Population Structure
To assess population structure in each species, we explored the
genome-wide SNP data using STRUCTURE v2.3.4 (Pritchard et al.2000). We ran STRUCTURE for K values 1 to 10 with 5 replicates
per K , where each replicate is a different sample of unlinked
SNPs, subsampled from the same linked SNP dataset. We ran STRUCTURE for
500,000 generations and discarded the first 10% as burn-in. The data
were modeled assuming admixture and correlated allele frequencies
between populations (Falush et al. 2003), while all other
parameters were kept as their default. For the sake of comparison to
results reported by Fernandez et al. (2021), Structure Harvester (Earl
& vonHoldt 2012) was then used to assess K using the Evanno
method (Evanno et al. 2005). We acknowledge that many empirical
studies have applied Evanno’s K and interpreted the results as a
measure of model fit, but the popularity of a given methods should not
be mistaken for its appropriateness. Evanno’s K lacks properties
that a parameter in evolutionary genetics should ideally possess, for
example it is extremely susceptible to uneven sampling (Puechmaille
2016) and reproducible inference is challenging (Novembre 2016). For
these reasons, we follow Pritchard et al . (2000) in treating
STRUCTURE as a tool for data exploration rather than phylogeographic
inference. Our inferences are derived from the results of the
model-based analyses described below.