Do Holarctic ant species exist? Trans-Beringian dispersal and homoplasy in the Formicidae

Aim: Continents harbour unique faunas, and only a small percentage of species naturally inhabit more than a single continent. This pattern is most evident in the insects, a morphologically small and extremely diverse group. Nevertheless, 12 species of ants have traditionally been recognized as native to both North America and Eurasia, the Holarctic region. Since intercontinental dispersal is presumably rare in ants, allopatric speciation in the absence of gene flow can be expected over evolutionary time. Here, we reassess the existence of Holarctic ant species and reconstruct their biogeographical history. Location: The Holarctic. Taxon: All known ant species with purportedly Holarctic distributions. Methods: We reconstructed the phylogenetic relationships, biogeographical history and reassessed the taxonomic status of all known ants with Holarctic distributions using genetic data based on one mitochondrial and three nuclear genes and an ancestral area reconstruction of 310 specimens and 73 species (the 12 Holarctic species plus outgroup taxa). Results: Contrary to the currently accepted hypothesis, only three ant species have Holarctic native ranges, while six taxa separate into distinct Palearctic and Nearctic species. Four species are shown to be recent introductions from Europe to North America by human activity, one of which was thought to be native. Genetic diversity is considerably higher within the North American than within European species as currently defined. Main conclusions: The Formicidae have repeatedly dispersed through Beringia, during and after land bridge formation, and in both directions between the Palearctic and Nearctic regions. However, only three cold-tolerant species crossed the Bering Strait in relatively recent time. Our results highlight the potential existence of many unknown Nearctic ant taxa. Reliance on an evolutionarily labile morphological character, erect hairs, seems to have obscured species delimitation in these ant taxa. Based on our investigation, the typical time for speciation in allopatry for ants is 2–5 Ma.