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28 July 2011, Ziheng Yang
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This examples dates divergences of sequences with sample dates, such as viral sequences.
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The control file is mcmctree.ctl, and you run the program mcmctree using it, for example, with
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..\..\bin\mcmctree (on Windows)
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../..\bin/mcmctree (on Macs or Linux)
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Please look at the control file mcmctree.ctl, the paml/mcmctree
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documentation in the doc/ folder, and perhaps also the step-by-step
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manual for running mcmctree.
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The following line in the control file specifies the TipDate model. The second number is the time unit.
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TipDate = 1 100 * TipDate (1) & time unit
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When you run the default analysis, you will see the following printout on the monitor.
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Date range: (94.00, 56.00) => (0, 0.38). TimeUnit = 100.00.
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The program scans the sequence names, and take the last field in the
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sequence name as the sampling date. The most recent sample date will
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be time zero, and the other times will be rescaled using the time
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unit. For example the sequences in the example dataset are from 1994
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to 1956, with 1994 becoming time 0, and 1956 becoming 0.38 as one time
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unit is specified to be 100 years.
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You should specify a prior on the age of the root of the tree.
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RootAge = B(0.8, 1.2, 0.01, 0.02) * root age constraints
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The above means that the age of root is between 0.8 and 1.2 time
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units, with those bounds violated with probability 1% and 2&.
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Other things that are important include the prior on the substitution rate
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rgene_gamma = 1 20 * gamma prior G(a, b) for rate for genes
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Note that the gamma distribution G(alpha, beta) has shape parameter
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alpha and scale parameter beta. You specify alpha depending on how
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much confidence you have (with alpha = 1 to be a diffuse prior and
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alpha = 5 or 10 to be informative priors, say), and the mean
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alpha/beta so that the rate is at the right range. Here alpha = 1
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means a diffuse prior, while alpha/beta = 0.05 means 0.05 changes per
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site per time unit, or 0.0005 changes per site per year (since the time
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The clock model may also be important.
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clock = 1 * 1: global clock; 2: independent rates; 3: correlated rates