Thesis #32 – Before the forces of natural selection plateau, it is possible for genetic drift, due to small population sizes among other possibilities, to weaken the ability of natural selection to distinguish among genetic variants affecting later adult life, leading to the evolution of even earlier plateaus in survival and reproduction.
But there are still other factors at work in the evolution of the cessation of aging. One of the most important of these is the role that population size can play in determining the window of adult ages over which natural selection is effective. This finding was discovered by Larry Mueller during his numerical studies of the implications of age-specific selection for the evolutionary transition from aging to later adult life. [See the Mueller and Rose (1996) article, listed in the bibliography.]
As mentioned before in the 55, there are a number of factors that serve to limit the power and efficacy of natural selection. One of the most important of these is the number of breeding individuals in a population, what is called the “effective population size” in evolutionary theory. Effective population size is quantitatively complicated, but basically it can be understood as the number of individuals who are contributing genes to the next generation. This concept also has age-specific quantitative features. In particular, the forces of natural selection are further attenuated with adult age in finite populations, compared to their theoretical power in populations that are infinitely large.
To give you some idea of what this means in human terms, the present human population size is about 7 billion people, with an effective population size of about half that, somewhere around 3 billion. At such a large effective population size, there is little attenuation in the forces of natural selection due to population size. But in our ancestral condition, prior to agricultural, we had a much smaller global population size, and we were split up into separate breeding groups, further reducing the effectiveness of natural selection, at every age.
Larry Mueller’s computer analysis shows that such small effective population sizes will lead to the evolution of much earlier mortality plateaus. Thus, in human terms, hunter-gatherer populations probably evolved mortality plateaus somewhere in their fifties or sixties, rather than the eighties or nineties that appear to be the case for agricultural populations on organic agricultural diets. In this respect, our ancestors were probably more like many other large mammal populations, especially mammalian predators, which rarely achieve the large population sizes of either widespread rodents, like rats, or ungulates, like caribou or bison, prior to European colonization of North America. Thus, thanks to the limits on their past population sizes, hunter-gatherer populations living their ancestral lifestyle probably undergo much earlier plateaus in their mortality rates, compared to agricultural populations that have risen to much higher population sizes, with greater efficiency of travel between populated areas. Thus European, Middle Eastern, and other Asian populations have probably had prolonged aging relative to that of non-agricultural populations, simply because of the much large population sizes of the former group. Or at least they probably do when subjected to the agricultural conditions under which the former group have evolved over the last ten to twenty thousand years. More on this later.