The Tree Below Shows the Estimated Percent Differences Accumulated in an Amino

The tree below shows the estimated percent differences accumulated in an amino acid sequence in a protein during different intervals of time. The intervals are indicated by horizontal lines, and each interval depicts the same length of time. X and Y are species that exist today, and CA is their now-extinct common ancestor. The range is the so-called 95% confidence interval, which means that there is only a 5% chance that the true value lays outside this range. These data conform to the expectations of a molecular clock.