Mutations and the age pattern of death

Tuljapurkar, Shripad

doi:10.1073/pnas.1307921110

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“…Intriguingly, the balance model, for proportional hazards in stationary populations, very likely reverse-engineered an earlier model of Williams and Taylor (1987): they did not describe its evolutionary process in detail, but their model also assumed stationarity, derived approximately the same equilibrium condition, and also did not yield any non-semelparous equilibrium for additive-hazards; regarding the latter, I suspect they slightly misspoke, and should have said no non-extinction equilibrium, with semelparity not being an equilibrium either. I argued that this joint finding of Williams and Taylor (1987) and of this thesis may shed light on the emergence of catastrophic senescence, or creeping walls of death, in models of mutation accumulation that assume additive hazards (Tuljapurkar, 2013).…”

mentioning

confidence: 72%

“…That is, as a life history evolves under the influence of the age-specific forces of selection-whatever genetic and ecological assumptions we use-the agespecific forces of selection evolve too (cf. Tuljapurkar, 2013). Thus we can examine an existing life history and from it calculate Hamilton's forces of selection, as we did for the example of the desert tortoise (Fig.…”

Section: 𝐹 𝑋mentioning

confidence: 99%

“…If mutation is unopposed by selection at post-reproductive ages, of course in the balance model infinite mortality will evolve. Catastrophic senescence further, however, invokes feedback over evolutionary time (Partridge & Barton, 1993, Box 2;Tuljapurkar, 2013;Wachter et al, 2013), 53 with the spread of deleterious alleles weaking selection against further deleterious alleles. Barton (1993), andWachter et al (2013), both used additive 53 From Partridge and Barton (1993, Box 2):…”

Section: "Walls Of Death" and "Catastrophic Senescence"mentioning

confidence: 99%

“…It emphasized the shortcoming most relevant to this thesis: that the forces of selection moulding the evolution of life histories are themselves functions of the life history, meaning that when a life history evolves its forces of selection must evolve too-complicating attempts to study how life histories may evolve in the long run (cf. Tuljapurkar, 2013). Chapter 1 then proposed the approach giving this thesis its title: to assume that life histories eventually reach a long-run evolutionary equilibrium where they (and their forces of selection) are not evolving any more; stipulating a model of the evolutionary process and its relevant genetic assumptions; and seeking simple, quantitative, equilibrium conditions that a life history would have to satisfy in order to no longer be evolving.…”

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confidence: 99%

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Equilibrium Conditions in the Evolution of Senescence

Bahry¹

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Senescence evolves because natural selection is less sensitive to late life than to early life.Hamilton formalized this as the sensitivities of fitness to small additive changes to age-specific mortality or fecundity; his framework has since been extended to alternative ecological and genetic assumptions. However, such forces of selection only explicitly model evolution in the short term; in the long term, as life histories evolve, their forces of selection evolve too. This thesis investigates long-term evolution of senescence by deriving conditions a population must satisfy in order to be in evolutionary equilibrium. It considers two models: a mutation-selection balance model; and an optimality model with a same-age log mortality cost of fecundity. The derived conditions are discussed, and heuristically compared to two species: Soay sheep, which senesce, and desert tortoises, which don't. Other, intermediate theoretical results are also given, including the force of selection on proportional hazards in stationary populations.iii INTRODUCTION: DEMOGRAPHY AND EVOLUTION OF SENESCENCE 1 ~This thesis contributes to the evolutionary theory of SENESCENCE, or "aging," using the tools of the theory of LIFE HISTORY EVOLUTION (Rose, 1991;Stearns, 1992;Fabian & Flatt, 2011;. 2 Life history traits are those traits that are most directly connected to demography and Darwinian fitness: survival and reproduction, and their pattern across the life span. This thesis therefore defines senescence in terms of increasing mortality over the lifespan, rather than any particular physiological change (Medawar, 1955). Not all species senesce, but many do: an 80year-old freshwater hydra is no more likely to die within the next week or year than a 20-yearold freshwater hydra, thus hydra do not senesce; an 80-year-old human is more likely to die within the next week or year than a 20-year-old human, thus humans do senesce (Finch, 1990;Jones et al., 2014; Fig. 1). This thesis also defines REPRODUCTIVE SENESCENCE as a decrease in reproductive output with age-with the caveat that this definition may require modification for species, such as humans and whales, whose menopause may be an adaptation to shift from giving birth to increased care for existing children and grandchildren (cf. Hawkes et al., 1998).

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mentioning

confidence: 72%

Section: 𝐹 𝑋mentioning

confidence: 99%