Figure 1.

Two models of protein sequence evolution. By comparing the likelihood values of these two models, one can judge whether r1 = r2 (r1 and r2 are the amino-acid substitution rates on branches H1 and H2, respectively). (a) The model assumes r1 = r2. (b) The model allows r1 and r2 to be different. H1 and H2: two human duplicate genes. Outgroup: the mouse ortholog.