Rooting molecular trees: problems and strategies

In constructing phylogenies from molecular data both the composition of the ingroup and the choice of outgroup can strongly affect the chances of obtaining the correct topology. This is because tree topology and uneven rates of molecular evolution affect the ability of tree-building algorithms to find the correct tree. Outgroups can be added in one of two ways, either to a single sister clade (preferably the closest) or through addition of single taxa from different clades. On theoretical and empirical grounds the former strategy is shown to be much more beneficial, both in terms of redressing balance and reducing stemminess. For parsimony to perform well some selection of sequence data, through omission or weighting, is also often necessary to reduce levels of homoplasy.