The  Genetic Structure of Populations Changes across Space and Time As
   population   geneticists   discovered  that  most  populations  harbor
   considerable  genetic  diversity,  they  also  found  that the genetic
   structure  of populations varies across space and time. To illustrate,
   we   consider   studies   on  Drosophila  pseudoobscura  conducted  by
   Theodosius Dobzhansky and his colleagues. This species is found over a
   wide  range  of  environmental  habitats,  including  the  western and
   southwestern  United  States. Although the flies throughout this range
   are   morphologically   similar,  Dobzhansky's  team  discovered  that
   populations  from different locations vary in the arrangement of genes
   on  chromosome  3.  They  found  several  different inversions in this
   chromosome  that can be detected by loop formations in larval polytene
   chromosomes.  Each  inversion  sequence  is  named after the locale in
   which it was first discovered (e.g., AR = Arrowhead, British Columbia,
   and  CH = Chiricahua Mountains, Arizona). The inversion sequences were
   compared with one standard sequence, arbitrarily designated ST.

   [Page  645] Figure 26-4 compares the frequencies of three arrangements