Correlation between the intensity of purifying selection on a gene in different populations between species (A) and within species (B). The number of gray squares represents the sample size in each population. (A) The estimated correlation coefficient in the comparison between population samples from S. cerevisiae and S. paradoxus. The gray arrows point to the sister population that was used to call SNPs as fixed (see Methods). (B) The estimated correlation coefficient based on fixed SNPs between two populations, X and Y, and fixed SNPs between the common ancestor of these populations and a third population, Z, from the same species. See text for further details. The central 80th percentiles of the correlation coefficient (in gray) were estimated by parametric bootstrap (with 100 replicates).
