Patterns of animal dispersal, vicariance and diversification in the Holarctic

Abstract

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-tEP). 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 (7045 Myr), suggesting that they resulted from division of cross-Atlantic rather than cross-Beringian distributions. Diversification in WN and W P increased in the Quaternary (< 3 Myr), whereas in E P and EN it decreased from a maximum in the Early-Mid Tertiary. 0 2001 The Linnean Society of London ADDITIONAL KEY WORDS: historical biogeographytrans-Atlantictrans-Beringiandisjunct.This incongruence is measured in abstract terms, such as items of error or amount of homoplasy. The incongruence can then be interpreted a posteriori in terms of biogeographic events, such as dispersal and extinction. The exact procedure for this a posteriori translation remains elusive. Furthermore, because no model is specified, it is difficult to predict the analytical behaviour of pattern-based methods and counter-intuitive results often occur (e.g. Ronquist, 1995Ronquist, , 1996.In contrast, event-based methods are explicitly derived from models of biogeographic processes. The relevant events are identified and associated with costs that are inversely related to the likelihood of t h e events. The analysis consists of a search for the reconstruction, which minimizes the total cost. This optimal reconstruction explicitly specifies the biogeographic events of interest, unlike pattern-based

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