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10 Intergenerational Transfers, Social Arrangements, Life Histories, and the Elderly--Ronald Lee
Pages 223-246

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From page 223...
... While reproduction involves costly transfers, it lies at the very heart of evolution and natural selection, and does not require explanation through kin selection or inclusive fitness.
From page 224...
... Yet intergenerational transfers can profoundly alter the energetic budget constraint faced by the individual organism over its lifecourse, since an offspring receiving transfers can consume more than it acquires through its own efforts, in which case adults must consume less than they acquire. My own research, much of which is joint with Cyrus Chu, and sometimes also with Hung-Ken Chien or Carl Boe, has focused on the role of intergenerational transfers in life history theory.
From page 225...
... analysis of how natural selection molds senescence. Hamilton's study used sensitivity analysis, asking how much a perturbation in fertility or mortality at a given age would affect reproductive fitness measured by the intrinsic rate of natural increase (stable population growth rate)
From page 226...
... Individuals of all ages live in food-sharing groups in which population age distributions are on average stable and depend on the levels of fertility and mortality determined by the model. Foraging success depends also on overall population density relative to a given resource, so population equilibrates eventually at a particular density, foraging productivity, fertility, mortality, and age distribution.
From page 227...
... T(a) is the share of lifetime net transfers to others remaining at age a.
From page 228...
... : "food provisioning is ubiquitous, generally biased in favour of helping families with large dependency loads and not limited to kin assistance." Food consumption and the transfers implicit in these sharing arrangements affect reproductive fitness because each individual's fertility and mortality are related to its food consumption. These relationships are calibrated on estimated relations in historical demographic data (Lee, 2008)
From page 229...
... In this case, there is no selective advantage to post-reproductive life since sharing is not at all kin-based. In this case, evolved adult mortality conforms to Hamilton's theory: It is shaped solely by remaining lifetime fertility.
From page 230...
... Presumably sharing raised fitness through maternal life insurance and through smoothing of dependency burdens. Social arrangements with broader sharing are able to reproduce successfully at higher densities.
From page 231...
... In other simulations in which childhood consumption was specified to raise adult foraging productivity, the outcome was the opposite of Figure 10-3: The large sharing groups went extinct first, the intermediate Third Cousin groups went extinct next, and the smallest and most variable
From page 232...
... OPTIMAL LIFE HISTORIES WITH INTERGENERATIONAL TRANSFERS To get at the more fundamental forces shaping the observed age patterns, I turn to a different approach: finding the optimal life history using analytic methods. This approach avoids the circularity of the mutation accumulation and sensitivity analysis approach.
From page 233...
... Despite the very different theoretical approaches and assumptions relative to the transfer model described earlier, there are many formal similarities in the results, and the central elements that emerge from the sensitivity approach occur again in the optimal life history approach. Here are some basic results on the age-shape of mortality across the life span (Chu et al., 2008)
From page 234...
... However, in optimal life history analysis. optimization is forwardlooking from each given age, unlike sensitivity analysis, which evaluates a perturbation's impact at birth.
From page 235...
... We show that evolution could move the human life history in this direction of further specialization, eventually reaching the reproductive "corner" outcome where the older woman has zero fertility (menopause) and the younger woman has higher fertility, shorter birth intervals, and greater reliance on the older woman's assistance to cope with the increased dependency burden.
From page 236...
... INTERGENERATIONAL TRANSFERS AND SEXUAL DIMORPHISM The classic theory of sexual dimorphism, dating to Darwin, begins with the observation that females invest much more heavily in reproduction than do males, starting with the trivial cost of sperm compared to the female egg, and continuing with the costs of pregnancy for viviparous species and birds, and then post-birth care, mammalian lactation, and transfers (although transfers are sometimes shared with males)
From page 237...
... . Intergenerational transfers evolve in species that have a high rate of time preference between early and adult years.
From page 238...
... Intergenerational transfers benefit the recipient but not the giver -- other than indirectly through reproductive fitness. Intergenerational transfers are never reciprocal, since by definition there is no quid pro quo.
From page 239...
... The aggregate credit balance is proportional to the difference in average ages of receiving and contributing, which is zero.
From page 240...
... In fact, every member of the population expects to make more transfers in the future than will be received. The aggregate credit balance is therefore negative, and indeed is proportional to the difference in average ages of receiving and donating, which is g -- one generation.
From page 241...
... DIRECT AND INDIRECT GENETIC EFFECTS AND INTERGENERATIONAL TRANSFERS Intergenerational transfers are all instances of indirect genetic effects, to the extent that the transfer behavior is genetically influenced and therefore heritable. Review articles on indirect gene effects often give maternal care of offspring, the most basic kind of intergenerational transfer, as a lead example of an indirect genetic effect (Wolf et al, 1998; Cheverud, 2003)
From page 242...
... The flatness may arise in an optimal life history because even in species with deterministic growth like mammals, the accumulation of skills and knowledge may continue after somatic growth has ceased, so the opportunity set may continue to expand (Chu et al., 2008, and references therein)
From page 243...
... These sensitivities might then be argued to shape the evolution of life histories. A matrix of effects from theory combined with focused experiments or field studies could be developed, as has been done in a partial way focusing on optimal clutch size for birds, as well as an attempt to estimate the matrix of effects empirically from observations on a group over time.
From page 244...
... Unlike the Leslie matrix, it would have no zeros because it would be filled by indirect effects operating through density, inter-age transfers, and competition for transferred resources. In this setup, genes have both direct and indirect genetic effects on reproductive fitness (Cheverud, 2003; Wolf, 2003)
From page 245...
... . The coevolution of intergenerational transfers and lon gevity: An optimal life history approach.
From page 246...
... . Intergenerational Transfers, Life Histories and the Evolution of Sociality.


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