In   spite   of   their   morphological   and  behavioral  divergence,
   demonstrating   significant   differences  between  these  species  in
   chromosomal  inversion patterns or protein polymorphisms is difficult.
   DNA hybridization studies carried out on the two species indicate that
   the sequence diversity between them is only about 0.55 percent (Figure
   26-12).   Thus,   nucleotide   sequence   diversity  may  precede  the
   development of protein or chromosomal polymorphisms.

   Figure   26-12.   Nucleotide  sequence  diversity  in  the  Drosophila
   planitibia  species complex. The shift to the left by the heterologous
   hybrids indicates the degree of nucleotide sequence divergence.

   Genetic  evidence suggests that the differences between D. heteroneura
   and  D.  silvestris  are  controlled  by  a relatively small number of
   genes.  For  example, as few as 15 to 19 major loci may be responsible
   for  the  morphological differences between the species, demonstrating
   that  the process of speciation likely involves only a small number of
   genes.