Abstract
From the standpoint of evolution, caring for old parents may be maladaptive, because they have ceased reproduction, so that the benefit for inclusive fitness may not be expected. Then why do we care for old parents? In this study, the evolution of care for the elderly was examined, by using an evolutionary genetic model, in which pleiotropic constraints between behaviors expressed in different social contexts among family members were assumed. It was suggested that establishing a solid relationship with parents during infancy should be selectively favorable, even though old parents have to be cared for in the future, but that caring for old parents may be excluded from the population if this behavior imposes high costs on reproduction of the younger generation. Despite the diminishing numbers of individuals within the population, care for the elderly may not be readily selected against, but at the same time this may not contribute to the rate of increase in population size. The significance of discussing the behavior of elderly caring from the standpoint of evolutionary genetics was emphasized.
Highlights
Senescence, which is defined as the decline in a wide range of bodily functions with age, is an almost universal phenomenon among organisms [1]
For the basic vital statistics of the hypothetical population used in this study, which is suffering from population aging and diminishing population size, the intrinsic rate of increase, generation time, and net reproduction rate in the 5-year time-unit were estimated as −0.081, 6.56, and 0.590, respectively
Under the conditions of population aging and diminishing population size, which countries such as Japan are or will be faced with, the relationship between changes in m(x) and corresponding changes in r increased with age, so that benefits contributed by old people to m(x) of younger individuals would have more positive effects on population increase than those contributed by any other young individual, if the amount of benefits is the same
Summary
Senescence, which is defined as the decline in a wide range of bodily functions with age, is an almost universal phenomenon among organisms [1]. In spite of its universality, the problem posed by this apparently deleterious phenomenon is to explain why organisms had evolved not to continue living and reproducing forever but to senesce and die . T. Miyo cept for solving this problem that the force of natural selection on deleterious genes declines with age. The later the deleterious genes are expressed, the weaker the force of natural selection. Deleterious genes expressed late in life would not be eliminated from the population, and the population tends to accumulate these deleterious mutations, resulting in senescence and ultimate death of organisms as an inevitable consequence of evolutionary processes within the population
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