When does the direction of natural selection depend on population size?

Daniel WEINREICH

Department of Ecology and Evolutionary Biology, and Center for Computational Molecular Biology, Brown University, Providence, Rhode Island

Classical evolutionary theory holds that the efficiency, but not the direction, of natural selection depends on population size. In small populations, drift overwhelms selection, rendering all fitness-affecting mutations selectively neutral. Yet, beneficial mutations never become deleterious and deleterious mutations never become beneficial. Remarkably, several mutations, including in modifiers of recombination and mutation rate, have now been shown to be favored at some population sizes but disfavored at others, contradicting established theory.  Previously, we have designated this phenomenon sign inversion. Here we show that all mutations susceptible to sign inversion can confer both fitness costs and benefits on their carriers that vary either between mutant lineages or across time within the same lineage. Using computer simulations, we demonstrate that both between-lineage and within-lineage variability can cause sign inversion and elucidate the common mechanism. Our results confirm that variability in the sign of selective effect is ultimately necessary for sign inversion, which occurs because selection against carriers bearing the cost and selection for carriers enjoying the benefit are overwhelmed by drift at different population sizes.

Recent publications:

1.     https://www.pnas.org/content/115/13/3422

2.     https://elifesciences.org/articles/51177

3.     https://www.genetics.org/content/203/2/923.article-info

Link to seminar: https://umontpellier-fr.zoom.us/webinar/register/WN_BNNG2ES2SgWAwk1xC7KrDg