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Density dependence triggers runaway selection of reduced senescence.

Seymour RM, Doncaster CP - PLoS Comput. Biol. (2007)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Mathematics, University College London, London, United Kingdom.

ABSTRACT
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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Graphs of Vital Rates(A) Relative fecundity functions mt(x) taking the form of Equation 15a at age t for x = 0, 0.1, 0.2, 0.5, 1, and 2, as indicated.(B) Total adult recruitment functions bt(x) = b0(x)mt(x), with b0(x) = 1 + x.(C) Survival functions st(x) taking the form of Equation 15b.(D) Mortality rate functions 							 derived from Equation 15b. The senescence rate is of the age-dependent form 							, with n = 5 (further detailed in Appendix A of Text S1). Other parameters are αb = αd = 0.5 and μ = 0.01. All graphs are on the same timescale.
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pcbi-0030256-g003: Graphs of Vital Rates(A) Relative fecundity functions mt(x) taking the form of Equation 15a at age t for x = 0, 0.1, 0.2, 0.5, 1, and 2, as indicated.(B) Total adult recruitment functions bt(x) = b0(x)mt(x), with b0(x) = 1 + x.(C) Survival functions st(x) taking the form of Equation 15b.(D) Mortality rate functions derived from Equation 15b. The senescence rate is of the age-dependent form , with n = 5 (further detailed in Appendix A of Text S1). Other parameters are αb = αd = 0.5 and μ = 0.01. All graphs are on the same timescale.

Mentions: For a given senescence rate , we assume that relative vitality is partitioned between births and deaths as follows: where 0 ≤ αb, αd ≤ 1 are fixed (age-independent) parameters with αb + αd = 1, μ = μ0 + g is the total age-independent mortality rate, and . A partition of this form is illustrated in Figure 3.


Density dependence triggers runaway selection of reduced senescence.

Seymour RM, Doncaster CP - PLoS Comput. Biol. (2007)

Graphs of Vital Rates(A) Relative fecundity functions mt(x) taking the form of Equation 15a at age t for x = 0, 0.1, 0.2, 0.5, 1, and 2, as indicated.(B) Total adult recruitment functions bt(x) = b0(x)mt(x), with b0(x) = 1 + x.(C) Survival functions st(x) taking the form of Equation 15b.(D) Mortality rate functions 							 derived from Equation 15b. The senescence rate is of the age-dependent form 							, with n = 5 (further detailed in Appendix A of Text S1). Other parameters are αb = αd = 0.5 and μ = 0.01. All graphs are on the same timescale.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230684&req=5

pcbi-0030256-g003: Graphs of Vital Rates(A) Relative fecundity functions mt(x) taking the form of Equation 15a at age t for x = 0, 0.1, 0.2, 0.5, 1, and 2, as indicated.(B) Total adult recruitment functions bt(x) = b0(x)mt(x), with b0(x) = 1 + x.(C) Survival functions st(x) taking the form of Equation 15b.(D) Mortality rate functions derived from Equation 15b. The senescence rate is of the age-dependent form , with n = 5 (further detailed in Appendix A of Text S1). Other parameters are αb = αd = 0.5 and μ = 0.01. All graphs are on the same timescale.
Mentions: For a given senescence rate , we assume that relative vitality is partitioned between births and deaths as follows: where 0 ≤ αb, αd ≤ 1 are fixed (age-independent) parameters with αb + αd = 1, μ = μ0 + g is the total age-independent mortality rate, and . A partition of this form is illustrated in Figure 3.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Mathematics, University College London, London, United Kingdom.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus