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In
ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
, ''r''/''K'' selection theory relates to the
selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strategie ...
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 Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure (e.g. time, energy, resources) that benefits offspring.Clutton-Brock, T.H. 1991. ''The Evolution of Parental Care''. Princeton, NJ: Princeton ...
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. 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 of ''r''/''K''-selection was coined by the ecologists
Robert MacArthur Robert Helmer MacArthur (April 7, 1930 – November 1, 1972) was a Canadian-born American ecologist who made a major impact on many areas of community and population ecology. Early life and education MacArthur was born in Toronto, Ontario, ...
and
E. O. Wilson Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, entomologist and writer. According to David Attenborough, Wilson was the world's leading expert in his specialty of myrmecology, the study of an ...
in 1967 based on their work on
island biogeography Insular biogeography or island biogeography is a field within biogeography that examines the factors that affect the species richness and diversification of isolated natural communities. The theory was originally developed to explain the pattern of ...
; although the concept of the evolution of life history strategies has a longer history (see e.g.
plant strategies Plant strategies include mechanisms and responses plants use to reproduce, defend, survive, and compete on the landscape. The term “plant strategy” has existed in the literature since at least 1965, however multiple definitions exist. Strategies ...
). The theory was popular in the 1970s and 1980s, when it was used as a
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate, ...
device, but lost importance in the early 1990s, when it was criticized by several empirical studies. 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. Some scientists now prefer to use the terms ''fast'' versus ''slow'' life history as a replacement for, respectively, ''r'' versus ''K'' reproductive strategy.


Overview

In ''r''/''K'' selection theory, selective pressures are
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
ed to drive
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
in one of two generalized directions: ''r''- or ''K''-selection. These terms, ''r'' and ''K'', are drawn from standard ecological
algebra Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics. Elementary a ...
as illustrated in the simplified Verhulst model of
population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of populations as dynamical systems. History Population dynamics has traditionally been the dominant branch of mathematical biology, which has ...
: \frac = rN \left(1 - \frac\right) where ''N'' is the
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
, ''r'' is the maximum growth rate, ''K'' is the
carrying capacity The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
of the local environment, and ''dN/dt'', the
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. F ...
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 German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...
''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 niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
s, and produce many
offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
, each of which has a relatively low probability of surviving to adulthood (i.e., high ''r'', low ''K'').For example: A typical ''r'' species is the dandelion (genus ''
Taraxacum ''Taraxacum'' () is a large genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The scientific and hobby study of the genus is known as taraxacology. The genus is native to Eurasia and Nor ...
''). In unstable or unpredictable environments, ''r''-selection predominates due to the ability to
reproduce Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...
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 Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
, small
body size Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...
, early maturity onset, short
generation time In population biology and demography, generation time is the average time between two consecutive generations in the lineages of a population. In human populations, generation time typically ranges from 22 to 33 years. Historians sometimes use this ...
, 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 Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
and
diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s, to
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s and
grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns an ...
es, to various
semelparous Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characteri ...
cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s and small
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, particularly
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s.


''K''-selection

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 Investment is the dedication of money to purchase of an asset to attain an increase in value over a period of time. Investment requires a sacrifice of some present asset, such as time, money, or effort. In finance, the purpose of investing is ...
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 : ''For a broader class of literature, see Academic publishing.'' Scientific literature comprises scholarly publications that report original empirical and theoretical work in the natural and social sciences. Within an academic field, scient ...
, ''r''-selected species are occasionally referred to as "opportunistic" whereas ''K''-selected species are described as "equilibrium". In stable or predictable environments, ''K''-selection predominates as the ability to
compete Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
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 Life expectancy is a statistical measure of the average time an organism is expected to live, based on the year of its birth, current age, and other demographic factors like sex. The most commonly used measure is life expectancy at birth ...
, 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. Organisms with ''K''-selected traits include large organisms such as
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...
s,
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s, and
whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s, but also smaller long-lived organisms such as
Arctic tern The Arctic tern (''Sterna paradisaea'') is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south a ...
s,
parrot Parrots, also known as psittacines (), are birds of the roughly 398 species in 92 genera comprising the order Psittaciformes (), found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoid ...
s and
eagle Eagle is the common name for many large birds of prey of the family Accipitridae. Eagles belong to several groups of genera, some of which are closely related. Most of the 68 species of eagle are from Eurasia and Africa. Outside this area, just ...
s.


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.Hrdy, Sarah Blaffer (2000), "Mother Nature: Maternal Instincts and How They Shape the Human Species" (Ballantine Books) Similarly,
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s such as
sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, ...
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. 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.


