In
ecologyEcology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
,
r/K selection theory relates to the
selectionNatural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
of combinations of traits in an organism that trade off between quantity or quality of offspring. The focus upon either increased quantity of offspring at the expense of individual
parental investmentIn evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
, or reduced quantity of offspring with a corresponding increased parental investment, is varied to promote success in particular environments. The theory was popular in the 1970's and 1980's when it was used as a heuristic device, but lost importance in the early 1990's as it was criticized by several empirical studies. The r/K selection paradigm has been replaced by a "life-history" paradigm. However, this continues to incorporate many of the themes important to the r/K paradigm.
The terminology of r/K-selection was coined by the ecologists
Robert MacArthurRobert Helmer MacArthur was an American ecologist who made a major impact on many areas of community and population ecology....
and
E. O. WilsonEdward Osborne Wilson is an American biologist, researcher , theorist , naturalist and author. His biological specialty is myrmecology, the study of ants....
based on their work on
island biogeographyIsland biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...
.
Overview
In r/K selection theory, selective pressures are
hypothesisA hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
ed to drive
evolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
in one of two generalized directions:
r- or
K-selection. These terms, r and K, are drawn from standard ecological
algebraAlgebra is the branch of mathematics concerning the study of the rules of operations and relations, and the constructions and concepts arising from them, including terms, polynomials, equations and algebraic structures...
, as illustrated in the simplified Verhulst model of
population dynamicsPopulation dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes...
:
where
r is the maximum growth rate of the
populationA population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
(
N), and
K is the
carrying capacityThe carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment...
of its local environmental setting. As the name implies,
r-selected species are those that place an emphasis on a high growth rate, and typically exploit less-crowded
ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s and produce many
offspringIn biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
, each of which has a relatively low probability of surviving to adulthood (i.e. high
r, low
K). 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
investIn evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e. low
r, high
K). In the
scientific literatureScientific literature comprises scientific publications that report original empirical and theoretical work in the natural and social sciences, and within a scientific field is often abbreviated as the literature. Academic publishing is the process of placing the results of one's research into the...
, r-selected species are occasionally referred to as "opportunistic", while K-selected species are described as "equilibrium".
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
fecundityFecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...
, small
body size, early maturity onset, short generation time, and the ability to
disperseBiological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
offspring widely.
Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms who exhibit r-selected traits can range from
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and diatoms, to insects and
weedA weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...
s, to various semelparous
cephalopodA cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s and
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, particularly small
rodentRodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s.
K-selection (stable environments)
In stable or predictable environments, K-selection predominates as the ability to
competeCompetition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...
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 expectancyLife expectancy is the expected number of years of life remaining at a given age. It is denoted by ex, which means the average number of subsequent years of life for someone now aged x, according to a particular mortality experience...
, and the production of fewer offspring, which 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
elephantElephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s,
treeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s,
humanHumans are the only living species in the Homo genus...
s and
whaleWhale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, but also smaller, long-lived organisms such as
Arctic TernThe Arctic Tern is a seabird of the tern family Sternidae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America...
s.
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,
reptileReptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s such as
sea turtleSea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...
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.
In ecological succession
In areas of major ecological disruption or sterilisation (such as after a major
volcanicVulcanism may refer to* Volcanism or volcanic activity.* Plutonism, a scientific theory of the Earth....
eruption, as at
KrakatoaKrakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...
or Mount Saint Helens), r- and K-strategists play distinct roles in the
ecological successionEcological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
that regenerates the
ecosystemAn ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
. 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
floraFlora is the plant life occurring in a particular region or time, generally the naturally occurring or indigenous—native plant life. The corresponding term for animals is fauna.-Etymology:...
and
faunaFauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
. The ability of an environment to increase energetic content, through photosynthetic capture of solar energy, increases with the increase in complex
biodiversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
as r species proliferate to reach a peak possible with K strategies.
Eventually a new equilibrium is approached (sometimes referred to as a
climax communityIn ecology, a climax community, or climatic climax community, is a biological community of plants and animals which, through the process of ecological succession — the development of vegetation in an area over time — has reached a steady state. This equilibrium occurs because the 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
landscapeLandscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...
. Traditionally, biodiversity was considered maximized at this stage, with introductions of new species resulting in the replacement and
local extinctionLocal extinction, also known as extirpation, is the condition of a species which ceases to exist in the chosen geographic area of study, though it still exists elsewhere...
of endemic species. However, the
Intermediate Disturbance HypothesisThe Intermediate Disturbance Hypothesis states that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem...
