In-Groups and Out-Groups

WEISHAMPEL, D.B. and HEINRICH, R.E., Cell Biology and Anatomy, Johns Hopkins University, School of Medicine, Baltimore, MD 21205. Accurate identification of the first or first two closest outgroups is necessary for the polarization of the characters pertaining to ingroup taxa. But what if ingroup-to-outgroup relationships are not known a priori? In order to assess the influence of misidentification of proximate outgroups on character polarity, we conducted perturbation analyses on 1) two idealized character sets drawn from a fully-resolved Hennigian comb and a fully dichotomized symmetrical "bush", both fully-resolved and consisting of 14 terminal taxa, and 2) the character set provided in Holtz's recent cladistic analysis of non-avian theropods. Terminal ingroup taxa were employed as "outgroups" in successive PAUP analyses in order to assess changes in tree topology, tree length, and consistency indices (C.I.) that result from alterations in character polarity. Perturbation analyses produced either a decrease or an increase in C.I. accompanied by changes in tree topology and tree length. Increases in C.I., which occur more frequently in trees characterized by extensive homoplasy, is especially significant in that an inappropriately-chosen outgroup may lead to the acceptance of an incorrect tree.