On the origin of an insular hybrid butterfly lineage.

A new species can form through hybridization between species. Hybrid speciation in animals has been intensely debated, partly because hard evidence for the process has been difficult to obtain. Here we report the discovery of a European hybrid butterfly lineage, a finding that can be considered surprising given the intense and long-term study of European butterflies. The lineage we describe is mainly inhabiting an island in the Baltic Sea and was previously designated as a subspecies (horkei) of one of the parental species (Aricia artaxerxes). By analysing whole-genome resequencing data and developing a novel cluster analysis based on historical recombination events (Fisher junctions), we determine that horkei originated by hybridization between the non-sister species A. artaxerxes and A. agestis. This hybridization event occurred approximately 54,000 years ago, predating the last glaciation of the current distribution range. Horkei must therefore have persisted long enough to be able to colonize its current range, despite that this area lies between the current distributions of the parental species. The hybrid origin, the maintenance of genomic integrity across times of dramatic climate change and the expression of a combination of parental traits suggest that horkei could be in the process of hybrid speciation.