One leg of each specimen was removed with sterile forceps, and genomic DNA extracted using the Qiagen DNeasy kit, according to the manufacturer's protocol (Qiagen, Hilden, Germany). A total of nine Iberian and nine Baltic DNA samples were chosen each from three populations, and their concentration measured on a NanoDrop Fluorospectrometer (NanoDrop, Wilmington, DE, USA). Equal DNA amounts were pooled for the two geographic regions, and each pool sequenced in 300 bp paired reads on one flow cell of an Illumina MiSeq. Library preparation and sequencing were conducted according to the manufacturer's protocols (Illumina, San Diego, CA, USA). The resulting DNA reads were quality trimmed with a minimum quality of 20 using Popoolation (Kofler et al. 2011). A de novo assembly was then generated including both libraries and using CLC genomic workbench with a word size of 45, a bubble size of 98, a minimum contig length of 1000 and including a scaffolding step (CLC Bio, Boston, MA, USA). We identified microsatellite markers in the genome assembly using Tandem Repeats Finder (Benson 1999). We then established a set of 14 polymorphic markers, which were genotyped for 260 specimens from all studied populations. Marker isolation, primer design, PCRs and genotyping were performed as described in Krehenwinkel and Tautz (2013). Microsatellite alleles were called using GeneMapper (Applied Biosystems, Foster City, CA, USA). Microsatellite analyzer (MSA) was used to generate population distance matrices (Nei's genetic distance) with 500 bootstrap replicates for the microsatellite data and to estimate genetic diversity (Dieringer and Schlötterer 2003). A neighbor‐joining tree was calculated based on the distance data with Phylip (Felsenstein 2002) and edited in MEGA 6. Pairwise F ST values between populations were calculated in Genepop on the web (Raymond and Rousset 2002). In addition to the distance based analysis, we performed genetic clustering analysis using STRUCTURE (Pritchard et al. 2000; Falush et al. 2003). STRUCTURE was run with an admixture model, k‐values from 1–10, 10 replicates per k‐value, a burnin period length of 100 000 and 100 000 MCMC replications after burnin. The number of populations was estimated with STRUCTURE HARVESTER (Earl and von Holdt 2011) using the approach described in Evanno et al. (2005). We performed two independent STRUCTURE analyses: one with all sampled populations and one with only native European populations, to exclude potential masking effects by highly divergent expanding populations.