title: The Rarity of Survival to Old Age Does Not Drive the Evolution of Senescence.
abstract: The evolution of senescence is often explained by arguing that, in nature, few individuals survive to be old and hence it is evolutionarily unimportant what happens to organisms when they are old. A corollary to this idea is that extrinsically imposed mortality, because it reduces the chance of surviving to be old, favors the evolution of senescence. We show that these ideas, although widespread, are incorrect. Selection leading to senescence does not depend directly on survival to old age, but on the shape of the stable age distribution, and we discuss the implications of this important distinction. We show that the selection gradient on mortality declines with age even in the hypothetical case of zero mortality, when survivorship does not decline. Changing the survivorship function by imposing age independent mortality has no affect on the selection gradients. A similar result exists for optimization models: age independent mortality does not change the optimal result. We propose an alternative, brief explanation for the decline of selection gradients, and hence the evolution of senescence.
authors.affiliation: Max Planck Odense Center on the Biodemography of Aging, Odense, Denmark.|Institute of Public Health, University of Southern Denmark, Winsløws Vej 9B, 5000 Odense C, Denmark.|Max Planck Research Group on Modeling the Evolution of Aging, Max Planck Institute for Demographic Research, Konrad Zuse Strasse 1, 18055 Rostock, Germany.|Leyden Academy of Vitality and Ageing, Poortgebouw LUMC, Rijnsburgerweg 10, 2333 AA Leiden, The Netherlands.
authors.forename: Maarten J
authors.initials: MJ
authors.lastname: Wensink
authors.affiliation: Max Planck Odense Center on the Biodemography of Aging, Odense, Denmark.|Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands.|Biology Department MS-34, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA.
authors.forename: Hal
authors.initials: H
authors.lastname: Caswell
authors.affiliation: Max Planck Odense Center on the Biodemography of Aging, Odense, Denmark.|Institute of Public Health, University of Southern Denmark, Winsløws Vej 9B, 5000 Odense C, Denmark.|Max Planck Research Group on Modeling the Evolution of Aging, Max Planck Institute for Demographic Research, Konrad Zuse Strasse 1, 18055 Rostock, Germany.|Biology Department, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark.
authors.forename: Annette
authors.initials: A
authors.lastname: Baudisch
keywords: Age distribution; Extrinsic mortality; Selection gradient; Senescence; Survivorship