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The theory of strategies r and K, a simplification of reality

In evolutionary ecology, one of the most widely accepted theories was the r/K theory of selection, developed by Wilson and MacArthur in 1967. This theory relates the number of offspring, and the energy investment that parents put into them, with selection. of inherited traits.

According to this theory, the net growth rate of a population would be related to two constants, the maximum growth rate of the species, represented by the letter “ r ” (from the German rate ), and the carrying capacity of the environment, i.e. , the maximum amount of resources it can provide, represented by the letter “ K ” (from German Kapazitätsgrenze ).

Faced with this fact, there would be two strategies that living beings can take, two directions in which selective pressure can direct the evolutionary process of a species. One focused on maximizing the reproductive rate, at the expense of less investment in the care of the offspring, and another in which there would be just enough offspring to reach the carrying capacity without exceeding it, with very few offspring and investing more effort. and resources in them.

Strategy r vs. Strategy K

Species that have evolved following the r -strategy have a massive number of offspring , with very short gestation periods. They spend little time caring for their young, and the parents themselves may function as significant factors in selection pressure from the start, either deliberately, or because of the very competition between the pups for a little more care and attention. resources than the rest, parents are faced with the question of which of their children to feed . In this way, each individual individual has a very low chance of success and most of the offspring die before leaving offspring.

In the theoretical model, they are species with a very high “ r ”, and a very low “ K ”. This type of strategy is very successful in unstable or unpredictable ecological niches, and is associated with very fertile species, of small size, with early maturity and high dispersal capacity. Among the r -strategist mammals are the rodents or, among the plants, the grasses.

On the contrary, the living beings that follow the K strategy invert those values . They are species that have a high “K” and a very low “r”. Large organisms with long life expectancies belong to this group, which produce very few offspring throughout their lives, but dedicate a lot of time and resources to their upbringing , so their offspring have a very high chance of success. It is not the parents who select which child to feed, since they only have one, and they can dedicate all their resources to it.

The K strategy is very successful in very stable ecosystems . Mammals with this type of strategy are, for example, elephants or large cetaceans. Among the plants, some stand out, such as the coconut palm.

neither black nor white

One of the difficulties of the r/K selection theory is that, in reality, very few species fall clearly into one of the two strategies , most are distributed along an intermediate spectrum; some that are “more of the K ” or “more of the r ” than others—a boar is more K than a rabbit, but more r than a rhinoceros—but they cannot be definitively cataloged in the pattern. And in fact, some species can’t even fit into that spectrum .

For example, a leatherback turtle can measure more than two meters in length and weigh up to 600 kilos. They are incredibly long-lived animals, which can exceed 80 years, and do not reach sexual maturity until they are approximately 8 years old. All of them are typical features of a species that follows a K strategy. And yet, every three or four years they lay more than 100 eggs, which hatch after two months. Thousands of eggs throughout a lifetime, most of which will not reach adulthood. A characteristic feature of the r strategy.

With many trees something similar happens. The olive tree is an extraordinarily slow-growing tree, which does not begin to bear olives until it is more than 5 or 6 years old, and which can live for more than a thousand years. Characteristics of a K strategist, who, however, can give more than 20,000 olives each year.

new theories

Physics has frictionless hockey pucks;
thermodynamics has Carnot machines; and ecology
evolutionary has r/K selection.

—Laurence D. Mueller (1997)


The r/K selection theory turned out to be very illustrative for concrete examples, but insufficient to explain natural diversity, let alone population dynamics . His models were very scarce because they oversimplified the enormous complexity of nature and its intricate relationships, to the point that their use to interpret certain results was considered archaic and quite naive.

Demographic models gradually replaced the r/K selection theory, incorporating not only growth rates and carrying capacity, but also patterns of mortality, population density, resource availability, environmental fluctuations, relationships with other beings alive and many other variables related to the evolutionary history of the population to be studied.

However, as long as it is taken into account that it is a simplification, the scenario provided by the r/K selection theory continues to have great educational value . It allows us to show trends, establish comparisons between species that exhibit different behaviors, or even teach, through the knowledge gaps it leaves, that science always establishes provisional knowledge that, in the future, can be corrected, improved, expanded or supplemented. with new data.


Bertram, J. et al. 2019. Density-dependent selection and the limits of relative fitness.

Theoretical Population Biology, 129, 81-92. DOI: 10.1016/j.tpb.2018.11.006 MacArthur, R. H. et al. 1967. The Theory of Island Biogeography. Princeton University Press.

Mueller, L. D. 1997. Theoretical and Empirical Examination of Density-Dependent Selection. Annual Review of Ecology and Systematics, 28, 269-288.

Pianka, E. R. 1970. On r- and K-Selection. The American Naturalist, 104(940), 592-597. DOI: 10.1086/282697

Reznick, D. et al. 2002. r- and K-selection revised: the role of population regulation in life-history evolution. Ecology, 83(6), 1509-1520. DOI: 10.1890/0012-9658(2002)083[1509:RAKSRT]2.0.CO;2

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