Live fast and die young, or slow and steady wins the race?
One of the best-known schemes for classifying life history diversity was also one of the first proposed. In 1967, Robert MacArthur and Edward O. Wilson coined the terms r-selection and Ê-selection to describe two ends of a continuum of life history patterns.
The “r” in r-selection refers to the intrinsic rate of increase of a population, a measure of how rapidly a population can grow. The term r- selection refers to selection for high population growth rates. This type of selection can occur in environments where population density is low—for example, in recently disturbed habitats that are being recolonized. In this type of habitat, genotypes that can grow and reproduce rapidly will be favored over those that cannot. In contrast, Ê-selection refers to selection for slower rates of increase, which occurs in populations that are at or approaching Ê, the carrying capacity or stable population size for the environment in which they live (see Concepts 10.3 and 10.5 for in-depth discussions of r and Ê). Ê-selection occurs under crowded conditions, in which genotypes that can efficiently convert food into offspring are favored. By definition, Ê-selected populations do not have high population growth rates, because they are already near the carrying capacity for their environment and competition for resources can be intense.One way to think of the r-Ê continuum is as a spectrum of population growth rates, from fast to slow. Organisms at the r-selected end of the continuum are often small and have short life spans, rapid development, early maturation, low parental investment, and high rates of reproduction. Examples of this “live fast, die young” end of the continuum include most insects, small short-lived vertebrates such as mice, and weedy plant species. In contrast, Ê-selected species tend to be long-lived, develop slowly, delay maturation, invest heavily in each offspring, and have low rates of reproduction. Examples of this “slow and steady” end of the continuum include large mammals such as elephants and whales, reptiles such as tortoises and crocodiles, and long-lived plant species such as oak and maple trees.
Like most classification schemes, the r-K continuum tends to emphasize the extremes. Most life histories are intermediate between these extremes, however, and hence the r-K approach is not informative in some situations. The distinction between r-selection and K-selection is perhaps most useful in comparing life histories in closely related species or species living in similar environments. For example, Braby (2002) compared three species of Australian butterflies in the genus Mycalesis. The species that occurs in the driest, least predictable habitats shows the most r-selected characteristics, including rapid development, early reproduction, production of many small eggs, and rapid population growth. In contrast, the two species found in more predictable, wet forest habitats have more K-selected characteristics.