Selection Against Demographic Stochasticity in Age-Structured Populations

TL;DR

This paper extends Gillespie's results to age-structured populations, showing that selection favors genotypes with lower demographic stochasticity, and provides a diffusion approximation consistent with simulations.

Contribution

It introduces a model linking demographic variance in age-structured populations to selection, generalizing previous work to include age-specific survival and fertility.

Findings

Selection favors genotypes with lower demographic stochasticity.

The diffusion approximation aligns with individual-based simulations.

Perturbation analysis helps assess sensitivity of fitness to life history changes.

Abstract

It has been shown that differences in fecundity variance can influence the probability of invasion of a genotype in a population, i.e. a genotype with lower variance in offspring number can be favored in finite populations even if it has a somewhat lower mean fitness than a competitor. In this paper, Gillespie's results are extended to population genetic systems with explicit age structure, where the demographic variance (variance in growth rate) calculated in the work of Engen and colleagues is used as a generalization of "variance in offspring number" to predict the interaction between deterministic and random forces driving change in allele frequency. By calculating the variance from the life history parameters, it is shown that selection against variance in the growth rate will favor a genotypes with lower stochasticity in age specific survival and fertility rates. A diffusion approximation for selection and drift in a population with two genotypes with different life history matrices (and therefore, different growth rates and demographic variances) is derived and shown to be consistent with individual based simulations. It is also argued that for finite populations, perturbation analyses of both the growth rate and demographic variances may be necessary to determine the sensitivity of "fitness" (broadly defined) to changes in the life history parameters.