Figure 1

Phylogenetic tree of Medicago species. A. The topology is from ref. [18]. The pattern of rhizobial specificity in extant species as well as reconstruction of ancestral states is from ref. [13]. The numbered branch labels indicate branch where most likely shifts in rhizobial specificity occurred. B. Phylogram with branch lengths estimated with the codon model (model M8). The numbered branches are those used for testing positive selection (1–5: same as in Figure 1A; 6: noticed for having an unexpected length). Asterisks indicate branches with an ω significantly higher than in the rest of the tree. The scale for branch length represents 0.1 nucleotide substitution per codon.