Life history theory is an analytical framework widely used in animal and human biology, psychology, and evolutionary anthropology which postulates that many of the physiological traits and behaviors of individuals may be best understood in terms of the key maturational and reproductive characteristics that define the life course.
Examples of these characteristics include:
Variations in these characteristics reflect differing allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions, especially growth, body maintenance, and reproduction. 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, resources spent growing to a larger body size cannot be spent increasing the number of offspring. In general terms the costs of reproduction may be paid in terms of energy being diverted away from body repair and maintenance and by reducing investment in immunological competence.
Thus the allocation of resources involves trade-offs. These trade-offs and strategies can be compared between species. Two of the most well-known trade-offs involve number of offspring (few or many) and timing of reproduction (accelerated maturation and reproduction versus delayed, allowing for larger size and more complex social supports). Anthony Zera and Zhangwn Zhao have recognized a prime example of trade-offs amongst female sand crickets. Female sand crickets are easily recognized as being long or short winged. Adult long winged females store excess triglycerides as an energy source to fuel well developed flight muscles. This trait is adaptive, providing the insect with the ability to find more suitable environments when necessary. The downfall of this is represented by the decreased rate of growth of ovaries. In the short winged females, triglyceride storage is limited and flight muscles are underdeveloped. Energy saved will be used to produce phospholipids, utilized in egg production. The following represents a trade-off experienced amongst female sand crickets, involving “dispersal ability versus early reproduction.” (Freeman 485-86) The extremes at the species level of these fundamental dimensions of reproduction were recognized long before life history theory, and are traditionally termed r/K selection theory. An r-selection strategy is the production of a large number of offspring (of whom only a minority may survive) as early in life as possible. The K-selection strategy is to produce a smaller number of "fitter" offspring with higher survival chances.
According to life history theory the individuals of a species are able to make limited shifts in reproductive strategies in response to the prevailing environments. Depending on abundance of resources and probable individual longevity, individuals consciously or unconsciously shift their reproductive strategy in one direction or the other to take advantage of available resources or to compensate for resource shortage or uncertainty.
The fetal origins hypothesis has subsequently been developed to include the idea of an 'appropriateness of fit' between the phenotype and its environment. This has resulted in increased attention being drawn to the problems which may occur if there is a mismatch between physiologic capacities established in early development and the environments in which they later function.
The life history theory is also being examined in recent studies involving the human immune system. Concepts from life-history theory are applied to highlight the major challenges and demands that are likely to shape immune function in a range of ecological contexts. Immune function is a major component of maintenance effort, and since resources are limited, trade-offs are expected between investment in maintenance and other critical life-history functions involving growth and reproduction. An adaptationist, life-history perspective helps make sense of the unusual developmental trajectory of immune tissues, and emphasizes that this complex system is designed to incorporate information from the surrounding ecology to guide its development. As a result, there is substantial population variation in immune development and function that is not considered by current biomedical approaches. In an attempt to construct a framework for understanding this variation, immune development is considered in relation to the competing life-history demands that define gestation, infancy, childhood, adolescence, and adulthood. Each life stage poses a unique set of adaptive challenges, and a series of hypotheses is proposed regarding their implications for immune development and function (McDade).
Thus the trade-offs identified by life history can be interpreted as implying that the present health consequences of differential fetal outcomes represent the resurfacing of costs deferred in early trade-offs. According to the fetal programming hypothesis permanent alterations in metabolic regulation under early conditions of adversity could be seen as adaptive in virtue the overarching need to produce an energy-sparing phenotype (Hales and Barker, 2001). The resulting conceptual problem has been to clarify why such permanent changes should be adaptive if they yield little in the way of future benefits, yet do exact a future cost. Perhaps the advantage is to trade off health in later life in order to allow an individual to grow to sexual maturity, mate successfully and perpetuate the bloodline. Once this is achieved, health in later life has minimal importance from an evolutionary perspective.
Analyzing trade-offs among life history traits has helped to explain many aspects of the extraordinary life history diversity known among living organisms. However, this view can sometimes obscure the fact that the life history of each organism unfolds in an ecological context that includes other individuals. In fact, the reproductive interests of males and females will often be different. Analyzing trade-offs among life history traits has helped to explain many aspects of the extraordinary life history diversity known among living organisms. However, this view can sometimes obscure the fact that the life history of each organism unfolds in an ecological context that includes other individuals. In fact, the reproductive interests of males and females will often be different. This presents a conflict of interest between males and females. Experiments performed by William Rice and colleagues present when the reproductive interests of female and male fruit flies are different, sexual selection has the tendency to favor adaptations that come about in one sex but are not beneficial to the other sex. One example of these adaptations is the biological makeup of male seminal fluid. This has evolved the ability to influence the behavior of females, like their rate of egg-laying or potential to remate with other males. The effects of these adaptations favor the male if his female mate is not monogamous, because this will increase the amount of fertilized eggs with his sperm. However, this could lead to effects that are not beneficial to females, since male seminal fluid displays toxic effects and increases the mortality of females. This will tend to favor the evolution of resistance in females, while males will develop adaptations to conquer the resistance of females. This process is called chase-away sexual selection (Freeman 516).
McDade, TW. American Journal of Phys. Anthropology; 2003.
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