External seminar - Guillaume Achaz (collège de France)

Abstract: Neutral theory assumes that in a population of size N, diversity results from an equilibrium between new mutations arising at rate μ and genetic drift that purge them at rate 1/N, predicting an equilibrium value proportional to Nμ. The difference between this expectation and the much lower observed molecular diversity is known as the Lewontin's paradox of variation. Here, we investigate the effect of genetic draft, a regime of evolution where recurrent sparse selective sweeps entirely drive the diversity of surrounding loci. More specifically, we focus on the neglected distant effect of selective sweeps on remote neutral loci, where the effect of a single sweep is almost negligible. We derived novel mathematical approximations of this underexplored regime and show that under weak genetic draft, diversity at neutral loci is a power law of the population size: N e^pi ~ N^(2A), for A < 0.5, where A is the ratio between recombination rate and coefficient of selection (A = c/s). Interestingly the Site Frequency Spectrum at neutral loci is identical to the one produced by genetic drift, as the underlying coalescent tree is an n-Kingman coalescent. In brief, weak genetic draft produces patterns of diversity that look entirely neutral, while being drastically reduced in magnitude. Ultimately, our study points to the need to explore evolutionary models for which diversity looks neutral but does not scale linearly with population size.
 

Preprint: https://www.biorxiv.org/content/10.1101/2023.07.19.549703v3