Transatlantic dispersal and introgression explain Holarctic disjunct distributions in Vanessa butterflies
Species with disjunct distributions have long puzzled evolutionary biologists and biogeographers. Long-distance dispersal is a key process leading to intra-specific disjunct distributions, which may trigger the early stages of allopatric speciation, ultimately resulting in inter-specific disjunctions. Vanessa butterflies display a wide array of movement behaviors, from low-dispersal species with restricted distributions to others performing annual long-range migratory cycles. The biogeographic history of Vanessa is intriguing, with cases of both intra- and inter-specific disjunctions. Vanessa atalanta is present in the Nearctic and Western Palearctic, but absent in Asia, and its sister species V. tameamea is endemic to Hawaii. Vanessa indica occurs only in Asia and its sister species, V. vulcania, is endemic to Macaronesia. Here, we investigate this conundrum through population genomics and demographic analyses of Vanessa species distributed in the Holarctic. Using ddRAD data from simples across its entire distributional range, we identify two structured populations in V. atalanta, separated by the Atlantic Ocean. Demographic simulations suggest a long-distance dispersal event from the Nearctic to Europe around the Last Glacial Maximum. When looking at hybridization between Holarctic species, we found evidence of introgression between the Palearctic population of V. atalanta and V. indica + V. vulcania. This is a strong indication that their distributions overlapped when V. atalanta colonized Europe. We hypothesize that V. atalanta caused a species displacement of V. indica from Europe to Asia, which would explain their current allopatric distribution. These results show the importance of long-distance dispersal for shaping biogeographic patterns, possibly mediated by species interactions.