Between Zeus and the Salmon The Biodemography of Longevity (1997) / Chapter Skim
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4 The Evolution of Senescence
4 The Evolution of Senescence
Pages 65-77
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From page 65... ...
I center my discussion on the concept of an evolutionary equilibrium for a life history, which includes the shape of fertility and mortality and possibly other features such as growth rates. A theory focused on equilibria is limited by the assumption that the forces of evolution have operated long enough that the characters we study are close to their equilibrium states.
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From page 66... ...
Unstable equilibria can also be interesting, typically in situations where one asks if a newly introduced phenotype can increase in frequency in a population from which it was previously absent. Such invasion analyses are used here to examine the effects of random environments on the evolution of mortality patterns.
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From page 67... ...
The object of the theory is to identify a subset ZE of the best competitors among the possible phenotypes Z The set ZE may contain several phenotypes that are equally competitive, or a single "best" phenotype; in either case, the ESS model consists of subset ZE How is competitive success (i.e., fitness)
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From page 68... ...
(This restriction applies to ESS and EGS models for life-history traits; obviously, an EGS model for modifiers of mutation rate will include mutational dynamics, as in Liberman and Feldman, 1986.) However, mutation is central to most arguments about the evolution of mortality rates, and it is clearly important to examine the joint dynamics of mutation and selection.
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From page 69... ...
In contrast to ESS models, this approach yields predictions for equilibrium phenotypes. These predictions can then be confronted with data in an unambiguous way.
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From page 70... ...
The Bacterium Limit For a different limit, let us constrain total reproduction, so that, for example, the total lifetime reproduction is fixed. Now assume that there are both beneficial and deleterious mutations affecting survival and fertility.
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From page 71... ...
OTHER EQUILIBRIA I now discuss several models that yield ESS patterns or mutation-selection equilibria that depart from the salmon type of mortality pattern, yielding patterns of the kind suggested by Carey et al.
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From page 72... ...
In essence, a catastrophic mortality decline is averted because of the accumulation of mutations that "hitch-hike" on a positive effect of selection early in life. Variable Environments A different explanation for the evolution of mortality rates is found in the analysis of life-history evolution in the common situation when vital rates depend on a randomly varying external environment.
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From page 73... ...
This paper concludes that phenotypic polymorphism for the length of reproductive life can be readily maintained by temporally varying selective regimes. The practical consequence of such polymorphism will be that mortality will not be observed to fall catastrophically but rather will show some kind of plateau at late ages.
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From page 74... ...
of reproduction can lead to equivalent fitness, just as in models with temporally varying vital rates. In other words, we have here another mechanism for the existence of equilibrium mortality patterns that are not salmonlike.
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From page 75... ...
It would be valuable to extend this theory to a model for quantitative traits using the methods of quantitative genetics and to examine the nature of equilibrium phenotypic distributions that would be predicted by such models. Size-Structured Models These models have been relatively little explored in the context of life-history evolution, although considerable work has been done on their application to the modeling of population dynamics.
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From page 76... ...
Such estimates would add to a broader and more accurate perspective on the evolution of mortality and fertility in humans. ACKNOWLEDGMENTS I thank the Morrison Institute for Population and Resource Studies, Marc Feldman, and Jean Doble at Stanford University; the National Institutes of Health for support from HD 16640 and HD 32124; the National Science Foundation for support from DEB 9420153; and Carl Boe, Jim Curtsinger, Ken Wachter, Jim Vaupel, Daniel Prom~slow, Jim Carey, Mike Rose, and Peter Abrams for useful 1- .
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From page 77... ...
Feldman 77 1982 On the evolutionary genetic stability of the sex-ratio. Theoretical Population Biology 21 :430-439.
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