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 dispersalvicariance 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). CrossBeringian 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 transBeringian 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.  2001 The Linnean Society of London