considerably weakens the predictive power of evolutionary life-history theory, because interactions between genotypes and environments for life-history traits are pervasive (e.g., Stearns, 1992; Chapman and Partridge, 1996). In addition, much demographic analysis of the impact of aging has relied on various measures of age-specific death rates late in life. The evolutionary theories of aging apply to fertility and survival rates jointly, and from an evolutionary perspective aging is complete when reproduction ceases, unless the old contribute to reproduction by relatives. Deriving any general predictions for death rates alone, especially for postreproductive death rates, is not simple.

The following sections briefly summarize these issues and point to some areas where further work could be profitable. In addition, the broader implications of evolutionary ideas for the aging process are discussed.

Age-Related Changes Within Individuals

Evolution of Life Histories and of Aging

The intrinsic state of individuals changes with age in humans and other organisms. These changes in state are reflected in changes in vital rates: survival and fertility. Evolutionary life-history theory has been directed at understanding the diverse array of age-related changes seen in different organisms.

From an evolutionary perspective, a life history is the combination of age-specific survival probabilities and fertilities characteristic of a type of organism. These produce the life table of survival probabilities to each age x (lx) and the fertilities at those ages (mx). The life history includes traits such as age at first breeding, number of breeding attempts, fecundity at each breeding attempt, and adult life span. The life histories observed in different species and, indeed, populations are extremely diverse, and the forces affecting their evolution are becoming well understood (e.g., Cole, 1954; Schaffer. 1974; Pianka and Parker, 1975; Charlesworth, 1980 and 1994; Stearns, 1992; Roff, 1992; McNamara and Houston, 1996; Tuljapurkar, in this volume). Natural selection is expected to act on the life history so as to incorporate into the population alleles that maximize the intrinsic rate of increase (r), which takes into account changes in the size of the population into which offspring are born or, in a population where numbers do not vary, lifetime reproductive success (R0). To reach this conclusion requires certain simplifying assumptions such as stable age structure, absence of frequency-dependent selection, and absence of any interaction between the effects of male and female genotype upon the fertility of mating pairs. Other more complex fitness criteria can be used if some of these assumptions are violated (Tuljapurkar, in this volume).

Maximization takes place in the face of two forces. The first is the set of constraints on the combinations of different life-history variables that can be realized. Some of these are internal to the organism, in the form of various



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