where all averages are across genes and PS, DS , PN and DN are the number of synonymous polymorphisms, synonymous substitutions, nonsynonymous polymorphisms and nonsynonymous substitutions, respectively. Confidence intervals of alpha were obtained through bootstraps (1,000 bootstraps) by randomly selecting genes with replacement as described in . In this case, substitutions (D) were considered as SNPs with an FSTvalue greater or equal to 0.9.