Shortest edit path between NS and NA. Trees depicting the m1, m2 SPR edits from Figure 3. Arc direction is from empty to filled circle. NS (t0) is the start tree (−ln L = 2761.72), on which we apply modification m1 (moving A/redknot/NJ/325/1989 H7N7 to A/HongKong/1774/99 H3N2). The log likelihood of the intermediate tree (t1, -ln L = 2761.87) is close to that of NS (this tree is inside NS’ CI). The second modification, m2, is applied to t1 and results in a topology identical to that of the NA tree (t2, -ln L = 2859.68). The rise in the negative log likelihood of the tree after m2 is 97.81 (the resulting tree is outside NS’ CI).