Bayesian Evolutionary Analysis Sampling Trees (BEAST) is a program for Bayesian Markov Chain Monte Carlo (MCMC) analysis of molecular sequences. It is oriented towards rooted, time-measured phylogenies inferred using strict or relaxed molecular clock models. It can be used as a method of reconstructing phylogenies but is also a framework for testing evolutionary hypotheses without conditioning on a single tree topology. BEAST averages over tree space, so that each tree is weighted proportional to its posterior probability.
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