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Life history theory is a theory of biological evolution that seeks to explain aspects of organisms' anatomy and behavior by reference to the way that their life histories - including their reproductive development and behaviors, life span and post-reproductive behavior - have been shaped by natural selection. These events, notably juvenile development, age of sexual maturity, first reproduction, number of offspring and level of parental investment, senescence and death, depend on the physical and ecological environment of the organism. Organisms have evolved a great variety of life histories, from Pacific salmon, which produce thousands of eggs at one time and then die, to human beings, who produce a few offspring over the course of decades. The theory depends on principles of evolutionary biology and ecology and is widely used in other areas of science. == Life history characteristics == Life history characteristics are traits that affect the life table of an organism, and can be imagined as various investments in growth, reproduction, and survivorship. The goal of life history theory is to understand the variation in such life history strategies. This knowledge can be used to construct models to predict what kinds of traits will be favoured in different environments. Without constraints, the highest fitness would belong to a Darwinian Demon, a hypothetical organism for whom such trade-offs do not exist. The key to life history theory is that there are limited resources available, and focusing on only a few life history characteristics is necessary. Examples of some major life history characteristics include: * Age at first reproductive event * Reproductive lifespan and ageing * Number and size of offspring Variations in these characteristics reflect different allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another. For example, birds with larger broods are unable to afford more prominent secondary sexual characteristics.〔Gustafsson, L., Qvarnström, A., and Sheldon, B.C. 1995. Trade-offs between life-history traits and a secondary sexual character in male collared flycatchers. Nature 375, 311 - 313〕 Life history characteristics will, in some cases, change according to the population density, since genotypes with the highest fitness at high population densities will not have the highest fitness at low population densities.〔Mueller, L.D., Guo, P., and Ayala, F.J. 1991. Density dependent natural selection and trade-offs in life history traits. Science, 253: 433-435.〕 Other conditions, such as the stability of the environment, will lead to selection for certain life history traits. Experiments by Michael R. Rose and Brian Charlesworth showed that unstable environments select for flies with both shorter lifespans and higher fecundity - in unreliable conditions, it is better for an organism to breed early and abundantly than waste resources promoting its own survival.〔Rose, M. and Charlesworth, B. A Test of Evolutionary Theories of Senescence. 1980. Nature 287, 141-142〕 Biological tradeoffs also appear to characterize the life histories of viruses, including bacteriophages. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Life history theory」の詳細全文を読む スポンサード リンク
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