 on in the tree and click Compute 
 | Calibrate Molecular Clock. The molecular clock dialog will then be shown 
 and if a branch in the tree is focused, the option for using a fixed evolutionary 
 rate will be enabled. If a node in the tree is focused, the option to 
 use a fixed time of divergence will be enabled (in which case you can 
 also set the scale bar title using the Time Unit edit box in this dialog).</p>
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