How much genetic divergence is necessary for two populations to maintain themselves as separate species? In part, the answer to this question depends upon how the species themselves were formed. In a study of allozyme polymorphism in allopatric populations (those found in different localities) within the Drosophila willistoni group in Venezuela, Ayala and colleagues showed that isolated populations of the same species were very similar genetically, with a mean genetic identity (the proportion of loci identical in two samples) of I = 0.970. However, the genetic identity was lower in populations that were practically reproductively isolated (termed "semispecies"; I ~ 0.8) and even lower in well-recognized, reproductively isolated sibling species (I = 0.517) and closely related, but nonsibling, species (I = 0.352).

Species formed in sympatry (i.e., in the same locality) might potentially be more similar genetically than those formed in allopatry, particularly if only a small number of loci are important for initial divergence. Sympatric speciation is most commonly associated with true fruit flies (Tephritidae) in the genus Rhagoletis. Bush in the 1960s suggested that speciation in these flies may have occurred following shifts in host use within the same habitat. He offered as an example of the process the host shift of the apple maggot, Rhagoletispomonella, from its native hawthorn host to introduced apples in New York State in the 1860s. There was considerable resistance initially to the concept of a nonallopatric mode of speciation. However, more recent work by Bush and colleagues has provided a convincing case for sympatric speciation in Rhagoletis, with differences between species being maintained through genetic control of emergence times on the different hosts. There also exists at least partial premating reproductive isolation associated with host-plant fidelity. Berlocher in 1976 measured the mean genetic identity between R. pomonella and two other different species of Rhagoletis that were likely formed in sympatry through host shifts and found the species to be nearly identical genetically (I = 0.980 and I = 0.989) and significantly more similar than were Rhagoletis species formed in allopatry and the Drosophila species noted above.

The conclusion from these studies is that the average genetic divergence itself is not critical for speciation, nor for the maintenance of reproductively isolated species, but rather that the effects of a small set of nonrandom loci may be important in species formation. Recent advances by Feder, facilitated through better understanding of the Rhagoletis genome, and by others studying Heliconius butterflies and pea aphids suggest that changes in only relatively few functional loci can lead to rapid speciation.

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