [[derivative]] of ''N'' with respect to ''t'' ([[time]]). The equation relates the rate of population change to the current population size and expresses the effect of the two parameters. As the name implies, so r-selected species emphasize high growth rate, typically exploit less-crowded [[ecological niche]]s, and produce many [[offspring]], each of which will likely die ere adulthood (i.e., high ''r'', low ''K'').<ref>For example: {{cite journal | last=Weinbauer | first=M.G. | coauthors=Höfle, M.G. | date=1 October 1998| title=Distribution and Life Strategies of Two Bacterial Populations in a Eutrophic Lake | journal=Appl. Environ. Microbiol. | volume=64 | pages=3776–3783 | pmid=9758799 | issue=10 | pmc=106546 }}</ref>
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| K-selected species display traits associated with living at densities close to carrying capacity, typically are strong competitors in crowded niches, and [[parental investment|invest]] more heavily in fewer offspring, each of which will likely mature (i.e., low ''r'', high ''K''). In [[scientific literature]], r-selected species are occasionally referred to as "opportunistic" whereas K-selected species are described as "equilibrium".<ref>For example: {{cite journal | last=Weinbauer | first=M.G. | coauthors=Höfle, M.G. | date=1 October 1998| title=Distribution and Life Strategies of Two Bacterial Populations in a Eutrophic Lake | journal=Appl. Environ. Microbiol. | volume=64 | pages=3776–3783 | pmid=9758799 | issue=10 | pmc=106546 }}</ref>
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| ===r-selection===
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| Environmental instability or unpredictability favors quick reproduction and renders useless competitive adaptations. Among the traits that are thought to characterize r-selection are high [[fecundity]], small [[allometry|body size]], early maturity, short generation time, and wide offspring dispersion.
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| Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms that exhibit r-selected traits can range from [[bacteria]] and [[diatoms]], to [[insects]] and [[weed]]s, to various [[semelparous]] [[cephalopod]]s and [[mammal]]s, particularly small [[rodent]]s.
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| ===K-selection===
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| In stable or predictable environments, K-selection predominates as the ability to [[Competition (biology)|compete]] successfully for limited resources is crucial and populations of K-selected organisms typically are very constant and close to the maximum that the environment can bear (unlike r-selected populations, where population sizes can change much more rapidly).
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| Traits that are thought to be characteristic of K-selection include large body size, long [[life expectancy]], and the production of fewer offspring, which often require extensive parental care until they mature. Organisms whose life history is subject to K-selection are often referred to as K-strategists or K-selected.<ref>[http://www.bio.miami.edu/tom/courses/bil160/bil160goods/16_rKselection.html "r and K selection"]. University of Miami Department of Biology. Retrieved February 4, 2011.</ref> Organisms with K-selected traits include large organisms such as [[elephant]]s, [[primate]]s and [[whale]]s, but also smaller, long-lived organisms such as [[Arctic Tern]]s.<ref>{{cite book|author1=John H. Duffus|author2=Douglas M. Templeton|author3=Monica Nordberg|title=Concepts in Toxicology|url=http://books.google.com/books?id=MK2WC70Iu3MC|year=2009|publisher=Royal Society of Chemistry|isbn=978-0-85404-157-2|page=[http://books.google.com.ph/books?id=MK2WC70Iu3MC&pg=PA171 171]}}</ref>
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| ===Continuous spectrum===
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| Although some organisms are identified as primarily r- or K-strategists, the majority of organisms do not follow this pattern. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.<ref>Hrdy, Sarah Blaffer (2000), "Mother Nature: Maternal Instincts and How They Shape the Human Species" (Ballantine Books)</ref>
