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1966 Evolution of demographic parameters
CHAPTER TWENTY FIVE
1966 Evolution of demographic parameters The concept William D. Hamilton (1966) sought to develop a simple understanding of how natural selection acts on equivalent changes in mortality as a function of age. He came up with the simple result that starting at sexual maturity, the strength of selection acting on equivalent changes in mortality will decrease with increasing age. Thus, senescence is a logical byproduct of natural selection.
The explanation Shortly after his work on kin selection, Hamilton (1966) explored the action of natural selection in age-structured populations. He adopted the Malthusian parameter (m), utilized by both by Norton (1928) and Fisher (1930), as the measure of fitness in an age-structured population, which is Z N the positive root, m, of the Lotka-Euler equation, emx lx fx ¼ 1, where 0
lx is the probability of surviving to age-x and fx is the fecundity of females aged-x. If age is broken into discrete classes, 1, 2, ., d, and px is the chance of an individual surviving from age-class x to age-class xþ1, lx ¼ p1p2 . px1. Hamilton looked at changes in px caused by mutations that increase or decrease survival by a factor 1d. From this type of analysis he found that similar changes in age-specific survival at different ages would always be most strongly favored (in the case of an increase in survival) at earlier ages. He concluded from this model that “A basis for the theory that senescence is an inevitable outcome of evolution is thus established”. A test of this theory is described in Chapter 50. Baudisch (2005) has recently suggested that there are other ways to compare mutations at different ages which won’t necessarily give the same results. For instance, age-specific survival may be increased by an exponential factor, e.g. p(1þd) . In this case, natural selection might resist senescent x decline if fecundity changes with age in an appropriate direction. There Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00025-9
© 2020 Elsevier Inc. All rights reserved.
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j
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Conceptual Breakthroughs in Evolutionary Ecology
are at present few well documented cases of organisms which do not show increasing mortality with age. Some studies of natural populations proport to show this (Jones et al., 2014), but such studies in nature encounter severe technical issues (see appendix). The evolutionary principles set forth by Hamilton have more recently been used to study late-life mortality plateaus. Carey et al. (1992) and Curtsinger et al. (1992) studied very large cohorts of laboratory fly populations and showed that although these populations displayed the expected increase in mortality through much of their adult life, at extremely old ages these mortality rates leveled off. These plateaus were an unexpected anomaly. However, Mueller and Rose (1996) demonstrated that such plateaus are an expected byproduct of the evolutionary forces identified by Hamilton plus random genetic drift. The impact of Hamilton’s (1966) work has been reviewed by Rose et al. (2007).
Impact: 10 The theory developed by Hamilton has had lasting impact and has been empirically supported in numerous experiments (Rose et al., 2007). As such it is one of the foundational breakthroughs in evolutionary biology.
References Baudisch, A., 2005. Hamilton’s indicators of the force of selection. Proc. Natl. Acad. Sci. U.S.A. 102, 8263e8268. Carey, J.R., Liedo, P., Orozco, D., Vaupel, J.W., 1992. Slowing of mortality rates at older ages in large medfly cohorts. Science 258, 457e461. Curtsinger, J.W., Fukui, H.H., Townsend, D.R., Vaupel, J.W., 1992. Demography of genotypes: failure of the limited life span paradigm in Drosophila melanogaster. Science 258, 461e463. Hamilton, W.D., 1966. The moulding of senescence by natural selection. J. Theor. Biol. 12, 12e45. Jones, O.R., Scheuerlein, A., Salguero-Gomez, R., Giovanni Camarda, C., Schaible, R., Casper, B.B., Dahlgren, J.P., Ehrlen, J., Garcıa, M.B., Menges, E.S., QuintanaAscencio, P.F., Caswell, H., Baudisch, A., Vaupel, J.W., 2014. Diversity of ageing across the tree of life. Nature 505, 169e173. Mueller, L.D., Rose, M.R., 1996. Evolutionary theory predicts late-life mortality plateaus. Proc. Natl. Acad. Sci. U.S.A. 93, 15249e15253. Rose, M.R., Rauser, C.L., Benford, G., Matos, M., Mueller, L.D., 2007. Hamilton’s forces of natural selection after forty years. Evolution 61, 1265e1276.