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.
Status of the theory
In the 1970s and 1980s, several studies failed to produce experimental corroboration of the r/K theory, and, as a result, r/K selection theory was discarded by biologists studying life-history evolution by the early nineties. Even though hundreds of papers used r/K selection theory to analyze life history data, and attempted to fit them into the r/K selection model, not a single study was able to demonstrate a correlation between fluctuation and adaptation or to show a tradeoff between r- and K-selected traits.
Although r/K selection theory became widely used during the 1970s, it also began to attract more critical attention. In particular, an influential review by the ecologist
Stephen C. StearnsStephen C. Stearns is an American biologist, the Edward P. Bass Professor of Ecology and Evolutionary Biology at Yale University...
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 lead 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 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" by theories of
Adaptive CapacityAdaptive capacity is the capacity of a system to adapt if the environment where the system exists is changing. It is applied to e.g., ecological systems and human social systems.As applied to ecological systems, the adaptive capacity is determined by :...
and
ResilienceResilience is the property of a material to absorb energy when it is deformed elastically and then, upon unloading to have this energy recovered. In other words, it is the maximum energy per unit volume that can be elastically stored...
promoted by C. Holling and Lance Gunderson, have revived interest in the theory, as a way of integrating social systems, economics and ecology.
Reznick and colleagues (2002) reviewed the controversy regarding the 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."
In humans
In his 1995 book
Race, Evolution, and BehaviorRace, Evolution, and Behavior: A Life History Perspective is a controversial book written by J...
, the
psychologistPsychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...
J. Philippe RushtonJean Philippe Rushton is a Canadian psychology professor at the University of Western Ontario who is most widely known for his work on racial group differences, such as research on race and intelligence, race and crime, and the application of r/K selection theory to humans in his book Race,...
argues that different human races exhibit reproductive strategies characteristic of r/K selection theory. Rushton presents 60 different behavioral and anatomical variables, and asserts that East Asians, who have slightly longer lifespans, physically mature slower and are less sexually permissive relative to other races, are at the K end of the spectrum, while Africans, for whom the opposite is true, are more r-strategist. Rushton's work has formed the theoretical basis for later studies from the hereditarian school of
Race and IntelligenceThe connection between race and intelligence has been a subject of debate in both popular science and academic research since the inception of intelligence testing in the early 20th century...
studies, for example in the work by
Richard LynnRichard Lynn is a British Professor Emeritus of Psychology at the University of Ulster who is known for his views on racial and ethnic differences. Lynn argues that there are hereditary differences in intelligence based on race and sex....
and
Tatu VanhanenTatu Vanhanen is a Professor Emeritus of Political Science at the University of Tampere in Tampere, Finland...
. Among Rushton's many critics are evolutionary biologist
Joseph L. GravesJoseph L. Graves, Jr. is Associate Dean for Research and Professor of Biological Studies at the Joint School for Nanoscience and Nanoengineering which is jointly administered by North Carolina Agricultural and Technical State University and UNC Greensboro...
who has done extensive testing of the r/k selection theory with species of
drosophilaDrosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
flies, argues that not only is r/K selection theory considered to be virtually useless when applied to human life history evolution, but Rushton himself does not apply the theory correctly, and displays a lack of understanding of evolutionary theory in general. Graves also states that the sources for the biological data gathered in support of Rushton's hypothesis were misrepresented and that much of his social science data was collected by dubious means. Other scholars have subsequently argued against Rushton's hypothesis on the basis that the concept of race is not supported by genetic evidence about the diversity of human populations.
See also
- Adaptive capacity
Adaptive capacity is the capacity of a system to adapt if the environment where the system exists is changing. It is applied to e.g., ecological systems and human social systems.As applied to ecological systems, the adaptive capacity is determined by :...
- Evolutionary game theory
Evolutionary game theory is the application of Game Theory to evolving populations of lifeforms in biology. EGT is useful in this context by defining a framework of contests, strategies and analytics into which Darwinian competition can be modelled. It originated in 1973 with John Maynard Smith...
- Life history theory
Life history theory posits that the schedule and duration of key events in an organism's lifetime are shaped by natural selection to produce the largest possible number of surviving offspring...
- Ruderal species
A ruderal species is a plant species that is first to colonize disturbed lands. The disturbance may be natural , or due to human influence – constructional , or agricultural .Ruderal species typically dominate the disturbed area...
- Trivers–Willard hypothesis