On the origin of an insular hybrid butterfly lineage

A new species can evolve when diverging lineages hybridize and give rise to a persistent and ecologically differentiated species. Hybrid speciation in animals has been intensely debated, partly because hard evidence for the process has been difficult to obtain. Recent access to large-scale, whole-genome sequencing data and development of novel analytical methods have made it more feasible to statistically test for hybrid origin of lineages. Here we report the discovery of a European hybrid butterfly lineage. The discovery of a hybrid lineage is surprising given the intense 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 analyzing whole-genome resequencing data, 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 distribution range, despite that this range lies between the current ranges of the parental species. The hybrid origin, the maintenance of genomic integrity through 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.