The  results  of the study indicate that a balance in the frequency of
   the  two inversions and the gene arrangements they contain is superior
   to  either  inversion  by  itself.  The  equilibrium  reached  in  the
   experiment  presumably  represents  the  highest  mean  fitness in the
   population under controlled laboratory conditions. This interpretation
   of the experiment suggests that natural selection is the driving force
   maintaining the diversity in chromosome 3 inversions.

   In  a  more  extensive study, Dobzhansky and his colleagues sampled D.
   pseudoobscura populations over a broad geographic range. They found 22
   different chromosome arrangements in populations from 12 locations. In
   Figure  26-7,  the  frequencies  of five of these inversions are shown
   according   to   geographic  location.  The  differences  are  largely
   quantitative,  with  most  populations  differing only in the relative
   frequencies  of  inversions. Collectively, Dobzhansky's data show that
   the  genetic  structure  of  D. pseudoobscura populations changes from
   place  to  place  and  from one time to another. At least some of this
   variation  in  population  genetic  structure is the result of natural