Is drift 'directional'? Unequal breeding sex ratio revisited.

This article focuses on the impact of unequal breeding sex ratio (BSR) on genetic drift and effective population size (Ne). It introduces a method based on binomial sampling to measure genetic drift through mean deviation (MD) of allele frequency changes, demonstrating that although unequal BSR can cause a rare neutral allele to increase in frequency, drift remains non-directional overall. The study finds that MD can be split into two equal but opposite components corresponding to allele frequency increases and decreases, with the direction of drift in individual cases predictable by the allele proportion in the rarer sex, allele frequency, and Ne. The research highlights that extreme BSRs, while seemingly directional in effect, do not violate the fundamental neutrality of drift, and the method offers a detailed framework for analyzing drift under unequal sex ratios.