Evolutionary inference from Q(ST)

Q(ST) is a commonly used metric of the degree of genetic differentiation among populations displayed by quantitative traits. Typically, Q(ST) is compared to F-ST measured on putatively neutral loci; if Q(ST) = F-ST, this is taken as evidence of spatially heterogeneous and diversifying selection. This paper reviews the uses, assumptions and statistics of Q(ST) and F-ST comparisons. Unfortunately, Q(ST)/F-ST comparisons are statistically challenging. For a single trait, Q(ST) must be compared not to the mean F-ST but to the distribution of F-ST values. The sources of biases and sampling error for Q(ST) are reviewed, and a new method for comparing Q(ST) and F-ST is suggested. Simulation results suggest that the distribution of neutral F-ST and Q(ST) values are little affected by various deviations from the island model. Consequently, the distributions of Q(ST) and F-ST are well approximated by the Lewontin-Krakauer prediction, even with realistic deviations from the island-model assumptions.