As a general phenomenon, Haldane's rule seems to be caused by several different phenomena. In some cases, it seems to be due to the type of recessivity of incompatibility interactions envisioned by Muller's theory. One obvious pattern that emerged was that closely related taxa had relatively similar sequences for a given protein, while more distantly related taxa had more divergent sequences for the same protein. This pattern forms the basis for phylogeny reconstruction using molecular sequences, which is considered in the next lecture. The figure below plots this data for a globin.
Time axis is in millions of years b.
Each point represents a comparison of the indicated species with humans. Although there is some scatter about the line, the data strongly suggest that number of substitutions separating a species pair is linearly proportional to the amount of time those species have been diverging. The linearity indicates that the rate of divergence is approximately constant, i.e. the molecular clock ticks with a constant rate. One is not restricted to examining just one protein to estimate the relationship between number of substitutions and divergence time. Again, the linearity, as well as the tightness, of the relationship between divergence time and number of substitutions is striking.