Among four taxa A, B, C, & D, there are three hypotheses of relationship:
            either A is most closely related to B, or to C, or to D
      We want to be able to evaluate hypotheses of the form:
       "X and Y are more closely related to each other than either is to Z"
            The alternative hypotheses can be shown as networks with branches and an internode

 Positions 1 - 4 are uninformative:
           They give no information about relationships, because
                   all hypotheses require the same number of changes,
                   so none is more parsimonious than the others.

Positions 5, 6 & 7 are informative:
         Two taxa share one state, other two share another
                They give information about relationships,
                because one hypothesis requires fewer changes than the others
                  & is therefore more parsimonious than the others

   Position 5 indicates that A & B are most closely related:
      The first hypothesis can explain the distribution of nucleotides with a single change,
         the latter two require two changes each.
     The first hypothesis is a more parsimonious explanation of the data than the others.

By the same logic:
   Position 6 indicates that A & C are most closely related.
   Position 7 indicates that A & D are most closely related.
    [Homework: for the three networks above,
        sketch the changes required by sites of Positions 6 & 7]