SPECIAL PAPER Directional asymmetry of long-distance dispersal and colonization could mislead reconstructions of biogeography Lyn G. Cook* and Michael D. Crisp INTRODUCTION It is commonly accepted that vicariance and long-distance dispersal (LDD) have both contributed to the current distributions of organisms (Raven, 1973; Wardle, 1978; McDowall, 2004; Thorne, 2004). Even taxa that appear to be poor dispersers have biogeographical patterns that require an inference of dispersal (e.g. amphibians, Vences et al., 2004; School of Botany and Zoology, The Australian National University, Canberra, Australia *Correspondence: Lyn Cook, School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia. E-mail: lyn.cook@anu.edu.au ABSTRACT Aim Phylogenies are increasingly being used to attempt to answer biogeographical questions. However, a reliance on tree topology alone has emerged without consideration of earth processes or the biology of the organisms in question. Most ancestral-state optimization methods have inherent problems, including failure to take account of asymmetry, such as unequal probabilities of losses and gains, and the lack of use of independent cost estimates. Here we discuss what we perceive as shortcomings in most current tree-based biogeography interpretation methods and show that consideration of processes and their likelihoods can turn the conventional biogeographical interpretation on its head. Location Southern hemisphere focus but applicable world-wide. Methods The logic of existing methods is reviewed with respect to their adequacy in modelling processes such as geographical mode of speciation and likelihood of dispersal, including directional bias. Published reconstructions of dispersal of three plant taxa between Australia and New Zealand were re-analysed using standard parsimony and maximum likelihood (ML) methods with rate matrices to model expected asymmetry of dispersal. Results Few studies to date incorporate asymmetric dispersal rate matrices or question the simplistic assumption of equal costs. Even when they do, cost matrices typically are not derived independently of tree topology. Asymmetrical dispersal between Australia and New Zealand could be reconstructed using parsimony but not with ML. Main conclusions The inadequacy of current models has important consequences for our interpretation of southern hemisphere biogeography, particularly in relation to dispersal. For example, if repeated directional dispersals and colonization in the direction of prevailing winds have occurred, with intervening periods of speciation, then there is no need to infer dispersals against those winds. Failure to take account of directionality and other biases in reconstruction methods has implications beyond the simple misinterpretation of the biogeography of a taxonomic group, such as calibration of molecular clocks, the dating of vicariance events, and the prioritization of areas for conservation. Keywords Ancestral areas, Antarctic Circumpolar Current, Australia, directional dispersal, maximum likelihood, maximum parsimony, New Zealand, southern hemi- sphere, trait optimization, West-wind Drift. Journal of Biogeography (J. Biogeogr.) (2005) 32, 741–754 ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi doi:10.1111/j.1365-2699.2005.01261.x 741