No evidence of selection
We found no evidence for either local adaptation or global selection that might be linked to the range expansion. For local adaptation we used FST to compare the expanded regions (Washington State and the eastern populations) to their nearest native range region (the Bay Area of California and the Pacific coast of southern California), but no obvious peaks stood out for either comparison (Fig. 2A–B). Chromosomal mean pFSTs were p = 0.7 ± 0.2 and no SNPs were significant after Bonferroni or false discovery rate corrections. Based on the most significant 1% of p-values from pFst, we found 6,079 SNPs that were shared between the two expanded region comparisons, only slightly higher than the expected number of high FST shared SNPs (5,599 SNPs). An alternative to divergence due to spatially varying selection is that with high gene flow homogenizing genetic diversity, selection from resources and environments in the expanded range would affect allele frequencies in the entire species. However, we also found no evidence of selective sweeps when analyzing the site frequency spectrum generated from all samples (Fig. 2C). In fact, the composite likelihood ratio (CLR) statistics representing the test for selective sweeps were all below 2.0, much lower than standard cutoffs found in many studies .