ABSTRACT
Population and quantitative genetics provide useful approximations to predict the evolution of populations and their multilocus adaptive dynamics. They are not supposed to hold under extreme parameter combinations, for which deviations need to be further quantified to provide insights into specific population dynamics. Here we focused on small selfing populations evolving under an under-explored High Drift-High Selection (HDHS) regime. We combined experimental data from the Saclay divergent selection experiments on maize flowering time, forward individual-based simulations, and theoretical predictions to dissect the evolutionary mechanisms at play in the observed selection response for a highly complex trait. We asked two main questions: How do mutations arise, spread, and reach fixation in populations evolving under HDHS? How does the interplay between drift and selection influence the response to selection ? We showed that the long-lasting response to selection in populations whose estimated effective population size ranged between 2.5 to 4 is due to the rapid fixation of de novo mutations. Among all fixed mutations, we found a clear signal of enrichment for beneficial mutations revealing a limited cost of selection in these populations. We argue that environmental stochasticity and variation in selection coefficients contribute to exacerbate mutational effects, thereby facilitating selection grasp and fixation of small-effect mutations. Hence the HDHS regime with non-limiting mutation highlights an interesting interplay between drift and selection that sustains a continuous response to selection. We discuss our results in the context of breeding populations and long-term survival of small selfing populations.
Competing Interest Statement
The authors have declared no competing interest.
