Density dependence triggers runaway selection of reduced senescence.
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Across a realistic spectrum of senescent age profiles, density regulation of recruitment can trigger runaway selection for ever-reducing senescence.The evolution of nonsenescence from senescence is robust to the presence of exogenous adult mortality, which tends instead to increase the age-independent component of vitality loss.We simulate examples of runaway selection leading to negligible senescence and even intrinsic immortality.
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PubMed Central - PubMed
Affiliation: Department of Mathematics, University College London, London, United Kingdom.
ABSTRACT
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In the presence of exogenous mortality risks, future reproduction by an individual is worth less than present reproduction to its fitness. Senescent aging thus results inevitably from transferring net fertility into younger ages. Some long-lived organisms appear to defy theory, however, presenting negligible senescence (e.g., hydra) and extended lifespans (e.g., Bristlecone Pine). Here, we investigate the possibility that the onset of vitality loss can be delayed indefinitely, even accepting the abundant evidence that reproduction is intrinsically costly to survival. For an environment with constant hazard, we establish that natural selection itself contributes to increasing density-dependent recruitment losses. We then develop a generalized model of accelerating vitality loss for analyzing fitness optima as a tradeoff between compression and spread in the age profile of net fertility. Across a realistic spectrum of senescent age profiles, density regulation of recruitment can trigger runaway selection for ever-reducing senescence. This novel prediction applies without requirement for special life-history characteristics such as indeterminate somatic growth or increasing fecundity with age. The evolution of nonsenescence from senescence is robust to the presence of exogenous adult mortality, which tends instead to increase the age-independent component of vitality loss. We simulate examples of runaway selection leading to negligible senescence and even intrinsic immortality. |
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Mentions: If D = 0, this is linear in x with slope C, and with D > 0, it is asymptotically linear as x → ∞. The exponential factor is important for small x, since it ensures that b0(x) is very flat, increasing only slowly near x = 0. In Appendix C of Text S1, it is shown that, for D > 0 and C sufficiently small, and with relative vitality given by Equation 12 (and for the more general forms considered in Appendix A of Text S1), R(x) in Equation 13 is monotonically decreasing in x, with R(x) → κCω0, a positive constant, as x → ∞ (where ). Clearly, if R(x) is decreasing and Cω0 ≥ 1, then R(x) > 1 for all x, and so there is a viable equilibrium population for every value of x, representing an evolutionary continuum from the most extreme compressed life history (x → ∞) to the most extreme spread, nonsenescent life history (x = 0). Examples of behaviors of R(x) for b0(x) in the family of Equation 14 are shown in Figure 2A–2C. Figure 2D shows an example in which b0(x) increases rapidly for small x, and then only slowly approaches its straight-line asymptote. The corresponding R(x) has a complicated shape, with two evolutionary optima, one near x = 0, and a more prominent one at a value near x = 1. |
View Article: PubMed Central - PubMed
Affiliation: Department of Mathematics, University College London, London, United Kingdom.