Life History Theory - Life History Characteristics

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 favored 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 aging
• 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. 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. 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 selected for flies with both shorter lifespans and higher fecundity.