r/K selection theory

In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of individual parental investment of r-strategists, or on a reduced quantity of offspring with a corresponding increased parental investment of K-strategists, varies widely, seemingly to promote success in particular environments. The concepts of quantity or quality offspring are sometimes referred to as "cheap" or "expensive", a comment on the expendable nature of the offspring and parental commitment made.^[1] The stability of the environment can predict if many expendable offspring are made or if fewer offspring of higher quality would lead to higher reproductive success. An unstable environment would encourage the parent to make many offspring, because the likelihood of all (or the majority) of them surviving to adulthood is slim. In contrast, more stable environments allow parents to confidently invest in one offspring because they are more likely to survive to adulthood.

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.

The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson in 1967^[2] based on their work on island biogeography;^[3] although the concept of the evolution of life history strategies has a longer history^[4] (see e.g. plant strategies).

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 it was criticized by several empirical studies.^[5][6] A life-history paradigm has replaced the r/K selection paradigm, but continues to incorporate its important themes as a subset of life history theory.^[7] Some scientists now prefer to use the terms fast versus slow life history as a replacement for, respectively, r versus K reproductive strategy.^[8]

Overview

 

A litter of rats 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. The same applies to mice.

In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection.^[2] These terms, r and K, are drawn from standard ecological algebra as illustrated in the simplified Verhulst model of population dynamics:^[9]

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

 

where N is the population, r is the maximum growth rate, K is the carrying capacity of the local environment, and dN/dt, the derivative of N with respect to time t, is the rate of change in population with time. Thus, the equation relates the growth rate of the population N to the current population size, incorporating the effect of the two constant parameters r and K. (Note that decrease is negative growth.) The choice of the letter K came from the German Kapazitätsgrenze (capacity limit), while r came from rate.

r-selection

r-selected species are those that emphasize high growth rates, typically exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K).^[10] A typical r species is the dandelion (genus Taraxacum).

In unstable or unpredictable environments, r-selection predominates due to the ability to reproduce rapidly. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Among the traits that are thought to characterize r-selection are 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 that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods and small mammals, particularly rodents.

K-selection

 

A bald eagle, an individual of a typical K-strategist species. K-strategists have longer life expectancies, produce fewer offspring and tend to be altricial, requiring extensive care by parents when young.

By contrast, K-selected species display traits associated with living at densities close to carrying capacity and typically are strong competitors in such crowded niches, that invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (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".^[10]

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 in number 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, 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.^[11] Organisms with K-selected traits include large organisms such as elephants, humans, and whales, but also smaller long-lived organisms such as Arctic terns,^[12] parrots and eagles.

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.^[13]

Similarly, reptiles such as sea turtles 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.

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.^[14]

Ecological succession

In areas of major ecological disruption or sterilisation (such as after a major volcanic eruption, as at Krakatoa or Mount St. 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.^[15]

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 endemic species.^[16] 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.

Application

While usually applied at the level of species, r/K selection theory is also useful in studying the evolution of ecological and life history differences between subspecies, for instance the African honey bee, A. m. scutellata, and the Italian bee, A. m. ligustica.^[17] At the other end of the scale, it has also been used to study the evolutionary ecology of whole groups of organisms, such as bacteriophages.^[18] Other researchers have proposed that the evolution of human inflammatory responses is related to r/K selection.^[19]

Some researchers, such as Lee Ellis, J. Philippe Rushton, and Aurelio José Figueredo, have attempted to applied r/K selection theory to various human behaviors, including crime,^[20] sexual promiscuity, fertility, IQ, and other traits related to life history theory.^[21][22] Rushton's developed "differential K theory" to attempt to explain variations in behavior across human races.^[22][23] Differential K theory has been debunked as being devoid of empirical basis, and has also been described as a key example of scientific racism.^[24][25][26]

Status

Although r/K selection theory became widely used during the 1970s,^{[27][28][29][30]} it also began to attract more critical attention.^{[31][32][33][34]} In particular, a review in 1977 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.^[35]

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.^[36] 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.^[37] Several other studies contradicting the predictions of r/K selection theory were also published between 1977 and 1994.^{[38][39][40][41]}

When Stearns reviewed the status of the theory again in 1992,^[42] 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.^[43]

More recently, the panarchy theories of adaptive capacity and 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.^[44]

Writing in 2002, Reznick and colleagues reviewed the controversy regarding r/K selection theory and concluded 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;^[31] Stearns 1976,^[45] 1977^[35]). The r- and K-selection paradigm was replaced by new paradigm that focused on age-specific mortality (Stearns, 1976;^[45] Charlesworth, 1980^[46]). 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.

— Reznick, Bryant and Bashey, 2002^[7]

Alternative approaches are now available both for studying life history evolution (e.g. Leslie matrix for an age-structured population) and for density-dependent selection (e.g. variable density lottery model^[47]).

