Long-branch abstractions

Recent attention has been focused on the sensitivities of various tree reco nstructing algorithms to sequence rate heterogeneity (long-branch attractio n). Phylogenetic conclusions from two recent empirical studies have been in dicted as artifacts attributable to long-branch attraction. Siddall et nl. (1995) concluded that Myxozoa are cnidarians and sister group to Polypodium based on 18S rDNA and morphology. Hanelt et nl. (1996) argued that this re sult is due to long-branch attraction. Whiting et al. (1997) concluded that the Strepsiptera are sister group to Diptera based on parsimony analysis o f 18S rDNA, 28S rDNA, and morphology. Huelsenbeck (1997) argued that this r esult also is attributable to long-branch attraction. We demonstrate that t he analyses and arguments dismissing these results as the effects of long-b ranch attraction are fundamentally flawed. The criteria employed by these a uthors were applied arbitrarily by them to the groups that they did not wan t, and yet using those same criteria, there is more reason to exclude other taxa besides Polypodium and there is more reason to disbelieve monophyly o f Diptera than monophyly of Strepsiptera with Diptera. Moreover, it is asse rted, long-branch attraction cannot explain the presence of nematocysts in Myxozoa and halteres in Strepsiptera. For these reasons, and in light of th e demonstration that long branches cannot attract each other in their mutua l absence, we conclude that the monophyly of Myxozoa + Polypodium and Strep siptera + Diptera is not due to long-branch attraction. We suggest that max imum likelihood methods are extremely sensitive to taxon and character samp ling and that these data sets are demonstrative of the long-branch repulsio n problem. (C) 1999 The Willi Hennig Society.