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| Similarly, [[reptile]]s such as [[sea turtle]]s display both r- and K-traits: although sea turtles are large organisms with long lifespans (provided they reach adulthood), they produce large numbers of unnurtured offspring. Mammalian males tend to be r-type reproducers, whereas females tend to have K characteristics.<ref>Hrdy, Sarah Blaffer (2000), "Mother Nature: Maternal Instincts and How They Shape the Human Species" (Ballantine Books)</ref>
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| The r/K dichotomy can be re-expressed as a continuous spectrum using the economic concept of discounted future returns, with r-selection corresponding to large discount rates and K-selection corresponding to small discount rates.{{fact|date=October 2013}}
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| ==Ecological succession==
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| In areas of major ecological disruption or sterilisation (such as after a major [[volcanism|volcanic]] eruption, as at [[Krakatoa]] or [[Mount Saint Helens]]), r- and K-strategists play distinct roles in the [[ecological succession]] that regenerates the [[ecosystem]]. Because of their higher reproductive rates and ecological opportunism, primary colonisers typically are r-strategists and they are followed by a succession of increasingly competitive [[flora]] and [[fauna]]. The ability of an environment to increase energetic content, through photosynthetic capture of solar energy, increases with the increase in complex [[biodiversity]] as r species proliferate to reach a peak possible with K strategies.<ref>{{cite book |first1=Lance H. |last1=Gunderson |first2=C.S. |last2=Holling |title=Panarchy: Understanding Transformations In Human And Natural Systems |url=http://books.google.com/books?id=DHcjtSM5TogC |year=2001 |publisher=Island Press |isbn=978-1-55963-857-9}}</ref>
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| Eventually a new equilibrium is approached (sometimes referred to as a [[climax community]]), with r-strategists gradually being replaced by K-strategists which are more competitive and better adapted to the emerging micro-environmental characteristics of the [[landscape]]. Traditionally, biodiversity was considered maximized at this stage, with introductions of new species resulting in the replacement and [[local extinction]] of [[Endemism|endemic]] species.<ref>{{cite journal | last1 = McNeely | first1 = J. A. | year = 1994 | title = Lessons of the past: Forests and Biodiversity | url = http://www.springerlink.com/content/t76125571tr97m64/ | journal = Biodiversity and Conservation | volume = 3 | pages = 3–20 | doi = 10.1007/BF00115329 }}</ref> However, the [[Intermediate Disturbance Hypothesis]] posits that intermediate levels of disturbance in a landscape create patches at different levels of succession, promoting coexistence of colonizers and competitors at the regional scale.
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| ==Status==
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| Although r/K selection theory became widely used during the 1970s,<ref>{{cite journal | last1 = Gadgil | first1 = M. | last2 = Solbrig | first2 = O.T. | year = 1972 | title = Concept of r-selection and K-selection — evidence from wild flowers and some theoretical consideration | journal = Am. Nat. | volume = 106 | issue =947 | pages = 14–31 |jstor=2459833}}</ref><ref>{{cite journal | last1 = Long | first1 = T. | last2 = Long | first2 = G. | year = 1974 | title = Effects of r-selection and K-selection on components of variance for 2 quantitative traits | journal = Genetics | volume = 76 | issue = 3| pages = 567–573 | pmid = 4208860 | pmc = 1213086 }}</ref><ref>{{cite journal | last1 = Grahame | first1 = J. | year = 1977 | title = Reproductive effort and r-selection and K-selection in 2 species of ''Lacuna'' (Gastropoda-Prosobranchia) | journal = Mar. Biol. | volume = 40 | issue = 3| pages = 217–224 | doi = 10.1007/BF00390877 }}</ref><ref>{{cite journal | last1 = Luckinbill | first1 = L.S. | year = 1978 | title = r and K selection in experimental populations of ''Escherichia coli'' | journal = Science | volume = 202 | issue = 4373| pages = 1201–1203 | doi = 10.1126/science.202.4373.1201 | pmid = 17735406 }}</ref> it also began to attract more critical attention.