A NEW MODEL FOR DISCRETE CHARACTER EVOLUTION

The paper provides an explicit justification for the principle that a uniform taxon should contribute only one datapoint in comparative anal yses with discrete variables. The justification is that phylogenetic p atterns in variables unincluded in the proposed test vitiate the assum ption of independence, both at the level of species and at the level o f branch segments. The consequence is that a uniform taxon cannot safe ly be counted as more than one datapoint. The arguments use a branchin g discrete Markov process in continuous time, with the new feature tha t the tested variables are only a subset of the evolving characters. T his model is proposed as a useful criterion for measuring the merit of proposed tests, and illustrates the necessity for models in evaluatin g comparative methods. (C) 1997 Academic Press Limited