TO TREE OR NOT TO TREE

The practice of tracking geographical divergence along a phylogenetic tree has added an evolutionary perspective to biogeographic analysis w ithin single species. In spite of the popularity of phylogeography, th ere is an emerging problem. Recurrent mutation and recombination both create homoplasy, multiple evolutionary occurrences of the same charac ter that are identical in state but not identical by descent. Homoplas ic molecular data are phylogenetically ambiguous. Converting homoplasi c molecular data into a tree represents an extrapolation, and there ca n be myriad candidate trees among which to choose. Derivative biogeogr aphic analyses of 'the tree' are analyses of that extrapolation, and t he results depend on the tree chosen. I explore the informational aspe cts of converting a multicharacter data set into a phylogenetic tree, and then explore what happens when that tree is used for population an alysis. Three conclusions follow: (i) some trees are better than other s; good trees are true to the data, whereas bad trees are not; (ii) fo r biogeographic analysis, we should use only good trees, which yield t he same biogeographic inference as the phenetic data, but little more; and (iii) the reliable biogeographic inference is inherent in the phe netic data, not the trees.