Molecular Evolutionary Analysis for Estimating the Strength of Fluctuating Selection among Individuals (FSI)

Fluctuating selection among individuals (FSI) refers to any mutation that exhibits different fitness effects among individuals. Thus, the selection nature of a mutation (deleterious, neutral, or beneficial) should be interpreted by the means of the population average. For instance, a neutral mutation on average could be slightly deleterious in some individuals, and slightly beneficial in others. It has been recently demonstrated that the effect of FSI is important in molecular evolution especially when the effective population size ( N _e ) is not small. Intriguingly, a novel pattern of molecular evolution called ‘selection duality’, i.e., mutations that are statistically slightly beneficial are subject to a negative selection, emerges under the condition that selective advantage is less than FSI. While FSI sheds some lights on the long-term neutralist-selectionist debate, an immediate question is how to calculate the strength of FSI-genetic drift relative to the N _e -genetic drift. In this article we develop a statistical method the relative FSI strength ( F ): if F is close to 0, the N _e -genetic drift is dominant; whereas the FSI-genetic drift is dominant if F is close to 1. One may tentatively set F =0.5 as an empirical criterion to weigh between those two genetic drifts. Our case study showed that the relative FSI-strength F is over 0.5 in most species, suggesting that the FSI-genetic drift, rather than the N _e -drift, plays a major role in metazoan genome evolution.