This function calculates the number of cherries (see definition below) on a phylogenetic tree, and tests the null hypotheses whether this number agrees with those predicted from two null models of trees (the Yule model, and the uniform model).
cherry(phy)an object of class "phylo".
A NULL value is returned, the results are simply printed.
A cherry is a pair of adjacent tips on a tree. The tree can be either rooted or unrooted, but the present function considers only rooted trees. The probability distribution function of the number of cherries on a tree depends on the speciation/extinction model that generated the tree.
McKenzie and Steel (2000) derived the probability distribution function of the number of cherries for two models: the Yule model and the uniform model. Broadly, in the Yule model, each extant species is equally likely to split into two daughter-species; in the uniform model, a branch is added to tree on any of the already existing branches with a uniform probability.
The probabilities are computed using recursive formulae; however, for both models, the probability density function converges to a normal law with increasing number of tips in the tree. The function uses these normal approximations for a number of tips greater than or equal to 20.
McKenzie, A. and Steel, M. (2000) Distributions of cherries for two models of trees. Mathematical Biosciences, 164, 81--92.