Studies  using  two  closely  related  species of the monkey flower, a
   plant  that  grows in the Rocky Mountains and areas west, confirm that
   species  can  be  separated  by  only  a  few genetic differences. One
   species,  Mimulus  cardinalis,  is fertilized by hummingbirds and does
   not   interbreed   with   Mimulus  lewisii,  which  is  fertilized  by
   bumblebees.   H.  D.  Bradshaw  and  his  colleagues  studied  genetic
   differences  related  to  reproduction  in  the  two species - namely,
   flower shape, size, and color and nectar production. For each trait, a
   difference  in  a  single  gene  provided  at  least 25 percent of the
   variation  observed  among  laboratory-created  hybrids. M. cardinalis
   makes  80 times more nectar than does M. lewisii, and a single gene is
   responsible  for  at  least  half  the  difference. A single gene also
   controls  a  large part of the differences in flower color between the
   two  species  (Figure  26-13).  In  this  case,  as  in  the  Hawaiian
   Drosophila,  species  differences  can be traced to a relatively small
   number of genes.

   [Page  651]  Figure  26-13.  Flowers of two closely related species of