PROBABILITIES OF EVOLUTIONARY TREES

Random sampling probabilities of evolutionary trees have been used to compare taxonomic classifications generated from different data sets, to test biogeographic hypotheses, and to study patterns of speciation. In this paper, I analyze a model in which speciation occurs by random bifurcation of existing lineages. A method of calculating the probabi lity of an evolutionary tree by counting the possible arrangements of its nodes, branches, and terminal taxa is presented. Probabilities tha t sets of species are distinct clades are also given. The results are applicable to any number of taxa. Probabilities of trees are not alter ed by the absence of extinct or unstudied taxa. Applications of the re sults to systematics and further problems in the probability of evolut ionary trees are discussed.