However,  more  than one nucleotide change may be required to change a
   given  amino acid. When the nucleotide changes necessary for all amino
   acid  differences  observed  in  a  protein  are  totaled, the minimal
   mutational   distance   between  the  genes  of  any  two  species  is
   established.  Column  (b)  in Table 26.4 shows such an analysis of the
   genes encoding cytochrome c. As expected, these values are larger than
   the  corresponding  number  of  amino acids separating humans from the
   other nine organisms listed.

   Fitch  used  data  on  the  minimal  mutational  distances between the
   cytochrome  c  genes of 19 organisms to reconstruct their evolutionary
   history.  The  result  is  an  estimate  of  the evolutionary tree, or
   phylogeny,  that  unites the species (Figure 26-18). The black dots on
   the  tips  of  the  twigs  represent  the  extant  species,  which are
   connected  to  the  inferred common ancestors represented by red dots.
   The  ancestral  species  evolved  and  diverged  to produce the modern
   species.  The  common  ancestors are connected to still earlier common
   ancestors, culminating in a single common ancestor for all the species