Parsimony Methods. NJ was originally described as a method for approximating a tree that minimizes the sum of least-squares branch lengths – the minimum – evolution criterion.
n-2 internal nodes; 2n-3 branches
Where N is the number of sites (characters) in the alignment and lj is the length for a single site j. This length lj is the amount of character change implied by a most parsimonious reconstruction that assigns a character-state xij to each node input data. Thus, for binary trees:
s to v;
s to w;
s to y‘;
r to v;
r to w;
r to y‘;
y to v;
y to w
both internal and external, also would be cut in a full round of TBR swapping.