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A K-reproductive strategy is a set of life history traits that would be selected for when a population reaches highest density. The terminology is taken from a parameter in population growth models, where carrying capacity (K) is the maximum population size that can be supported by an environment. The opposite strategy is an r reproductive strategy, after the parameter for the maximum intrinsic rate of natural increase (rmax) at theoretically zero density.
Origin of the Concept
The terms r- and K-strategy originated from “r-selection” and “K-selection,” which were first coined by MacArthur and Wilson (1967). On a hypothetical island with a starting point of abundant resources, these resources would become limited as the island is fully occupied. Using this example, the authors proposed that different selection regimes would operate as resource limitation experienced by a population changes between two opposing ends of density dependence – low to high density. Under this scenario, high density and thus high level of competition over resources would favor traits that maximize carrying capacity, hence the K-selection.
Pianka (1970) expanded the concept of r- and K-selection to develop a theory of life history strategies. He considered that the life history of an organism is a position on a continuum between a quantitative extreme (the r-endpoint) and a qualitative extreme (the K-endpoint). Importantly, Pianka developed a set of predictions for the life history correlates expected under K- and r-selection. In a high-density environment, K-selection would favor traits that allow for organisms to persist in the face of scarce resources such as high competitive ability, delayed maturation, longer lifespan, larger body size, and iteroparity. A suite of opposite traits would be expected under a low-density environment through r-selection. These alternative sets of life history traits became known as “K-strategies” and “r-strategies.”
The idea of r- and K-selection, and Pianka’s broadening of the idea into life history strategies, have been influential to the study of life history evolution. Most of all, it appealed to the desire to enumerate a simple yet generalizable explanation for life history diversity. Here the degree of density dependence was used as a major axis to explain the covariation of certain life history traits across species. For example, based on an inverse correlation between rmax and body size, Pianka compared insects and vertebrates for their nonoverlapping distribution of body size to suggest bimodality in life history traits among relatively r- and K-selected organisms. Similarly, later studies incorporated the comparative approach for different species or different populations of the same species. But differences in the density experienced by organisms were often only assumed or inferred but not directly tested for its causal contribution to life history differences.
Numerous empirical and theoretical studies have pointed out the limited applicability of r- and K-selection theory. First, in experimental evolution studies, fruit flies (Mueller 1988) or pitcher plant mosquitoes (Bradshaw and Holzapfel 1989) bred under high or low densities evolved differences in competitive ability, consistent with MacArthur and Wilson’s predictions. However, their life history traits did not differ by breeding lines as Pianka would predict. Second, mathematical models of selection have shown limitations of the r- and K-selection theory. For example, the theory assumes that r and K approximate fitness, but this assumption would be valid only when population growth rates follow the logistic equation and not when we take into account of factors such as population dynamics or age structure. Current theories, including those put forward by Charlesworth (1980) and Stearns (1992), further incorporate age-specific mortality or density-independent factors such as environmental stability to understand the causal link between the environment and an optimal life history. Theoretical consideration has also provided no support for the idea that density-dependent selection will explain the evolution of semelparity vs. iteroparity.
Due to these problems, the theory of r- and K-selection was gradually replaced by models that describe specific conditions of population density dependence and the magnitude of environmental effects. As accurate measures of these factors are hard to evaluate in natural populations, challenges remain for current paradigms of life history evolution.
- Charlesworth, B. (1980). Evolution in age-structured populations. Cambridge: Cambridge University Press.Google Scholar
- MacArthur, R. H., & Wilson, E. O. (1967). The theory of island biogeography. Princeton, New Jersey: Princeton University Press.Google Scholar
- Stearns, S. C. (1992). The evolution of life histories (Vol. 249). Oxford: Oxford University Press.Google Scholar