2.4.2 Identifying potential outliers
A modified version of POWSIM (Ryman &
Palm, 2006) was used to investigate whether the distribution ofF ST values in our Pool-seq data was consistent
with the expectation for selectively neutral loci, or if there was
deviation from such a distribution which could reflect directional
selection (cf. Lamichhaney et al., 2012).
We compared the observed genome-wide distribution ofF ST with a simulated “expected” distribution
under drift only. The observed distribution was based on per SNP
estimates (1 bp; n =12,177,462; meanF ST=0.083). We used a conservative approach
defining outliers as SNPs with an observed F STabove the largest value of the expected distribution
(F ST>0.864; details in Appendix S3).
We also defined outliers as those SNPs with anF ST above a thresholds corresponding to the
99.5th percentile (e.g.
Fabian et al., 2012;
Keehnen, Hill, Nylin, & Wheat, 2018) of
the genome-wide F ST distribution. A threshold of
the 97.5th percentile was used forF ST based on 5 kb windows (e.g.
Gagnaire et al., 2018;
Keehnen et al., 2019).