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.