See also

• Evolutionary game theory
• Life history theory
• Minimax/maximin strategy
• Ruderal species
• Semelparity and iteroparity
• Trivers–Willard hypothesis

References

1. "r and K selection". www.bio.miami.edu. Retrieved 2020-10-27.
2. Pianka, E.R. (1970). "On r and K selection". American Naturalist. 104 (940): 592–597. doi:10.1086/282697. S2CID 83933177.
3. MacArthur, R.; Wilson, E.O. (1967). The Theory of Island Biogeography (2001 reprint ed.). Princeton University Press. ISBN 978-0-691-08836-5.
4. For example: Margalef, R. (1959). "Mode of evolution of species in relation to their places in ecological succession". XVTH International Congress of Zoology.
5. Roff, Derek A. (1993). Evolution Of Life Histories: Theory and Analysis. Springer. ISBN 978-0-412-02391-0.
6. Stearns, Stephen C. (1992). The Evolution of Life Histories. Oxford University Press. ISBN 978-0-19-857741-6.
7. Reznick, D; Bryant, MJ; Bashey, F (2002). (PDF). Ecology. 83 (6): 1509–1520. doi:10.1890/0012-9658(2002)083[1509:RAKSRT]2.0.CO;2. Archived from the original (PDF) on 2010-12-30. Retrieved 2013-05-11.
8. Jeschke, Jonathan M.; Kokko, Hanna (2009). "The roles of body size and phylogeny in fast and slow life histories". Evolutionary Ecology. 23 (6): 867–878. doi:10.1007/s10682-008-9276-y. S2CID 38289373.
9. Verhulst, P.F. (1838). "Notice sur la loi que la population pursuit dans son accroissement". Corresp. Math. Phys. 10: 113–121.
10. 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. Bibcode:1998ApEnM..64.3776W. doi:10.1128/AEM.64.10.3776-3783.1998. PMC 106546. PMID 9758799.
11. "r and K selection" 2014-09-05 at the Wayback Machine. University of Miami Department of Biology. Retrieved February 4, 2011.
12. John H. Duffus; Douglas M. Templeton; Monica Nordberg (2009). Concepts in Toxicology. Royal Society of Chemistry. p. 171. ISBN 978-0-85404-157-2.
13. Hrdy, Sarah Blaffer (2000), "Mother Nature: Maternal Instincts and How They Shape the Human Species" (Ballantine Books)
14. Reluga, T.; Medlock, J.; Galvani, A. (2009). "The discounted reproductive number for epidemiology". Mathematical Biosciences and Engineering. 6 (2): 377–393. doi:10.3934/mbe.2009.6.377. PMC 3685506. PMID 19364158.
15. Gunderson, Lance H.; Holling, C.S. (2001). Panarchy: Understanding Transformations In Human And Natural Systems. Island Press. ISBN 978-1-55963-857-9.
16. McNeely, J. A. (1994). "Lessons of the past: Forests and Biodiversity". Biodiversity and Conservation. 3: 3–20. CiteSeerX 10.1.1.461.5908. doi:10.1007/BF00115329. S2CID 245731.
17. Fewell, Jennifer H.; Susan M. Bertram (2002). "Evidence for genetic variation in worker task performance by African and European honeybees". Behavioral Ecology and Sociobiology. 52 (4): 318–25. doi:10.1007/s00265-002-0501-3. S2CID 22128779.
18. Keen, E. C. (2014). "Tradeoffs in bacteriophage life histories". Bacteriophage. 4 (1): e28365. doi:10.4161/bact.28365. PMC 3942329. PMID 24616839.
19. VAN BODEGOM, D.; MAY, L.; MEIJ, H. J.; WESTENDORP, R. G. J. (2007). "Regulation of Human Life Histories: The Role of the Inflammatory Host Response". Annals of the New York Academy of Sciences. 1100 (1): 84–97. Bibcode:2007NYASA1100...84V. doi:10.1196/annals.1395.007. PMID 17460167. S2CID 43589115.
20. Ellis, Lee (1987-01-01). "Criminal behavior and r/K selection: An extension of gene‐based evolutionary theory". Deviant Behavior. 8 (2): 149–176. doi:10.1080/01639625.1987.9967739. ISSN 0163-9625.
21. Figueredo, Aurelio José; Vásquez, Geneva; Brumbach, Barbara Hagenah; Schneider, Stephanie M. R. (2007-03-01). "The K-factor, Covitality, and personality". Human Nature. 18 (1): 47–73. doi:10.1007/bf02820846. ISSN 1045-6767. PMID 26181744. S2CID 10877330.
22. Weizmann, Fredric; Wiener, Neil I.; Wiesenthal, David L.; Ziegler, Michael (1990). "Differential K theory and racial hierarchies". Canadian Psychology. 31 (1): 1–13. doi:10.1037/h0078934.
23. Peregrine, P (2003). "Cross-cultural evaluation of predicted associations between race and behavior". Evolution and Human Behavior. 24 (5): 357–364. doi:10.1016/s1090-5138(03)00040-0.
24. Winston, Andrew S. (29 May 2020). "Scientific Racism and North American Psychology". Oxford Research Encyclopedias: Psychology. doi:10.1093/acrefore/9780190236557.013.516. ISBN 978-0-19-023655-7.
25. Weizmann, Frederic; Wiener, Neil I.; Wiesenthal, David L.; Ziegler, Michael (1989). "Scientific racism in contemporary psychology". International Journal of Dynamic Assessment & Instruction. 1:1: 81–93.