Ecological succession

In areas of major ecological disruption or sterilisation (such as after a major
volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...
eruption, as at
Krakatoa Krakatoa (), also transcribed (), is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group (Krakatoa archipelago) comprising four islands. Tw ...
or
Mount St. Helens Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
), ''r''- and ''K''-strategists play distinct roles in the
ecological succession Ecological succession is the process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire) or more or less. Bacteria allows for the cycling of nutrients such as ca ...
that regenerates the
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
. 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 Flora is all the plant life present in a particular region or time, generally the naturally occurring (indigenous) native plants. Sometimes bacteria and fungi are also referred to as flora, as in the terms '' gut flora'' or '' skin flora''. E ...
and
fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
. The ability of an environment to increase energetic content, through photosynthetic capture of solar energy, increases with the increase in complex
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
as ''r'' species proliferate to reach a peak possible with ''K'' strategies. Eventually a new equilibrium is approached (sometimes referred to as a
climax community In scientific ecology, climax community or climatic climax community is a historic term for a community of plants, animals, and fungi which, through the process of ecological succession in the development of vegetation in an area over time, hav ...
), 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 A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or man-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes the ...
. Traditionally, biodiversity was considered maximized at this stage, with introductions of new species resulting in the replacement and
local extinction Local extinction, also known as extirpation, refers to a species (or other taxon) of plant or animal that ceases to exist in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinct ...
of
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
species. 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''. At the other end of the scale, it has also been used to study the
evolutionary ecology Evolutionary ecology lies at the intersection of ecology and evolutionary biology. It approaches the study of ecology in a way that explicitly considers the evolutionary histories of species and the interactions between them. Conversely, it can ...
of whole groups of organisms, such as
bacteriophages A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacterio ...
. Other researchers have proposed that the evolution of human
inflammatory response Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molecu ...
s is related to ''r''/''K'' selection. Some researchers, such as
Lee Ellis Altis Lee Ellis (born March 1, 1942) is an American sociologist who was a professor of sociology at Minot State University from 1976 to 2007. He was also an assistant and associate professor there from 1976 to 2009, after which he served as a visi ...
,
J. Philippe Rushton John Philippe Rushton (December 3, 1943 – October 2, 2012) was a Canadian psychologist and author. He taught at the University of Western Ontario until the early 1990s, and became known to the general public during the 1980s and 1990s for resea ...
, and Aurelio José Figueredo, have applied ''r''/''K'' selection theory to various human behaviors, including
crime In ordinary language, a crime is an unlawful act punishable by a State (polity), state or other authority. The term ''crime'' does not, in modern criminal law, have any simple and universally accepted definition,Farmer, Lindsay: "Crime, definit ...
,
sexual promiscuity Promiscuity is the practice of engaging in sexual activity frequently with different partners or being indiscriminate in the choice of sexual partners. The term can carry a moral judgment. A common example of behavior viewed as promiscuous by man ...
, fertility, IQ, and other traits related to
life history theory Life history theory is an analytical frameworkVitzthum, V. (2008). Evolutionary models of women's reproductive functioning. ''Annual Review of Anthropology'', ''37'', 53-73 designed to study the diversity of life history strategies used by differen ...
. Rushton's work resulted in him developing " differential ''K'' theory" to attempt to explain many variations in human behavior across geographic areas, a theory which has been criticized by many other researchers.


Status

Although ''r''/''K'' selection theory became widely used during the 1970s, it also began to attract more critical attention. In particular, a review by the ecologist
Stephen C. Stearns Stephen C. Stearns (born December 12, 1946, in Kapaau, Hawaii and raised in Hawi, Hawaii) is an American biologist, and the Edward P. Bass Professor of Ecology and Evolutionary Biology Emeritus at Yale University. He is known for his work in lif ...
drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it. 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. A 1982 study by Templeton and Johnson showed that in a population of ''
Drosophila mercatorum ''Drosophila mercatorum'' is a species of fruit fly in the genus ''Drosophila'', ''repleta'' subgroup, described by Patterson and Wheeler in 1942.''Systema Dipterorum''. Pape T. & Thompson F.C. (eds), 2011 Thought to be native to South America, ...
'' under ''K''-selection the population actually produced a higher frequency of traits typically associated with ''r''-selection. Several other studies contradicting the predictions of ''r''/''K'' selection theory were also published between 1977 and 1994. When Stearns reviewed the status of the theory in 1992, 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. More recently, the
panarchy Panarchy may refer to: * Panarchy (Dartmouth), student society at Dartmouth College * Panarchy (ecology) A social-ecological system consists of 'a bio-geo-physical' unit and its associated social actors and institutions. Social-ecological system ...
theories of
adaptive capacity Adaptive capacity relates to the capacity of systems, institutions, humans and other organisms to adjust to potential damage, to take advantage of opportunities, or to respond to consequences. In the context of ecosystems, adaptive capacity is deter ...
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. Writing in 2002, Reznick and colleagues reviewed the controversy regarding ''r''/''K'' selection theory and concluded that: Alternative approaches are now available both for studying life history evolution (e.g.
Leslie matrix The Leslie matrix is a discrete, age-structured model of population growth that is very popular in population ecology named after Patrick H. Leslie. The Leslie matrix (also called the Leslie model) is one of the most well-known ways to describe ...
for an age-structured population) and for density-dependent selection (e.g. variable density lottery model).


See also

*
Evolutionary game theory Evolutionary game theory (EGT) is the application of game theory to evolving populations in biology. It defines a framework of contests, strategies, and analytics into which Darwinian competition can be modelled. It originated in 1973 with John Ma ...
*
Life history theory Life history theory is an analytical frameworkVitzthum, V. (2008). Evolutionary models of women's reproductive functioning. ''Annual Review of Anthropology'', ''37'', 53-73 designed to study the diversity of life history strategies used by differen ...
* Minimax/maximin strategy *
Ruderal species A ruderal species is a plant species that is first to colonize disturbed lands. The disturbance may be natural for example, wildfires or avalanchesor the consequences of human activities, such as construction ( of roads, of buildings, mining, et ...
*
Semelparity and iteroparity Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characteri ...
*
Trivers–Willard hypothesis In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard in 1973, suggests that female mammals adjust the sex ratio of offspring in response to maternal condition, ...


References

{{DEFAULTSORT:R K Selection Theory Ecological theories Evolutionary biology concepts Mating systems Population ecology Race and intelligence controversy Selection