Data from: Genome-wide differentiation in closely related populations: the roles of selection and geographic isolation

Population divergence in geographic isolation is due to a combination of
factors. Natural and sexual selection may be important in shaping patterns
of population differentiation, a pattern referred to as ‘isolation by
adaptation’ (IBA). IBA can be complementary to the well-known pattern of
‘isolation by distance’ (IBD), in which the divergence of closely related
populations (via any evolutionary process) is associated with geographic
isolation. The barn swallow Hirundo rustica complex comprises six closely
related subspecies, where divergent sexual selection is associated with
phenotypic differentiation among allopatric populations. To investigate
the relative contributions of selection and geographic distance to
genome-wide differentiation, we compared genotypic and phenotypic
variation from 350 barn swallows sampled across eight populations (28
pairwise comparisons) from four different subspecies. We report a draft
whole-genome sequence for H. rustica, to which we aligned a set of 9493
single nucleotide polymorphisms (SNPs). Using statistical approaches to
control for spatial autocorrelation of phenotypic variables and geographic
distance, we find that divergence in traits related to migratory behaviour
and sexual signalling, as well as geographic distance, together explain
over 70% of genome-wide divergence among populations. Controlling for IBD,
we find 42% of genomewide divergence is attributable to IBA through
pairwise differences in traits related to migratory behaviour and sexual
signalling alone. By (i) combining these results with prior studies of how
selection shapes morphological differentiation and (ii) accounting for
spatial autocorrelation, we infer that morphological adaptation plays a
large role in shaping population-level differentiation in this group of
closely related populations.