Outlier Detection
For S. umbilicalis , 143 loci (1.2%) were identified by at least
three methods, with 52 and 14 of these loci identified by four and five
methods, respectively (Table 3). Observed heterozygosity was lower than
expected and FIS was positive for all populations (Table
S6). FST values ranged from 0-0.368 at outlier loci
compared to 0-0.026 at neutral loci, and 80.3% and 81.8% of these
pairwise comparisons were significant (p=0.001) in the neutral and
outlier datasets, respectively (Fig. 2A; Table S7). For N.
lapillus , 24 loci (0.6%) were identified by at least three methods,
with only one locus identified by four methods (Table 3). Observed
heterozygosity was higher in N. lapillus and values were similar
to expected heterozygosity across all sites (Table S8).
FIS was lower in N. lapillus in comparison toS. umbilicalis (Table S8). FST values ranged from
0.015-0.637 in outlier loci compared to 0.033-0.362 in neutral loci, and
all pairwise comparisons were significant (p=0.001), with the exception
of the north Wales sites (Llanddulas & Great Orme West, p=0.211) in the
neutral marker dataset (Fig. 3A, Table S9). All downstream analyses of
adaptive population structure were carried out on these outlier
datasets. Our resulting neutral marker datasets contained 11,457 and
3,820 loci for S. umbilicalis and N. lapillus ,
respectively.