13.2.3 Life History Variation & r/K selection theory NEET

13.2.3 Life History Variation
  • Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in the habitat in which they live. 
  • Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. 
  • Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times during their lifetime (most birds and mammals). 
  • Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals). 
  • So, which is desirable for maximising fitness? 
  • Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.
  • Evolution of life history traits in different species is currently an important area of research being conducted by ecologists.
r/K selection theory
  • A population ecology concept is r/K selection theory, one of the first predictive models in ecology used to explain life-history evolution. 
  • The premise behind the r/K selection model is that natural selection pressures change according to population density. 
  • For example, when an island is first colonized, density of individuals is low. 
  • The initial increase in population size is not limited by competition, leaving an abundance of available resources for rapid population growth. 
  • These early phases of population growth experience density-independent forces of natural selection, which is called r-selection. 
  • As the population becomes more crowded, it approaches the island's carrying capacity, thus forcing individuals to compete more heavily for fewer available resources. 
  • Under crowded conditions, the population experiences density-dependent forces of natural selection, called K-selection.
  • In the r/K-selection model, the first variable r is the intrinsic rate of natural increase in population size and the second variable K is the carrying capacity of a population.
  • Different species evolve different life-history strategies spanning a continuum between these two selective forces. 
  • An r-selected species is one that has high birth rates, low levels of parental investment, and high rates of mortality before individuals reach maturity. 
  • Evolution favours high rates of fecundity in r-selected species. 
  • Many kinds of insects and invasive species exhibit r-selected characteristics. 
  • In contrast, a K-selected species has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. 
  • Humans and elephants are examples of species exhibiting K-selected characteristics, including longevity and efficiency in the conversion of more resources into fewer offspring.
r- selected species
K- selected species
'r' is the intrinsic rate of natural increase in population size
'K' is the carrying capacity of a population.
An r-selected species is one that has high birth rates
low levels of parental investment, and high rates of mortality before individuals reach maturity. 
Evolution favours high rates of fecundity in r-selected species.
a K-selected species has low rates of fecundity
Low levels of parental investment in the young, and high rates of mortality as individuals mature.
high levels of parental investment in the young, and low rates of mortality as individuals mature.
Many kinds of insects and invasive species exhibit r-selected characteristics
Humans and elephants are examples of species exhibiting K-selected characteristics

including longevity and efficiency in the conversion of more resources into fewer offspring.

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