A  Method  for  Estimating  Evolutionary  Trees from Genetic Data Many
   methods use data on genetic differences to estimate phylogenies. It is
   beyond  the  scope  of this chapter to review all of them. Instead, we
   shall present just one method, called the unweighted pair group method
   using  arithmetic  averages,  or  UPGMA.  This  method is not the most
   powerful  one  for  estimating  phylogenies from genetic data, but, in
   spite   of  its  lengthy  name,  it  is  intuitively  straightforward.
   Furthermore, UPGMA works reasonably well under many circumstances.

   [Page  654]  The  starting  point  for  UPGMA  is  a  table of genetic
   distances  among  a group of species [Figure 26-19(a)]. To demonstrate
   the  technique,  we  will  use  data from DNA hybridization studies by
   Charles Sibley and Jon Alquist, who computed genetic distances between
   humans  and  four species of apes: the common chimpanzee, the gorilla,
   the siamang, and the common gibbon.

   Figure  26-19.  Phylogeny  reconstruction  by  UPGMA.  (This  item  is
   displayed on page 655 in the print version) [View full size image]