Between Zeus and the Salmon: The Biodemography of Longevity
Citation: Wachter (1997) Between Zeus and the Salmon: The Biodemography of Longevity.
Theories of Longevity Evolutionary theory of senescence: natural selection clears away genes that compromise reproduction or survival to and through ages of reproduction but leaves genes with bad effects at older ages alone. Mutation-accumulation theory: mutations deleterious to survival at older, postreproductive or postnuturant ages should accumulate over eons, since selection is not clearing them away. Antagonistic pleitropy: there are genes that have positive effects on net reproduction and negative effects on postreproductive survival, similar to disposable soma theory (Kirkwood 1977) in which organisms allocate energy between functions of reproduction and maintenance. Hayflick limit: certain types of mammalian cells transplanted to cultures only divide up to a limited # of times. Vaupel's more optimistic view: the same mechanisms built in to guarantee fail-safe completion of the reproductive "mission" may endow the body with residual post-completion life; preadaption the system honed by evolution to solve one problem turns out, serendipitously, to be a full-fledged good start on the solution to another problem. Empirical Challenges 1.) Hazard functions measured at extreme ages in large population from several profoundly different species do not rise indefinitely with age. In some other cases, they rise, but at decreasing rates. 2.) Human death rats in some developed societies are falling, even among the oldest-old. 3.) Selective breeding of flies, mice, and other laboratory animals has shown an extraordinary plasticity of life span in the face of selection. 4.) From studies of Drosophilia and nematodes on the relationship between heterogeneity and hazard rates at extreme ages...the observed levels of hazards at advanced ages is not produced solely by the selective effect of genetically frailer individuals dying earlier, leaving genetically more robust individuals to die at lower rates at later ages; the leveling occurs in pure-bred strains, among populations of individuals who are nearly genetically identical. The Elderly in Nature There are exceptional cases of postreproductive survival. Intergenerational transfers are also important. For example, menopause might have evolved in response to tradeoffs favoring the cessation of one's own reproduction and the channeling of effort into protecting and endowing the reproductive changes of one's offspring. Theoretical Departures Explorations of homeostatic feedback (both demographic and biological) and fluctuating environments (e.g., benefiting postponing fertility, etc.) are departing from older evolutionary theories of longevity. There also appears to be a historically contingent character of evolutionary change. Evolution may be selecting for plasticity of response to times of fest and times of famine, rather than for optimum vigor under fixed conditions.