<ref name=wilbur74>{{cite journal | last=Wilbur | first=H.M. | year=1974 | title=Environmental certainty, trophic level, and resource availability in life history evolution | journal=American Naturalist | volume=108 | pages=805–816 | jstor=2459610 |last2=Tinkle |first=2D.W. |last3=Collins |first3=J.P. | issue=964 }}</ref><ref>{{cite journal | last1 = Barbault | first1 = R. | year = 1987 | title = Are still r-selection and K-selection operative concepts? | journal = Acta Oecologica-Oecologia Generalis | volume = 8 | pages = 63–70 }}</ref><ref>{{cite journal | last1 = Kuno | first1 = E. | year = 1991 | title = Some strange properties of the logistic equation defined with r and K – inherent defects or artifacts | journal = Researches on Population Ecology | volume = 33 | pages = 33–39 | doi = 10.1007/BF02514572 }}</ref><ref>{{cite journal | last1 = Getz | first1 = W.M. | year = 1993 | title = Metaphysiological and evolutionary dynamics of populations exploiting constant and interactive resources – r-K selection revisited | journal = Evolutionary Ecology | volume = 7 | issue = 3| pages = 287–305 | doi = 10.1007/BF01237746 }}</ref> In particular, a review by the ecologist [[Stephen C. Stearns]] drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it.<ref>{{cite journal|author=Stearns, S.C.|year=1977|title=Evolution of life-history traits – critique of theory and a review of data|journal=Ann. Rev. of Ecology and Systematics|volume=8|pages=145–171|url=http://faculty.washington.edu/kerrb/Stearns1977.pdf|format=PDF|doi=10.1146/annurev.es.08.110177.001045}}</ref>
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| In 1981, a review of the r/K selection literature by Parry demonstrated that there was no agreement among researchers using the theory about the definition of r and K selection, which led him to question whether the assumption of a relation between reproductive expenditure and packaging of offspring was justified.<ref>{{cite journal |last=Parry |first=G.D. |title=The Meanings of r- and K-selection |journal=Oecologia |volume=48 |issue=2 |pages=260–4 |year=March 1981 |doi=10.1007/BF00347974 |url=http://link.springer.com/article/10.1007/BF00347974}}</ref> A 1982 study by Templeton and Johnson, showed that in a population of ''[[Drosophila mercatorum]]'' under K selection the population actually produced a higher frequency of traits typically associated with r selection.<ref>{{cite book |last1=Templeton A.R. |first2=J.S. |last2=Johnson |chapter=Life History Evolution Under Pleiotropy and K-selection in a Natural Population of Drosophila mercatorum |pages=225–239 |editor1-first=J.S.F. |editor1-last=Barker |editor2-first=W.T. |editor2-last=Starmer |title=Ecological genetics and evolution: the cactus-yeast-drosophila model system |url=http://books.google.com/books?id=soDwAAAAMAAJ |year=1982 |publisher=Academic Press |isbn=978-0-12-078820-0}}</ref> Several other studies contradicting the predictions of r/K selection theory were also published between 1977 and 1994.<ref>{{cite journal |first1=Terry W. |last1=Snell |first2=Charles E. |last2=King |title=Lifespan and Fecundity Patterns in Rotifers: The Cost of Reproduction |journal=Evolution |volume=31 |issue=4 |pages=882–890 |date=December 1977 }}</ref><ref>{{cite journal |first1=Charles E. |last1=Taylor |first2=Cindra |last2=Condra |title=r- and K-Selection in Drosophila pseudoobscura |journal=Evolution |volume=34 |issue=6 |pages=1183–93 |date=November 1980 }}</ref><ref>{{cite journal |last1=Hollocher |first1=H. |last2=Templeton |first2=A.R. |title=The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. VI. The non-neutrality of the Y chromosome rDNA polymorphism |journal=Genetics |volume=136 |issue=4 |pages=1373–84 |date=April 1994 |pmid=8013914 |pmc=1205918 |url=http://www.genetics.org/cgi/pmidlookup?view=long&pmid=8013914}}</ref><ref>{{cite journal |last1=Templeton |first1=A.R. |last2=Hollocher |first2=H. |last3=Johnston |first3=J.S. |title=The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. V. Female phenotypic expression on natural genetic backgrounds and in natural environments |journal=Genetics |volume=134 |issue=2 |pages=475–85 |date=June 1993 |pmid=8325484 |pmc=1205491 |url=http://www.genetics.org/cgi/pmidlookup?view=long&pmid=8325484}}</ref>
