Between Zeus and the Salmon The Biodemography of Longevity (1997) / Chapter Skim
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1 Between Zeus and the Salmon: Introduction
1 Between Zeus and the Salmon: Introduction
Pages 1-16
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From page 1... ...
As I see it, for demographers today, the golden challenge is to make the right judgment call predicting our children's life spans. Will the recent pace of gains in life expectancy and active life expectancy extend to the next generation, or are we approaching the point of diminish
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From page 2... ...
It seems to me, as I shall describe, that the newest work in biodemography is lowering our comfort level with accounts involving limits to life expectancy and programmed senescence and enhancing our openness to models and hunches that treat life spans as highly plastic. I begin by reviewing ideas from the evolutionary theory of longevity that have coexisted amicably with a pessimistic demographic stance in regard to open-ended further progress against old-age mortality.
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In its simple forms, this mutation-accumulation theory predicts sharply rising hazard functions beyond some threshold age. Antagonistic pleiotropy, as it pertains to senescence, occurs if there are genes that have positive effects on net reproduction and negative effects on postreproductive survival.
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While this process may be implicated in some localized phenomena of aging, any pervasive role for the Hayflick limit on cells in determining the senescent mortality of organisms remains uncertain, as Caleb Finch discusses in the concluding chapter. Nonetheless, as the most frequently cited result in all of gerontology, the Hayflick limit has contributed powerfully to a general sense that the study of longevity is a study of limits, tradeoffs, and diminishing returns.
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In this section I review the new empirical results. First is the discovery that hazard functions measured at extreme ages in large populations from several profoundly different species do not rise indefinitely with age.
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Some levelingoff of the hazard function at extreme ages has been detected in the best-recorded recent data on human populations, and decreasing hazards at extreme ages are implied by the survival of the world's oldest well-documented individuals. A second set of empirical results that challenge the ethos of limit theories are careful measurements that show human death rates in developed societies to be falling even among the oldest of the oldest-old.
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THE ELDERLY IN NATURE These new empirical results promote a sense of a genetic heritage produced by evolution that is permissive or even positive, as far as old-age survival is concerned, in contrast to the sense of limits characteristic of earlier writings in the evolutionary theory of senescence. Paralleling this work is a renewed appreciation of the roles of the elderly in nature.
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From page 8... ...
Meticulous field work has given us estimates of interage and intergenerational transfers in several hunter-gatherer societies. Ronald Lee shows in Chapter 11 that these estimates fit in well with the patterns of downward age-specific resource flows measured by himself and others in agricultural societies, which have given way to upward flows in economically more developed societies.
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The population age pyramid of prehistoric men and women is a crucial unknown in this debate. Ethnographers studying the remaining hunter-gatherer populations of today find significant postreproductive survival.
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distribution might be shown to arise as a limiting form of hazard function for organisms with a wide range of detailed mortality dynamics. The trouble so far with complex-system models is that their assumptions can be fairly freely chosen to produce any desired hazard function.
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From page 11... ...
These factors probably express themselves mainly in cultural and social arrangements, but those then change the parameters for further biological evolution. There is a fuzzy boundary between factors that can plausibly affect our evolving gene pool and factors that operate through culturally transmitted practices, and there is a potent analogy between genetic evolution and cultural evolution.
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We reach unprecedented average sizes, as well as unprecedented average life spans, perhaps in concert. This phenomenon is easier to understand if evolution has been selecting for plasticity of response to times of feast and times of famine, rather than for optimum vigor under fixed conditions.
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Tests of predictions from the evolutionary theory of senescence are beginning to capitalize on the quantitative specificity permitted by QTL analysis. We should soon be learning, for some laboratory organisms, whether it is commonplace for portions of the genome that correlate positively with net reproduction to correlate negatively with older-age survival rates, as antagonistic pleiotropy posits.
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I have already mentioned the hope that research will soon sort out the roles of environmental heterogeneity and individual life-course dynamics in the observed leveling of hazard functions. In the last generation of demographic models, treatment of heterogeneity (the frailty specification)
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Genetic indicators offer the best hope for obtaining usable instrumental variables and so for beginning to sort out the tangle of selection bias and causation. The most urgent need for genetic indicators in large national surveys is for confirmatory studies of causal effects of genes on medical conditions.
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From page 16... ...
Orozco, and J Vaupel 1992 Slowing of mortality rates at older ages in large medfly cohorts.
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