26. "Statement from the Department of Psychology regarding research conducted by Dr. J. Philippe Rushton". Department of Psychology, University of Western Ontario.
27. Gadgil, M.; Solbrig, O.T. (1972). "Concept of r-selection and K-selection — evidence from wild flowers and some theoretical consideration" (PDF). Am. Nat. 106 (947): 14–31. doi:10.1086/282748. JSTOR 2459833. S2CID 86412666.
28. Long, T.; Long, G. (1974). "Effects of r-selection and K-selection on components of variance for 2 quantitative traits". Genetics. 76 (3): 567–573. doi:10.1093/genetics/76.3.567. PMC 1213086. PMID 4208860.
29. Grahame, J. (1977). "Reproductive effort and r-selection and K-selection in 2 species of Lacuna (Gastropoda-Prosobranchia)". Mar. Biol. 40 (3): 217–224. doi:10.1007/BF00390877. S2CID 82459157.
30. Luckinbill, L.S. (1978). "r and K selection in experimental populations of Escherichia coli". Science. 202 (4373): 1201–1203. Bibcode:1978Sci...202.1201L. doi:10.1126/science.202.4373.1201. PMID 17735406. S2CID 43276882.
31. Wilbur, H.M.; Tinkle, D.W.; Collins, J.P. (1974). "Environmental certainty, trophic level, and resource availability in life history evolution". American Naturalist. 108 (964): 805–816. doi:10.1086/282956. JSTOR 2459610. S2CID 84902967.
32. Barbault, R. (1987). "Are still r-selection and K-selection operative concepts?". Acta Oecologica-Oecologia Generalis. 8: 63–70.
33. 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. doi:10.1007/BF02514572. S2CID 9459529.
34. Getz, W.M. (1993). "Metaphysiological and evolutionary dynamics of populations exploiting constant and interactive resources – r-K selection revisited". Evolutionary Ecology. 7 (3): 287–305. doi:10.1007/BF01237746. S2CID 21296836.
35. Stearns, S.C. (1977). (PDF). Annu. Rev. Ecol. Syst. 8: 145–171. doi:10.1146/annurev.es.08.110177.001045. Archived from the original (PDF) on 2008-12-16.
36. Parry, G.D. (March 1981). "The Meanings of r- and K-selection". Oecologia. 48 (2): 260–4. Bibcode:1981Oecol..48..260P. doi:10.1007/BF00347974. PMID 28309810. S2CID 30728470.
37. Templeton A.R.; Johnson, J.S. (1982). "Life History Evolution Under Pleiotropy and K-selection in a Natural Population of Drosophila mercatorum". In Barker, J.S.F.; Starmer, William T. (eds.). Ecological genetics and evolution: the cactus-yeast-drosophila model system. Academic Press. pp. 225–239. ISBN 978-0-12-078820-0.
38. Snell, Terry W.; King, Charles E. (December 1977). "Lifespan and Fecundity Patterns in Rotifers: The Cost of Reproduction". Evolution. 31 (4): 882–890. doi:10.2307/2407451. JSTOR 2407451. PMID 28563718.
39. Taylor, Charles E.; Condra, Cindra (November 1980). "r- and K-Selection in Drosophila pseudoobscura". Evolution. 34 (6): 1183–93. doi:10.2307/2408299. JSTOR 2408299. PMID 28568469.
40. Hollocher, H.; Templeton, A.R. (April 1994). "The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. VI. The non-neutrality of the Y chromosome rDNA polymorphism". Genetics. 136 (4): 1373–84. doi:10.1093/genetics/136.4.1373. PMC 1205918. PMID 8013914.
41. Templeton, A.R.; Hollocher, H.; Johnston, J.S. (June 1993). "The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. V. Female phenotypic expression on natural genetic backgrounds and in natural environments". Genetics. 134 (2): 475–85. doi:10.1093/genetics/134.2.475. PMC 1205491. PMID 8325484.
42. Stearns, S.C. (1992). The Evolution of Life Histories. Oxford University Press. ISBN 978-0-19-857741-6.
43. Graves, J. L. (2002). "What a tangled web he weaves Race, reproductive strategies and Rushton's life history theory". Anthropological Theory. 2 (2): 2 131–154. doi:10.1177/1469962002002002627. S2CID 144377864.
44. Gunderson, L. H. and Holling C. S. (2001) Panarchy: Understanding Transformations in Human and Natural Systems Island Press. ISBN 9781597269391.
45. Stearns, S.C. (1976). "Life history tactics: a review of the ideas". Quarterly Review of Biology. 51 (1): 3–47. doi:10.1086/409052. PMID 778893. S2CID 37813334.
46. Charlesworth, B. (1980). Evolution in age structured populations. Cambridge, UK: Cambridge University Press.
47. Bertram, Jason; Masel, Joanna (October 2019). "Density-dependent selection and the limits of relative fitness". Theoretical Population Biology. 129: 81–92. doi:10.1016/j.tpb.2018.11.006. PMID 30664884.