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| When Stearns reviewed the status of the theory in 1992<ref>{{cite book | title=The Evolution of Life Histories | last=Stearns | first=S.C. | year=1992 | publisher=Oxford University Press | isbn=978-0-19-857741-6 }}</ref> he noted that from 1977 to 1982 there was an average of 42 references to the theory per year in the BIOSIS literature search service, but from 1984 to 1989 the average dropped to 16 per year and continued to decline. He concluded that r/K theory was a once useful heuristic that no longer serves a purpose in life history theory.<ref>{{cite journal | last=Graves | first=J. L. | year=2002 | url=http://ant.sagepub.com/content/2/2/131.short | title=What a tangled web he weaves Race, reproductive strategies and Rushton's life history theory | journal=Anthropological Theory | volume=2 | pages=2 131–154 | doi=10.1177/1469962002002002627 | issue=2 }}</ref>
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| More recently, the [[Panarchy#Panarchy_in_systems_theory|panarchy]] theories of [[adaptive capacity]] and [[Resilience (ecology)|resilience]] promoted by [[C. S. Holling]] and Lance Gunderson have revived interest in the theory, and use it as a way of integrating social systems, economics and ecology.<ref>Gunderson, L. H. and Holling C. S. (2001) [http://books.google.co.nz/books?hl=en&lr=&id=o4u89akUhJMC&oi=fnd&pg=PR7&dq=%22Panarchy:+Understanding+Transformations+in+Human+and+Natural+Systems%22&ots=ec5F9tgS-H&sig=9fzjcjMRO6N8sCWRieYofxzxUno#v=onepage&q=%22Panarchy%3A%20Understanding%20Transformations%20in%20Human%20and%20Natural%20Systems%22&f=false ''Panarchy: Understanding Transformations in Human and Natural Systems''] Island Press. ISBN 9781597269391.</ref>
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| In 2002, Reznick and colleagues reviewed the controversy regarding r/K selection theory and wrote that: "The distinguishing feature of the r- and K-selection paradigm was the focus on density-dependent selection as the important agent of selection on organisms’ life histories. This paradigm was challenged as it became clear that other factors, such as age-specific mortality, could provide a more mechanistic causative link between an environment and an optimal life history (Wilbur et al. 1974; Stearns 1976, 1977). The r- and K-selection paradigm was replaced by new paradigm that focused on age-specific mortality (Stearns, 1976; Charlesworth, 1980). This new life-history paradigm has matured into one that uses age-structured models as a framework to incorporate many of the themes important to the r–K paradigm."<ref name=ReznickEA2002>Reznick D, Bryant MJ and Bashey F (2002) [http://www2.hawaii.edu/~taylor/z652/Reznicketal.pdf "r-and K-selection revisited: the role of population regulation in life-history evolution"] ''Ecology'', '''83''' (6): 1509–1520.
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| {{doi|10.1890/0012-9658(2002)083[1509:RAKSRT]2.0.CO;2}}</ref>
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| ==See also==
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| * [[Evolutionary game theory]]
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| * [[Ruderal species]]
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| * [[Trivers–Willard hypothesis]]
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| ==References==
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| {{Reflist|30em}}
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| {{modelling ecosystems|expanded=other}}
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| {{DEFAULTSORT:R K Selection Theory}}
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| [[Category:Ecological theories]]
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| [[Category:Mating systems]]
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| [[Category:Population ecology]]
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| [[Category:Race and intelligence controversy]]
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| [[Category:Selection]]
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