In  many  cases, we would like to estimate not only which members of a
   set  of  species  are most closely related, but also when their common
   ancestors  lived. Sometimes we can do so, thanks to molecular clocks -
   amino  acid  sequences  or  nucleotide sequences in which evolutionary
   changes accumulate at a constant rate over time.

   Research  by  Walter  M. Fitch and colleagues on the influenza A virus
   shows  how  molecular  clocks  are  used.  Fitch  and  his  associates
   sequenced  part  of  the  hemagglutinin gene from flu viruses that had
   been   isolated  at  different  times  over  a  20-year  period.  They
   calculated  the  number  of  nucleotide  differences among the various
   viruses  and  constructed an evolutionary tree [Figure 26-20(b)]. Most
   strains  have  gone  extinct,  leaving  no  descendants among the more
   recently  isolated  viruses.  Fitch's group then plotted the number of
   nucleotide  substitutions  between the first virus and each subsequent
   virus  against  the  year  in  which  the  virus  was isolated [Figure
   26-20(a)].  The  points  all  fall  very  close  to  a  straight line,
   indicating that nucleotide substitutions in this gene have accumulated