Semipermeable species boundaries create opportunities for gene flow and adaptive potential

Hybridisation and gene flow can have both deleterious and adaptive
consequences for natural populations and species. To better understand the
extent of hybridisation in nature and the balance between its beneficial
and deleterious outcomes in a changing environment, information on
naturally hybridising non-model organisms is needed. This requires the
characterisation of the structure and extent of natural hybrid zones. Here
we study natural populations of five keystone mound-building wood ant
species in the Formica rufa group across Finland. No genomic studies
across the species group exist, and the extent of hybridisation and
genomic differentiation in sympatry is unknown. Combining genome-wide and
morphological data, we demonstrate more extensive hybridisation than was
previously detected between all five species in Finland. Specifically, we
reveal a mosaic hybrid zone between F. aquilonia, F. rufa and F.
polyctena, comprising further generation hybrid populations. Despite this,
we find that Formica rufa, F. aquilonia, F. lugubris, and F. pratensis
form distinct gene pools in Finland. We also find that hybrids occupy
warmer microhabitats than the non-admixed populations of cold-adapted F.
aquilonia, and suggest that warm winters and springs, in particular, may
benefit hybrids over F. aquilonia, the most abundant F. rufa group species
in Finland. In summary, our results indicate that extensive hybridisation
may create adaptive potential that could promote wood ant persistence in a
changing climate. Additionally, they highlight the potentially significant
ecological and evolutionary consequences of extensive mosaic hybrid zones,
within which independent hybrid populations face an array of ecological
and intrinsic selection pressures.