Mapping the Peaks: Fitness Landscapes of the Fittest and the Flattest

Abstract

Background Populations exposed to a high mutation rate harbor abundant deleterious genetic variation, leading to depressed mean fitness. This reduction in mean fitness presents an opportunity for selection to restore adaptation through the evolution of mutational robustness. In extreme cases, selection for mutational robustness can lead to "flat" genotypes (with low fitness but high robustness) out-competing "fit" genotypes with high fitness but low robustness-a phenomenon known as "survival of the flattest". While this effect was previously explored using the digital evolution system Avida, a complete analysis of the local fitness landscapes of "fit" and "flat" genotypes has been lacking, leading to uncertainty about the genetic basis of the survival of the flattest effect.

Moderate Amounts of Epistasis are Not Evolutionarily Stable in Small Populations

“…we also observe something akin to truncation selection [9,10]. In drift robustness, we observe both an increase in neutral mutations as well as an increase in strongly deleterious and lethal mutations, mediated by strong negative epistasis (q > 1).…”

“…Populations can adapt to high mutation rates and/or small population sizes by evolving "mutational robustness" [5] or "drift robustness" [6][7][8]. Populations evolve mutational robustness by moving onto flat fitness peaks, where they experience a reduction in maximum fitness counterbalanced by an increased fraction of new mutations that are either neutral or have a small fitness effect [9,10]; this phenomenon is often referred to as the "survival-of-the-flattest" effect [9]. Robustness to drift, on the other hand, appears to involve favoring fitness peaks that have steep flanks, enabled by mutations that are synergistic in their deleterious effect [6,7], while reducing (rather than increasing) the likelihood of mutations with small effect, and increasing the fraction of mutations that are lethal.…”

“…Robustness to drift, on the other hand, appears to involve favoring fitness peaks that have steep flanks, enabled by mutations that are synergistic in their deleterious effect [6,7], while reducing (rather than increasing) the likelihood of mutations with small effect, and increasing the fraction of mutations that are lethal. Interestingly, a recent re-analysis of the survival-of-the-flattest effect has shown that an increase in the fraction of lethal mutations is also seen in the response to high mutation rates [10], suggesting that resistance to drift and resistance to mutations are intertwined (see also [8]).…”