The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
be replaceable at any one time. Although the elements of the set vary from time to time and from lineage to lineage, a total of 44 amino acids are permanently unreplaceable. Moreover, the number of different amino acids that can occur at any particular variable site is very small, limited to 2–4 alternatives. The conclusion is that molecular clocks have complex features that must be ascertained before drawing out inferences about the topology and timing of historical relationships.
