Fig. 2.

Phylogeny of the wren family Troglodytidae and representative species from related oscine passerine families showing recurrent nonsynonymous substitutions at the first position of codon 55 in the β^A-globin gene. Blue and red tick marks indicate the minimum number of changes that are consistent with maximum-likelihood estimates of ancestral states at each node (see pie diagrams). An ancestral CpG dinucleotide was eliminated multiple times independently by nonsynonymous G→A transition substitutions (which produced β55Val→Ile replacements in Andean house wrens and several other passerine lineages) and G→C transversion substitutions (which produced β55Val→Leu replacements in multiple lineages). The third position of the codon immediately preceding β55 was cystosine (C) in all examined species except for Cinclus leucocephalus, which had thymine (T) and is part of a clade in which the CpG dinucleotide had already been eliminated by a CpG→CpC substitution. Depending on the methylation status of the cytosine, the rate of elimination of the CpG dinucleotide by point mutations at either site is expected to be ∼10- to 15-fold higher than the mean mutation rate for non-CpG nucleotide sites. Maximum-likelihood analyses confirmed the expectation that the rate of substitutions that eliminated the CpG dinucleotide (CpG→CpA and CpG→CpC) was significantly higher than the rate of non-CpG substitutions at the same site.