Trans-Atlantic Dispersal and Introgression Explain Holarctic Disjunct Distributions in Vanessa Butterflies.

Species with disjunct distributions have long puzzled evolutionary biologists and biogeographers. Long-distance dispersal can play a pivotal role in generating intra-specific disjunct distributions, initiating early stages of allopatric speciation and leading to eventual interspecific disjunctions. Vanessa butterflies exhibit diverse movement behaviours, from low-dispersal species with restricted distributions to others that engage in annual extensive migratory cycles. The biogeographic history of Vanessa presents intriguing cases of both intra- and interspecific disjunctions. Vanessa atalanta is present in the Nearctic and Western Palearctic but is absent in Asia, while 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 atalanta using ddRAD data from 70 samples across its entire distributional range, identifying two genetically differentiated populations separated by the Atlantic Ocean. Demographic simulations and phylogenetic analyses suggest that these originated via long-distance dispersal from the Nearctic to Europe around the Last Glacial Maximum. Hybridisation tests revealed introgression between the Palearctic population of V. atalanta and V. indica, indicating that their distributions overlapped during V. atalanta's colonisation of Europe. We hypothesise that V. atalanta caused a species displacement of V. indica from Europe to Asia, explaining their current allopatric distributions-a scenario that is supported by ecological niche modelling. Together, our results illustrate the role of long-distance dispersal and species interactions in shaping complex biogeographic patterns.