Extensive hybridisation between multiple differently adapted species may aid persistence in a changing climate

Hybridisation and gene flow can have both deleterious and adaptive consequences for natural populations and species. To better understand the extent and consequences of hybridisation in nature, information on naturally hybridising non-model organisms is required, including characterising the structure and extent of natural hybrid zones. Here we study natural populations of five keystone mound-building wood ant (Formica rufa group) species 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 show that Formica rufa, F. aquilonia, F. lugubris, and F. pratensis form distinct gene pools in Finland. We demonstrate more extensive hybridisation than previously thought between all five species and reveal a mosaic hybrid zone between F. aquilonia, F. rufa and F. polyctena. We show that hybrids between these climatically differently adapted species occupy warmer habitats than the cold-adapted parent F. aquilonia. This suggests hybrids occupy a different microclimatic niche compared to the locally abundant parent. We propose that wood ant hybridisation may increase with a warming climate, and warm winters, in particular, may provide a competitive advantage for the hybrids over F. aquilonia in the future. In summary, our results demonstrate how extensive hybridisation may help persistence in a changing climate. Additionally, they provide an example on how mosaic hybrid zones can have significant ecological and evolutionary consequences because of their large extent and independent hybrid populations that face both ecological and intrinsic selection pressures.