Thesis #48 – Agricultural populations have also undergone substantial increases in population size compared to those of their ancestral hunter-gatherer populations, which increased the effectiveness of natural selection at later adult ages, resulting in the evolution of a delay in the cessation of aging under agricultural conditions.

In addition to the massive change in natural selection arising from altered diet and activity, agricultural civilizations feature radically increased effective population sizes, especially since the advent of recorded history.  If this concept is elusive to you, think about the following question:  how many people of the opposite sex could you have met, copulated with, and had children with during the last twenty years?  I don’t mean what multiple of individuals, I mean even if it were just one person, how many people on the planet, conceivably, could you have mated with?

The answer is around one or two billion, allowing for the too young and the too old.  This is because we have had access to efficient air travel over the last twenty years, and for much of this period travel has been relatively unrestricted, leaving aside police-states like North Korea, Cuba, and Iran.  That, crudely, is the scale of effective population size of our species at present.

During the hunter-gatherer phase of our evolutionary history, the total number of individuals available for mating to any one individual was orders of magnitudes less.  Travel was not very efficient, as we had neither horses nor roads.  And our population densities were vastly less.  Perhaps you would have had a “dating pool” of some thousands.

In agricultural civilizations, such as those of ancient Rome or China throughout the last two millennia, there were good roads and wide circulation of merchants, soldiers, and slaves within and just beyond the perimeters of such societies.  There were hundreds of thousands of individuals with whom a person might conceive children in their lifetime.  Marriage would have been socially or legally constrained by conventions of nationality, class, or caste.  But a significant amount of fertilization would have involved slaves, prostitutes, concubines, mistresses, rape, and other “less official” forms of mating, enough to ensure a very large effective population size.

The issue here is closely related to the premises with which the 55 began.  Natural selection works with full effectiveness only in large populations, and at early ages.  Species with population sizes like those of hunter-gatherer humans had the effectiveness of natural selection greatly constrained by population size, because we were not a particularly abundant species compared to those of most insects, or even those of many small mammals.  Large mammals have some of the smaller population sizes among all animal species.  It was only with the advent of agricultural civilization that human populations became large, and these populations have furthermore been much more integrated in terms of opportunities for mating.