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a Fibrinopeptides ° Hemoglobin o Cytochrome c + Unknown date of divergence a Fibrinopeptides ° Hemoglobin o Cytochrome c + Unknown date of divergence

200 400 600 800 1000 Millions of years since divergence

Figure 8.13 (left) Rates of protein evolution as amino acid changes per 100 residues in fibrinopeptides, hemoglobin, and cytochrome c over very long periods of time. Rates of divergence are linear over time for each protein, as expected for a molecular clock. Different proteins have different clock rates due to different mutation rates and degrees of functional constraint imposed by natural selection. Amino acid changes between pairs of taxa with unknown divergence times are plotted on dashed lines with the same slope as lines through points for taxa with estimated divergence times. The six points with unknown divergence times for hemoglobin represent divergence of ancestral globins into hemoglobins and myoglobins in the earliest animals, events that the molecular clock estimates to have happened between 1.1 billion and 800 million years ago. Data from Dickerson (1971).

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