Between Zeus and the Salmon The Biodemography of Longevity (1997) / Chapter Skim
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5 Evolutionary Biology and Age-Related Mortality
5 Evolutionary Biology and Age-Related Mortality
Pages 78-95
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From page 78... ...
This decline is generally taken to be indicative of a decline in intrinsic state, known as aging or senescence. The exact form of the survival curve differs considerably among human populations living in different areas and at different times (Finch, 1990; Wilmoth, in this volume)
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From page 79... ...
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.
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From page 80... ...
In an environment with relatively high, externally imposed adult mortality, for instance, effort expended on reproduction is expected to increase even if survival probability and future fertility are thereby reduced by physiological costs, because prospects for future reproduction are anyway poor for environmental reasons. The life histories of many organisms include a period of aging or senescence, terms for the deterioration in performance observed later in the adult life span.
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From page 81... ...
Under some circumstances, a condition is predicted to improve for part of the adult period, for instance as a result of growth or learning, which could lead to gains in future reproductive prospects through improved survival probability and fertility (Hamilton, 1966; Charlesworth, 1980 and 1994~. Aging is not an inevitable feature of life-history optimization and, for it to appear, the age of expression of the beneficial effects of the mutant must precede the age of expression of the deleterious effects.
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From page 82... ...
What do these evolutionary theories allow us to predict about age-related changes in death rate in the later part of life in any population, including human populations? This question raises an important empirical issue, which is how the intrinsic deterioration that causes death rate to increase and fertility to decline at later ages can be detected and compared between different populations.
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From page 83... ...
how use of this measure, instead of measures of rates of change in age-specific death rates, would affect conclusions about the pattern of change of intrinsic state in relation to age, and their concordance with evolutionary theories of aging. Aging, the process we are trying to measure, occurs either because of laterexpressed costs of processes beneficial earlier in life or because mutation pressure is depressing the life history below the optimal one.
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From page 84... ...
The important point is that evolutionary theories of aging do not necessarily predict Gompertzian-type increases in postreproductive mortality rates. Nor does the mutation-accumulation theory necessarily predict catastrophic increases in mortality when reproduction ceases, especially as mutations that produce aging also
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From page 85... ...
. Forinstance, higher externally imposed death rates of adults are expected to cause evolution of higher rates of aging for both survival and fertility.
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From page 86... ...
Typically, measurements of intrinsic change have been made in benign environments, very different from that in which the life history evolved, a problem because life-history traits and the correlations between them are often extremely sensitive to environmental change (e.g., Stearns, 1992; Chapman and Partridge, 1996~. The study organism may, through phenotypic plasticity, alter its own intrinsic life history as a proximate response to, for example, changes in nutrition or even the removal of external hazard.
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From page 87... ...
In any discussion of the implications of evolutionary knowledge for age-related patterns of human death rates, we therefore need to bear in mind that part of the picture is missing, and that this could alter our conclusions. However, the physical frailty of the oldest old may have greatly limited their contribution to the reproductive success of younger kin, so the difficulty may not be too serious.
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From page 88... ...
Life histories, like any other trait evolving under natural selection, are expected to evolve toward a form optimal only for the environments in which selection occurs. There has been a great deal of theoretical and empirical work on how sensitivity to the environment of life-history traits, their "norm of reaction" or "phenotypic plasticity," is expected to evolve for the range of environments naturally encountered (e.g., Gomulkiewicz and Kirkpatrick, 1992; Scheiner, 1993; de Jong,1995~.
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From page 89... ...
can, paradoxically, extend life span (e.g., Luckinbill and Clare, 1986; Chapman and Partridge, 1996~. However, at the same time lifetime reproductive success is reduced because of a reduction in fecundity.
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From page 90... ...
If mortality rates were higher among the sexually active than among the celibate males before the reversal of reproductive status occurred, then the survivors of sexual activity would presumably be, on average, less frail than the survivors of celibacy. It is therefore possible that in the above experiment sexual activity did have lingering effects on male death rate, but that these were masked by the effects of the variation in frailty (Prowse and Partridge, 1997~.
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From page 91... ...
Another prediction is that, because the rate of aging evolves in response to the external hazards, we would expect the different systems underlying continuing survival and fertility to wear out in parallel, because of the common pattern of natural selection maintaining them in the face of damage. In consequence, we would not expect to find major genes that reduce the aging rate, unless they also had a deleterious effect on some other life-history aspect.
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From page 92... ...
However, refinement of theory and data should greatly improve our understanding of these issues, and there may be other ways in which evolutionary thinking can inform future work on these issues. For instance, it would be helpful, at a physiological level, to know what the main tradeoffs are in mammalian life histories.
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From page 93... ...
Lithgow, and T.E. Johnson 1994 Mortality rates in genetically heterogeneous populations of Caenorhabditis elegans.
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From page 94... ...
1995b The inbreeding decline and average dominance of genes affecting male life-history characters in Drosophila melanogaster. Genetical Research 65(1)
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From page 95... ...
1974 Selection for optimal life histories: the effects of age structure. Ecology 5:291-303.
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