Population type influences the rate of ageing

Andrew Dj Overall,Richard Ga Faragher

doi:10.1038/s41437-019-0187-1

Abstract

Mutation accumulation is one of the major genetic theories of ageing and predicts that the frequencies of deleterious alleles that are neutral to selection until post-reproductive years are influenced by random genetic drift. The effective population size (Ne) determines the rate of drift and in age-structured populations is a function of generation time, the number of newborn individuals and reproductive value. We hypothesise that over the last 50,000 years, the human population survivorship curve has experienced a shift from one of constant mortality and no senescence (known as a Type-II population) to one of delayed, but strong senescence (known as a Type-I population). We simulate drift in age-structured populations to explore the sensitivity of different population ‘types’ to generation time and contrast our results with predictions based purely on estimates of Ne. We conclude that estimates of Ne do not always accurately predict the rates of drift between populations with different survivorship curves and that survivorship curves are useful predictors of the sensitivity of a population to generation time. We find that a shift from an ancestral Type-II to a modern Type-I population coincides with an increase in the rate of drift unless accompanied by an increase in generation time. Both population type and generation time are therefore relevant to the contribution mutation accumulation makes to the genetic underpinnings of senescence.

Highlights

IntroductionIf the population is stable, survivorship curves describe how the numbers of individuals of a cohort decline with time
The survivorship curve is a useful visualisation of the frequency distribution of the age classes of a population (Rauschert 2010) and is calculated as lx = nx/n0, where nx is the number of individuals in the study population who survive to the beginning of age category x and n0 is the number of newborns
The two main genetic causes of senescence, antagonistic pleiotropy and mutation accumulation, have different expectations with regard to the frequency distribution of mutations that are deleterious to the older age classes of a species (Rodrguez et al 2016)

Summary

Introduction

If the population is stable, survivorship curves describe how the numbers of individuals of a cohort decline with time. The survival curve of modern humans is described as a classic “Type-I”, where the probability of survival is high until relative old age, whereby it declines rapidly, which is typical of many large mammals. Our closest living relative species, the chimpanzee, has a survivorship curve that is variable, depending on whether the population is wild or captive (Thompson et al 2007), but the wild examples are arguably closer to a TypeII (Hill et al 2001; Bronikowski et al 2016), which is described by a constant proportion of individuals dying over time.

Methods

Results

Conclusion