Fig. 1.

Different evolutionary scenarios for two binary sites. Each site can have {0,1} states. Thus, the pair of sites can have {00,01,10,11} states. We show in bold font the observed states at the leaves of the tree, and in italics font the states after the first split (all scenarios are identical up to that point). Disks represent unobserved true evolutionary changes: Half-disks are single changes; full-disks are double changes. Also shown is the MI for each pair of scenarios. No other covariation measures were included because they are meaningless in two-site scenarios.