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.