ROOTING WITH MULTIPLE OUTGROUPS - CONSENSUS VERSUS PARSIMONY

Using outgroup(s) is the most frequent method to root trees. Rooting t hrough unconstrained simultaneous analysis of several outgroups is a f avoured option because it serves as a test of the supposed monophyly o f the ingroup. When contradiction occurs among the characters of the o utgroups, the branching pattern of basal nodes of the rooted tree is d ependent on the order of the outgroups listed in the data matrix, that is, on the prime outgroup (even in the case of exhaustive search). Di fferent equally parsimonious rooted trees (=cladograms) can be obtaine d by permutation of prime outgroups. An alternative to a common implic it practice (select one outgroup to orientate the tree) is that the ac cepted cladogram is the strict consensus of the different equally pars imonious rooted trees. The consensus tree is less parsimonious but is not hampered with extra assumption such as the choice of one outgroup (or more) among the initial number of outgroup terminals. It also does not show sister-group relations that are ambiguously resolved or not resolved at all. (C) 1998 The Willi Hennig Society.