Question

Why do different proteins appear to evolve different rates?!

Answer 1

Each protein has its own evolution rate, and that rate is constant across phylogenies (i.e., hemoglobin does not evolve at the same rate as cytochrome c, but hemoglobins from humans, mice, etc. do have comparable rates of evolution.). Not all regions within a protein mutate at the same rate; functionally important areas mutate more slowly and amino acid substitutions involving similar amino acids occurs more often than dissimilar substitutions.

The structure of some proteins is rigidly defined by the function they perform--any mutation that causes even a small change in the molecular structure will impair the protein's function. While Other proteins can tolerate a large amount of change and still carry out their function..