The Yule model and the coalescent model are two neutral stochastic models for generating
trees in phylogenetics and population genetics, respectively. Although these models
are quite different, they lead to identical distributions concerning the probability
that pre-specified groups of taxa form monophyletic groups (clades) in the tree. We
extend earlier work to derive exact formulae for the probability of finding one or
more groups of taxa as clades in a rooted tree, or as `clans' in an unrooted tree.
Our findings are relevant for calculating the statistical significance of observed
monophyly and reciprocal monophyly in phylogenetics.