Patterns of animal dispersal, vicariance and diversification in the Holarctic

We analysed patterns of animal dispersal, vicariance and diversification in the Holarctic based on complete phylogenies of 57 extant non-marine taxa, together comprising 770 species, documenting biogeographic events from the Late Mesozoic to the present. Four major areas, each corresponding to a historically persistent landmass, were used in the analyses: eastern Nearctic (EN), western Nearctic (WN), eastern Palaeoarctic (EP) and western Palaeoarctic (WP). Parsimony-based tree fitting showed that there is no significantly supported general area cladogram for the dataset. Yet, distributions are strongly phylogenetically conserved, as revealed by dispersal-vicariance analysis (DIVA). DIVA-based permutation tests were used to pinpoint phylogenetically determined biogeographic patterns. Consistent with expectations, continental dispersals (WP↔EP and WN↔EN) are significantly more common than palaeocontinental dispersals (WN↔EP and EN↔WP), which in turn are more common than disjunct dispersals (EN↔EP and WN↔WP). There is significant dispersal asymmetry both within the Nearctic (WN→EN more common than EN→WN) and the Palaeoarctic (EP→WP more common than WP→EP). Cross-Beringian faunal connections have traditionally been emphasized but are not more important than cross-Atlantic connections in our data set. To analyse changes over time, we sorted biogeographic events into four major time periods using fossil, biogeographic and molecular evidence combined with a ‘branching clock’. These analyses show that trans-Atlantic distributions (EN—WP) were common in the Early-Mid Tertiary (70–20 Myr), whereas trans-Beringian distributions (WN—EP) were rare in that period. Most EN—EP disjunctions date back to the Early Tertiary (70–45 Myr), suggesting that they resulted from division of cross-Atlantic rather than cross-Beringian distributions. Diversification in WN and WP increased in the Quaternary (< 3 Myr), whereas in EP and EN it decreased from a maximum in the Early-Mid Tertiary.