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 170 species, documenting biogeographic events from the Late Mesozoic to the present. Four major areas, each corresponding to a his torically persistent landmass, were used in the analyses: eastern Nearctic (EN), western Nearctic (WN), eastern Palaeoarctic (EP) and western Palaeoar ctic (WP). Parsimony-based tree fitting showed that there is no significant ly supported general area cladogram for the dataset. Yet, distributions are strongly phylogenetically conserved, as revealed by dispersal-vicariance a nalysis (DIVA). DIVA-based permutation tests were used to pinpoint phylogen etieally determined biogeographic patterns. Consistent with expectations, c ontinental dispersals (WP <----> EP and WN <----> EN) are significantly mor e common than palaeocontinental dispersals (WN <----> EP and EN <----> WP), which in turn are more common than disjunct dispersals (EN <----> EP and W N <----> WP). There is significant dispersal asymmetry both within the Near ctic (WN --> EN more common than EN --> WN) and the Palaeoarctic (EP --> WP more common than WP --> EP). Cross-Beringian faunal connections have tradi tionally been emphasized but are not more important than cross-Atlantic con nections in our data set. To analyse changes over time, we sorted biogeogra phic events into four major time periods using fossil, biogeographic and mo lecular 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 th at period. Most EN-EP disjunctions date back to the Early Tertiary (70-45 M yr), suggesting that they resulted from division of cross-Atlantic rather t han cross-Beringian distributions. Diversification in WN and WP increased i n the Quaternary (< 3 Myr), whereas in EP and EN it decreased from a maximu m in the Early-Mid Tertiary. (C) 2001 The Linnean Society of London.