Evidence for a species core
Despite the lack of population structure or signals of selection, we did find expected core-edge patterns of genetic diversity. We found higher nucleotide diversity (pairwise theta) in regions in the native range compared to the expanded ranges (Fig. 3A), consistent with classic core-edge expectations, though the magnitude of these differences is small. The Pacific Coast region, in the native range, had the highest average nucleotide diversity at 1.88x10-3 while the eastern and Washington state expanded regions had the lowest values at 1.74x10-3 and 1.77x10-3, respectively. Decreased diversity in the expanded regions did not appear to be driven by increased relatedness among sampled individuals (Fig. S5A). However, some counties in the northern and eastern expanded regions appeared to have lower heterozygosity, which could contribute to the observed pattern (Fig. S5B). Using the directionality index, ψ, calculated pairwise across six broad regions, we identified the greater San Francisco Bay Area and the California Central Valley regions as sources of gene flow and the expanded range in Washington state as a majority sink of gene flow (Fig. 3B, Table S4). However, all of the ψ values were below 0.05 (absolute value), which is far lower than the cutoff used in the original paper, an absolute value of 3, and those seen in other studies using this method . The non-significant directionality pairwise comparisons are consistent with our high levels of gene flow.