To  test  the  hypothesis  that  this  cyclic  change is a response to
   natural  selection,  Dobzhansky  and  his  group  devised a laboratory
   experiment. They constructed large population cages from which samples
   of  D.  pseudoobscura  could be removed periodically and studied. They
   began  with a population that was 88 percent CH and 12 percent ST. The
   flies  were  maintained  at 25°C and sampled over a 1-year period. As
   shown in Figure 26-6, the frequency of ST increased gradually until it
   was  present  at  a level of 70 percent. At that point, an equilibrium
   between  ST and CH was reached. When the same experiment was performed
   at  16°C,  no change in inversion frequency occurred. The researchers
   concluded that the equilibrium reached at 25°C was in response to the
   elevated temperature, the only variable in the experiment.

   Figure  26-6.  Increase  in  the ST arrangement of D. pseudoobscura in
   population cages under laboratory conditions.

   The  results  of the study indicate that a balance in the frequency of
   the  two inversions and the gene arrangements they contain is superior