R-selection

In ecology, r/K selection theory relates to the selection of traits (in organisms) that allow success in particular environments. The theory originates from work on island biogeography by the ecologists Robert MacArthur and E. O. Wilson (MacArthur & Wilson, 1967).

Overview
In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two stereotyped directions: r- or K-selection. These terms, r and K, are derived from standard ecological algebra, as illustrated in the simple Verhulst equation of population dynamics:

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

Where $$r$$ is the growth rate of the population ($$N$$), and $$K$$ is its carrying capacity.

Typically, r-selected species produce many offspring, each of which is unlikely to survive to adulthood, while K-selected species invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.

r/K selection and environmental stability
In unstable or upredictable environments r-selection predominates, as the ability to reproduce quickly is crucial, and there is little advantage in adaptations that permit successful competition with other organisms (since the environment is likely to change again). Traits that are thought to be characteristic of r-selection include: high fecundity; small size; 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 with r-selected traits range from bacteria and diatoms, through insects and weeds, to various semelparous cephalopods and mammals.

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 are typically very constant and close to the maximum that the environment can bear. Traits that are thought to be characteristic of K-selection include: large size; long life span; and the production of fewer offspring that are well cared for. 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 elephants, humans, trees and whales.

r/K as a continuous spectrum
It should be noted that, although some organisms are primarily r- or K-strategists, the majority of organisms fall between these two ecological extremes, and may display traits considered characteristic of both ends of the r-K spectrum. For instance, trees have traits such as longevity and strong competitiveness that characterise them as K-strategists. However, in reproduction, trees typically produce thousands of offspring and disperse them widely, traits characteristic of r-strategists.

J. Philippe Rushton's highly controversial application of this theory to different human races is considered scientific racist pseudoscience by most biologists.