1966 Evolution of demographic parameters The concept William D. Hamilton (1966) sought to develop a simple understanding of how natural selection acts on equivalent changes in mortality as a function of age. He came up with the simple result that starting at sexual maturity, the strength of selection acting on equivalent changes in mortality will decrease with increasing age. Thus, senescence is a logical byproduct of natural selection.
The explanation Shortly after his work on kin selection, Hamilton (1966) explored the action of natural selection in age-structured populations. He adopted the Malthusian parameter (m), utilized by both by Norton (1928) and Fisher (1930), as the measure of fitness in an age-structured population, which is Z N the positive root, m, of the Lotka-Euler equation, emx lx fx ¼ 1, where 0
lx is the probability of surviving to age-x and fx is the fecundity of females aged-x. If age is broken into discrete classes, 1, 2, ., d, and px is the chance of an individual surviving from age-class x to age-class xþ1, lx ¼ p1p2 . px1. Hamilton looked at changes in px caused by mutations that increase or decrease survival by a factor 1d. From this type of analysis he found that similar changes in age-specific survival at different ages would always be most strongly favored (in the case of an increase in survival) at earlier ages. He concluded from this model that “A basis for the theory that senescence is an inevitable outcome of evolution is thus established”. A test of this theory is described in Chapter 50. Baudisch (2005) has recently suggested that there are other ways to compare mutations at different ages which won’t necessarily give the same results. For instance, age-specific survival may be increased by an exponential factor, e.g. p(1þd) . In this case, natural selection might resist senescent x decline if fecundity changes with age in an appropriate direction. There Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00025-9
© 2020 Elsevier Inc. All rights reserved.
55
j
56
Conceptual Breakthroughs in Evolutionary Ecology
are at present few well documented cases of organisms which do not show increasing mortality with age. Some studies of natural populations proport to show this (Jones et al., 2014), but such studies in nature encounter severe technical issues (see appendix). The evolutionary principles set forth by Hamilton have more recently been used to study late-life mortality plateaus. Carey et al. (1992) and Curtsinger et al. (1992) studied very large cohorts of laboratory fly populations and showed that although these populations displayed the expected increase in mortality through much of their adult life, at extremely old ages these mortality rates leveled off. These plateaus were an unexpected anomaly. However, Mueller and Rose (1996) demonstrated that such plateaus are an expected byproduct of the evolutionary forces identified by Hamilton plus random genetic drift. The impact of Hamilton’s (1966) work has been reviewed by Rose et al. (2007).
Impact: 10 The theory developed by Hamilton has had lasting impact and has been empirically supported in numerous experiments (Rose et al., 2007). As such it is one of the foundational breakthroughs in evolutionary biology.
References Baudisch, A., 2005. Hamilton’s indicators of the force of selection. Proc. Natl. Acad. Sci. U.S.A. 102, 8263e8268. Carey, J.R., Liedo, P., Orozco, D., Vaupel, J.W., 1992. Slowing of mortality rates at older ages in large medfly cohorts. Science 258, 457e461. Curtsinger, J.W., Fukui, H.H., Townsend, D.R., Vaupel, J.W., 1992. Demography of genotypes: failure of the limited life span paradigm in Drosophila melanogaster. Science 258, 461e463. Hamilton, W.D., 1966. The moulding of senescence by natural selection. J. Theor. Biol. 12, 12e45. Jones, O.R., Scheuerlein, A., Salguero-Gomez, R., Giovanni Camarda, C., Schaible, R., Casper, B.B., Dahlgren, J.P., Ehrlen, J., Garcıa, M.B., Menges, E.S., QuintanaAscencio, P.F., Caswell, H., Baudisch, A., Vaupel, J.W., 2014. Diversity of ageing across the tree of life. Nature 505, 169e173. Mueller, L.D., Rose, M.R., 1996. Evolutionary theory predicts late-life mortality plateaus. Proc. Natl. Acad. Sci. U.S.A. 93, 15249e15253. Rose, M.R., Rauser, C.L., Benford, G., Matos, M., Mueller, L.D., 2007. Hamilton’s forces of natural selection after forty years. Evolution 61, 1265e1276.