Genetic Differentiation During the Speciation Process in Drosophila

• Published in: Evolution 1974-12, Vol.28 (4), p.576-592
• Main Authors: Ayala, Francisco J., Tracey, Martin L., Hedgecock, Dennis, Richmond, Rollin C.
• Format: Article
• Language: English
• Subjects: Drosophila; Evolution; Evolutionary genetics; Genetic distance; Genetic loci; Genetic variation; Population genetics; Sibling species; Speciation; Taxa
• ISSN: 0014-3820; 1558-5646
• DOI: 10.1111/j.1558-5646.1974.tb00791.x

We have studied allelic variation at 36 gene loci coding for enzymes in natural populations of 14 taxa of the Drosophila willistoni group. Five different levels of evolutionary divergence can be recognized in these taxa. The average amounts of genetic differentiation, D, between populations are for each level as follows: geographic populations, 0.031 ± 0.007; subspecies, 0.230 ± 0.016; semispecies, 0.226 ± 0.033; sibling species, 0.581 ± 0.039; morphologically different species, 1.056 ± 0.068. D may be interpreted as the number of electrophoretically detectable allelic substitutions per locus that have accumulated during the evolution of two taxa since they separated from a common ancestor. The model of geographic speciation recognizes two stages in the formation of species. In the first stage, allopatric populations become genetically sufficiently different for natural selection to favor the development of reproductive isolation between the populations if they were to come into geographic contact. The second stage occurs after sympatry when reproductive isolation is being completed. Two pairs of subspecies in the D. willistoni group represent populations in the first stage of speciation. On the average about 0.23 allelic substitutions per locus have accumulated in the separate evolutions of the subspecies. The second stage of the speciation process can be recognized in the semispecies of D. paulistorum. Genetic differentiation is not significantly greater, on the average, between semispecies than between subspecies. Thus, our results indicate that a substantial degree of genetic differentiation occurs during the first stage of speciation, but little change is required during the second stage when reproductive isolation is being completed. Species are independent evolutionary units, and thus they continue to diverge genetically through time. In spite of their morphological similarity, any two sibling species have accumulated, on the average, about 0.58 allelic substitutions per locus during their separate evolutions; and about twice as many allelic substitutions have accumulated in the evolution of morphologically distinguishable species.