In selection theory, selective pressures are
hypothesised to drive
evolution in one of two generalized directions: - or -selection. : \frac{\text{d}N}{\text{d}t} = r\ N \left( 1 - \frac{\ N\ }{ K }\right) where is the
population, is the maximum
growth rate, is the
carrying capacity of the local environment, and (the
derivative of population size with respect to time ) is the rate of change in population with time. Thus, the equation relates the growth rate of the population to the current population size, incorporating the effect of the two constant parameters and . (Note that when the population size is greater than the carrying capacity then 1 - N/K is negative, which indicates a population decline or negative growth.) The choice of the letter came from the
German Kapazitätsgrenze (capacity limit), while came from
rate.
r-selection -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 , low ). A typical species is the dandelion (genus
Taraxacum). In unstable or unpredictable environments, -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 -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 -selection are often referred to as -strategists or -selected. Groups of organisms known for exhibiting -selected traits are
bacteria,
diatoms,
insects,
grasses,
cephalopods,
fowl, and
rodents.
K-selection , an individual of a typical -strategist species. -strategists have longer life expectancies, produce fewer offspring, and when young tend to be
altricial requiring extensive care by parents. By contrast, -selected species display traits associated with living at densities close to carrying capacity and typically are strong competitors in such crowded niches, that
invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low , high ). In
scientific literature, -selected species are occasionally referred to as "opportunistic" whereas -selected species are described as "equilibrium". Organisms with -selected traits include large organisms such as
elephants,
sharks,
humans, and
whales, but also smaller long-lived organisms such as
Arctic terns,
parrots, and
eagles.
Continuous spectrum Although some organisms are identified as primarily - or -strategists, the majority of organisms do not follow this pattern. For instance, trees have traits such as longevity and strong competitiveness that characterise them as -strategists. In reproduction, however, trees typically produce thousands of offspring and disperse them widely, traits characteristic of -strategists. Similarly,
reptiles such as
sea turtles display both - and -traits: Although sea turtles are large organisms with long lifespans (provided they reach adulthood), they produce large numbers of unnurtured offspring. The dichotomy can be re-expressed as a continuous spectrum using the economic concept of
discounted future returns, with -selection corresponding to large discount rates and -selection corresponding to small discount rates. ==Ecological succession==