mrbayes - 3.2.7a-7+b2 main

Bayesian inference of phylogeny is based upon a quantity called the posterior
probability distribution of trees, which is the probability of a tree
conditioned on the observations. The conditioning is accomplished using
Bayes's theorem. The posterior probability distribution of trees is
impossible to calculate analytically; instead, MrBayes uses a simulation
technique called Markov chain Monte Carlo (or MCMC) to approximate the
posterior probabilities of trees.