Allele size standardization
Genotype data from different studies obtained with the aforementioned six microsatellite sets were combined after standardizing allele sizes using the Genalex add-in for Excel (version 6.503) (Peakall & Smouse, 2006; Peakall & Smouse, 2012). Standardization was necessary, because allele size estimates have been observed to differ up to 7 bp among platforms (LaHood, Moran, Olsen, Grant, & Park, 2002). In our case they differed up to 11 bp (microsatellite set B relative to set A:BM3517 : 8 bp, INRA006 : 11 bp, ≤5 bp in the remaining cases). For various populations, two microsatellite sets were available (sets A-D in Tables 1 and 2), which permitted standardization by shifting allele sizes by a constant amount in one of the two sets. With two microsatellites sets (sets E and F in Tables 1 and 2) standardization was only possible in comparison with another microsatellite set applied to another population. The size shift we used was the one that resulted in the highest Pearson correlation in allele frequencies and lowest F ST between two sets. Given large enough sample sizes, their values are expected to be close to 1 and 0, respectively. Generally, the obtained Pearson correlations were >0.8 and the F ST values <0.01. Using the standardized data to regress pairwise multilocus F ST values on geographic distance explained 78% of the genetic variation among populations in East and southern Africa (Figure S1), thereby providing further support for the accuracy of the allele size standardization. A detailed description of the allele size standardization is given in Text S1.