Abstract: An outgroup roots a network to form a tree and/or to infer hypothetical ancestral character states. Usually, multiple taxa of a closely related sister group of the ingroup are selected. To empirically evaluate the choice of outgroup, we implemented three strategies of outgroup selection: a single taxon from the sister group, multiple taxa within the sister group, and multiple taxa from successive sister groups. Subsequently, we evaluated their effects on tree topologies within the family Halictidae (Hymenoptera: Apoidea) incorporating three tree reconstruction methods: maximum likelihood, maximum parsimony and Bayesian inference. The use of multiple taxa within the sister group produced more consistent results than the other two outgroup strategies. The tree topologies were generally consistent with the putative tree topology of Halictidae. Compared with the other two tree reconstruction methods, maximum parsimony produced more consistent results with different outgroup strategies, yet often obtained less resolution.

Key words: Halictidae, homoplasy, monophyly, outgroup, sister group, phylogeny