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In ecology, r/K selection theory relates to the selection of combinations of traits that trade off the quantity and quality of offspring to promote success in particular environments. The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson[1] based on their work on island biogeography.[2]

Contents

Overview

In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection.[1] These terms, r and K, are derived from standard ecological algebra, as illustrated in the simple Verhulst equation of population dynamics:[3]

 \frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)

where r is the growth rate of the population (N), and K is the carrying capacity of its local environmental setting. Typically, r-selected species exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood. In contrast, K-selected species are strong competitors in crowded niches, and invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood. In the scientific literature, r-selected species are occasionally referred to as "opportunistic", while K-selected species are described as "equilibrium".[4]

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r-selection (unstable environments)

In unstable or unpredictable environments, r-selection predominates as the ability to reproduce quickly is crucial. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Traits that are thought to be characteristic of r-selection include: high fecundity, small body size, early maturity onset, short generation time, and the ability to disperse offspring widely. Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms with r-selected traits range from bacteria and diatoms, through insects and weeds, to various semelparous cephalopods and mammals, especially small rodents.

K-selection (stable environments)

In stable or predictable environments, K-selection predominates as the ability to 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). Traits that are thought to be characteristic of K-selection include: large body size, long life expectancy, and the production of fewer offspring that 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. Organisms with K-selected traits include large organisms such as elephants, trees, humans and whales, but also smaller, long-lived organisms such as Arctic Terns. The climate that demonstrate predictably variable characteristics of a stable climate is the oceanic climate. The most stable climate in this sense would be found in Northwest Europe. The warm Gulf Stream and the cool temperature of the northern latitude form a natural front in which predictably variable characteristics are found to the greatest extent. The very location would be southern Lofoten which is also known for the greatest positive temperature anomaly in the world.

As a continuous spectrum

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. Similarly, reptiles such as sea turtles display both r- and K-traits: although large organisms with long lifespans (should they reach adulthood), they produce large numbers of unnurtured offspring.

In ecological succession

In areas of major ecological disruption or sterilisation (such as after a major 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.[5] 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. Typically, biodiversity is maximised at this stage, with introductions of new species resulting in the replacement and local extinction of endemic species.[6]

Status of the theory

Although r/K selection theory became widely used during the 1970s,[7][8][9][10] it also began to attract more critical attention.[11][12][13][14] In particular, an influential review by the ecologist Stephen Stearns drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it.[15] Nonetheless, the concept of r/K selection is still used in biological and ecological research.[16][17][18][19]

In Humans

In the 1995 book Race, Evolution, and Behavior, J. Philippe Rushton, a psychologist and head of the Pioneer Fund, argues that different human races exhibit reproductive strategies characteristic of r/K selection theory. Rushton asserts that Asians, who have slightly longer lifespans and lower rates of reproduction relative to other races, are at the K end of the spectrum, while Africans, for whom the opposite is true, are more r-strategists[20].

Rushton's research has been heavily criticized,[21] and other studies have contradicted many of his claims.[22]

See also

References

  1. ^ a b Pianka, E.R. (1970). On r and K selection. American Naturalist 104, 592-597.
  2. ^ MacArthur, R. and Wilson, E.O. (1967). The Theory of Island Biogeography, Princeton University Press (2001 reprint), ISBN 0-691-08836-5M.
  3. ^ Verhulst, P.F. (1838). Notice sur la loi que la population pursuit dans son accroissement. Corresp. Math. Phys. 10, 113-121.
  4. ^ For example: Weinbauer, M.G.; Höfle, M.G. (1 October 1998). "Distribution and Life Strategies of Two Bacterial Populations in a Eutrophic Lake". Appl. Environ. Microbiol. 64 (10): 3776–3783. PMID 9758799. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=106546.  
  5. ^ Gunderson, L. & Holling, C.S. (Eds) (2001), "Panarchy: Understanding Transformations in Human and Natural Systems" (Island Press)
  6. ^ McNeely, J. A. (1994) Lessons of the past: Forests and Biodiversity. Biodiversity and Conservation 3, 3-20.
  7. ^ Gadgil, M. and Solbrig, O.T. (1972). Concept of r-selection and K-selection - evidence from wild flowers and some theoretical consideration. Am. Nat. 106, 14.
  8. ^ Long, T. and Long, G. (1974). Effects of r-selection and K-selection on components of variance for 2 quantitative traits. Genetics 76, 567-573.
  9. ^ Grahame, J. (1977). Reproductive effort and r-selection and K-selection in 2 species of Lacuna (Gastropoda-Prosobranchia). Mar. Biol. 40, 217-224.
  10. ^ Luckinbill, L.S. (1978). r and K selection in experimental populations of Escherichia coli. Science 202, 1201-1203.
  11. ^ Wilbur, H.M. (1974). "Environmental certainty, trophic level, and resource availability in life history evolution". American Naturalist 108: 805–816. doi:10.1086/282956.  
  12. ^ Barbault, R. (1987). Are still r-selection and K-selection operative concepts? Acta Oecologica-Oecologia Generalis 8, 63-70.
  13. ^ Kuno, E. (1991). Some strange properties of the logistic equation defined with r and K - inherent defects or artifacts. Researches on Population Ecology 33, 33-39.
  14. ^ Getz, W.M. (1993). Metaphysiological and evolutionary dynamics of populations exploiting constant and interactive resources - r-K selection revisited. Evolutionary Ecology 7, 287-305.
  15. ^ Stearns, S.C. (1977). "Evolution of life-history traits - critique of theory and a review of data" (PDF). Ann. Rev. Of Ecology and Systematics 8: 145–171. doi:10.1146/annurev.es.08.110177.001045. http://faculty.washington.edu/kerrb/Stearns1977.pdf.  
  16. ^ Caroli, L.; Capizzi, D. and Luiselli, L. (2000). "Reproductive Strategies and Life-history Traits of the Savi's Pine Vole, Microtus savii". Zoological Science 17: 209–216. doi:10.2108/zsj.17.209.  
  17. ^ Chikatsu, N.; Nakamura, Y., Sato, H., Fujita, T., Asano, S. and Motokura, T. (2002). "p53 mutations and tetraploids under r- and K-selection". Oncogene 21: 3043–3049. doi:10.1038/sj/onc/1205413.  
  18. ^ Okada, H.; Harada, H., Tsukiboshi, T. and Araki, M. (2005). "Characteristics of Tylencholaimus parvus (Nematoda: Dorylaimida) as a fungivorus nematode". Nematology 7: 843–849. doi:10.1163/156854105776186424.  
  19. ^ Hamer, A.J.; Mahony, M.J. (2007). "Life history of an endangered amphibian challenges the declining species paradigm". Australian Journal of Zoology 55: 79–88. doi:10.1071/ZO06093.  
  20. ^ Rushton, J. P. (1997). Race, Evolution, and Behavior: A Life History Perspective, Charles Darwin Research Institute, ISBN 0-9656836-2-1.
  21. ^ Barash, D.P. (1995). "Book review: Race, Evolution, and Behavior". Animal Behaviour 49: 1131–1133.  
  22. ^ Cernovsky, Z.Z. (1995). "On the similarities of American blacks and whites: A reply to J.P. Rushton". Journal of Black Studies 25: 672–679. http://www.euvolution.com/euvolution/blackwhite.html.  

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