FIG. 21.15 The molecular clock. Different genes evolve at different rates because of differences in the intensity of negative selection. After Fig. 20- p. 733, in A. J. F. Griffiths, S. R. Wessler, S. B. Carroll, and J. Doebley, 2012, Introduction to Genetic Analysis, 10th ed., New York: W. H. Freeman.
After Fig. 20-