Inecology,r/K selection theory relates to theselection of combinations oftraits 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 reduced individualparental investment ofr-strategists, or on a reduced quantity of offspring with a corresponding increased parental investment ofK-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.
The terminology ofr/K-selection was coined by the ecologistsRobert MacArthur andE. O. Wilson in 1967[2] based on their work onisland 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 aheuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies.[5][6] Alife-history paradigm has replaced ther/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 termsfast versusslow life history as a replacement for, respectively,r versusK reproductive strategy.[8]
Inr/K selection theory, selective pressures arehypothesised to driveevolution in one of two generalized directions:r- orK-selection.[2] These terms,r andK, are drawn from standard ecologicalformula as illustrated in the simplifiedVerhulst model ofpopulation dynamics:[9]
whereN is thepopulation,r is the maximumgrowth rate,K is thecarrying capacity of the local environment, and dN / dt (thederivative of population sizeN with respect to timet) is the rate of change in population with time. Thus, the equation relates the growth rate of the populationN to the current population size, incorporating the effect of the two constant parametersr andK.(Note that when the population size is greater than the carrying capacity then 1 - N/K is negative, which indicates a population decline or negative growth.) The choice of the letterK came from theGermanKapazitätsgrenze (capacity limit), whiler came fromrate.
r-selected species are those that emphasize high growth rates, typically exploit less-crowdedecological niches, and produce manyoffspring, each of which has a relatively low probability of surviving to adulthood (i.e., highr, lowK).[10] A typicalr species is the dandelion (genusTaraxacum).
In unstable or unpredictable environments,r-selection predominates due to the ability toreproduce 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 characterizer-selection are highfecundity, smallbody size, early maturity onset, shortgeneration time, and the ability todisperse offspring widely.
Organisms whose life history is subject tor-selection are often referred to asr-strategists orr-selected. Organisms that exhibitr-selected traits can range frombacteria anddiatoms, toinsects andgrasses, to varioussemelparouscephalopods, certain families of birds, such asdabbling ducks, and smallmammals, particularlyrodents.
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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, thatinvest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., lowr, highK). Inscientific literature,r-selected species are occasionally referred to as "opportunistic" whereasK-selected species are described as "equilibrium".[10]
In stable or predictable environments,K-selection predominates as the ability tocompete successfully for limited resources is crucial and populations ofK-selected organisms typically are very constant in number and close to the maximum that the environment can bear (unliker-selected populations, where population sizes can change much more rapidly).
Traits that are thought to be characteristic ofK-selection include large body size, longlife expectancy, and the production of fewer offspring, which often requireextensive parental care until they mature. Organisms whose life history is subject toK-selection are often referred to asK-strategists orK-selected.[11] Organisms withK-selected traits include large organisms such aselephants,sharks,humans, andwhales, but also smaller long-lived organisms such asArctic terns,[12]parrots, andeagles.
Although some organisms are identified as primarilyr- orK-strategists, the majority of organisms do not follow this pattern. For instance, trees have traits such as longevity and strong competitiveness that characterise them asK-strategists. In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic ofr-strategists.[13]
Similarly,reptiles such assea turtles display bothr- andK-traits: Although sea turtles are large organisms with long lifespans (provided they reach adulthood), they produce large numbers of unnurtured offspring.
Ther/K dichotomy can be re-expressed as a continuous spectrum using the economic concept ofdiscounted future returns, withr-selection corresponding to large discount rates andK-selection corresponding to small discount rates.[14]
In areas of major ecological disruption or sterilisation (such as after a majorvolcanic eruption, as atKrakatoa orMount St. Helens),r- andK-strategists play distinct roles in theecological succession that regenerates theecosystem. Because of their higher reproductive rates and ecological opportunism, primary colonisers typically arer-strategists and they are followed by a succession of increasingly competitiveflora andfauna. The ability of an environment to increase energetic content, through photosynthetic capture of solar energy, increases with the increase in complexbiodiversity asr species proliferate to reach a peak possible withK strategies.[15]
Eventually a new equilibrium is approached (sometimes referred to as aclimax community), withr-strategists gradually being replaced byK-strategists which are more competitive and better adapted to the emerging micro-environmental characteristics of thelandscape. Traditionally, biodiversity was considered maximized at this stage, with introductions of new species resulting in the replacement andlocal extinction ofendemic species.[16] However, theintermediate 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.
While usually applied at the level of species,r/K selection theory is also useful in studying the evolution of ecological andlife 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 theevolutionary ecology of whole groups of organisms, such asbacteriophages.[18] Other researchers have proposed that the evolution of humaninflammatory responses is related tor/K selection.[19]
Some researchers, such asLee Ellis,J. Philippe Rushton, andAurelio José Figueredo, have attempted to applyr/K selection theory to various human behaviors, includingcrime,[20]sexual promiscuity, fertility,IQ, and other traits related tolife history theory.[21][22] Rushton developed "differentialK theory" to attempt to explain variations in behavior acrosshuman races.[22][23] DifferentialK theory has been debunked as being devoid of empirical basis, and has also been described as a key example ofscientific racism.[24][25][26]
Althoughr/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 ecologistStephen 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 ther/K selection literature by Parry demonstrated that there was no agreement among researchers using the theory about the definition ofr- andK-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 ofDrosophila mercatorum underK-selection the population actually produced a higher frequency of traits typically associated withr-selection.[37] Several other studies contradicting the predictions ofr/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 thatr/K theory was a once useful heuristic that no longer serves a purpose in life history theory.[43]
More recently, thepanarchy theories ofadaptive capacity andresilience promoted byC. 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 regardingr/K selection theory and concluded that:
The distinguishing feature of ther- andK-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]). Ther- andK-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 ther–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 densitylottery model[47]).