 used by the Maximum Parsimony method to search for the best tree.  It is defined as the sum of the minimum numbers of substitutions over all sites for the given topology.   To compute the tree length for the unweighted parsimony method, we use the procedure described in Fitch (1971), which is based on the two rules described below.  For a given site these rules are applied to each node and the sum of substitutions over all nodes and over all sites are taken.  Note that the estimation of the minimum number 