r/K selection theory

In ecology, r/K selection theory relates to the selection 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 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 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 ecology, r/K selection theory relates to the selection 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 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 terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson in 1967 based on their work on island biogeography; although the concept of the evolution of life history strategies has a longer history (see e.g. plant strategies). The theory was popular in the 1970s and 1980s, when it was used as a heuristic 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 many of its important themes. In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection. These terms, r and K, are drawn from standard ecological algebra as illustrated in the simplified Verhulst model of population dynamics: where N is the population, r is the maximum growth rate, K is the carrying capacity of the local environment, and dN/dt, the derivative 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 Kapazitätsgrenze (capacity limit), while r came from rate. r-selected species are those that emphasize high growth rates, typically exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K). A typical r species is the dandelion (genus Taraxacum). In unstable or unpredictable environments, r-selection predominates due to the ability to reproduce 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, small body size, early maturity onset, 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 that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods and small mammals, particularly rodents. As with K-selection, below, the r/K paradigm (Differential K theory) has controversially been associated with human behavior and separately evolved populations.