Brief Overview of Life History Theory

In evolutionary terms, the ability to reproduce is the most important capacity living organisms possess, allowing for passing genes on to the next generations. For animals that reproduce more than once, like many mammals, reproductive effort involves investment in multiple breeding episodes over the life course. For long- lived primates, like humans, successful adaption to a given set of environmental circumstances is measured in terms of lifetime reproductive success or the number of healthy, potentially reproductive offspring produced relative to the success of other individuals in the population.

Reproduction is costly for humans and other long-lived primates. Not only does reproductive effort involve pregnancy and lactation, it may also include long-term provisioning of mates or nurturing offspring long past weaning. A variety of factors contribute to lifetime reproductive success in humans and nonhuman primates, including the timing of birth, maturation, and spacing of reproductive episodes, the amount of energy invested in procuring mates and producing offspring, and the energetic expenditures and risks involved in sustaining pregnancy, producing milk, and other forms of offspring nurture and care. The resources needed for reproduction may be in short supply or available only seasonally. Longer reproductive life spans also necessitate maintaining healthy bodies and nonreproductive structures between breeding periods over the long haul, as well as obtaining adequate nutrition to support reproductive organs and offspring during the demands of breeding. Complex social networks and learning systems often must be negotiated or mastered. Reproductive individuals can incur significant risks in the form of predation, competition with conspecifics, physiological stress, exposure to diseases or accident, or the loss of energy stores vital for future health and reproductive investment.

The trajectory of the life span—the timing of maturation, births, and deaths—is referred to by evolutionary biologists as the life history. Rates of birth and mortality (vital rates), in turn, are referred to as life history traits. Evolutionary theory predicts that these demographic variables represent key aspects of an animal’s phenotype that have been shaped by natural selection and contribute to lifetime reproductive success, just as other key phenotypic characteristics contribute to an individual’s evolutionary success relative to others in a population. Natural selection is expected to produce organisms with the most successful adaptive “strategies” or complex sets of trade-offs among all of the benefits, costs, and risks associated with reproduction and associated phenomena. Life history trade-offs can be extremely complex and difficult to measure directly, particularly in human or other primate populations, since physiological measurements and experimental manipulations are difficult to carry out.

Some of the physiological structures, hormones, and growth factors necessary for reproduction in mammals—mammary glands, gonads, and sex steroid hormones like estrogens, progesterone, and testosterone—carry special physiological risks (e.g., an increased risk of cancer), as well as providing obvious reproductive benefits. Traits that provide developmental or reproductive benefits early in life may become liabilities later on and may contribute to the aging process (Finch and Rose 1995; Finch 2007; Cohen and Holmes 2014; Williams 1957). While natural selection is expected to optimize reproductive traits and factors during the reproductive life span, no direct evolutionary benefits accrue by surviving or remaining healthy past the period in which reproduction is possible. Traits are only adaptive insofar as they contribute to lifetime reproductive success, synonymous with Darwinian fitness. Variation in the human reproductive life span itself, therefore, reflects past natural selection as well as shorter-term responses to evolutionary change in response to novel environments.

 
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