The views of Mayr, Dobzhansky, and Bush may not be as trichotomous as they seem with respect to Rhagoletis. Geographic isolation appears to have established an initial kernel of genetic differentiation that was later expanded on and contributed to sympatric host shifts and new fly taxa. Thus, although geographic context is critical for understanding speciation, allopatry and sympatry should not always be considered as diametrically opposed modes of divergence along an axis of spatial isolation. Differentiation and processes occurring in isolation and contact can interact and compliment each other to accentuate species formation, arguing for a more pluralistic view of modes of speciation (Mallet, 2005). In the case of R. pomonella, the relationship involves a likely sequence of geographic isolation, life-history adaptation, secondary contact, differential introgression, inversion clines, and sympatric host shifts. The evolution of reinforcement can be viewed in an analogous manner, involving non-host-related traits affecting prezygotic isolation rather than ecological adaptation per se. Also, there is no reason to presume that host-related differences that originated in sympatry cannot be solidified by periods of geographic isolation between host-associated populations, although such allopatry is not required to complete the speciation process. Thus, during the time course of differentiation, populations can assume characteristics of both allopatric and sympatric modes of divergence, with phenotypic and genetic elements interacting to further the speciation process.
The connectivity of speciation mode is perhaps best epitomized for R. pomonella if one views the phylogeography of the fly as reflecting sequential adaptation to spatially more finely packaged phenological host niches. At the coarsest level, Altiplano, Sierra Oriental, and Northern R. pomonella populations initially became differentially adapted to temporal and spatial disjunctions in hawthorn fruiting time through a “modular” genetics associated with inversions affecting diapause. After secondary introgression from Mexico, the modular gene blocks became arrayed in the formof broad inversion clines in the North in response to latitudinal variation in hawthorn fruiting time. Last, life-history variation inherent in the clines was extracted on a microgeographic scale [primarily by shifts in allele (inversion) frequencies] to facilitate sympatric shifts and specialization of R. pomonella in the United States to a number of cooccurring host plant species with differing fruiting times. However, host specificity does not appear to be a factor reproductively isolating Altiplano and Sierra flies. Here, geography may act as habitat fidelity does in sympatry, limiting migration and facilitating divergence.
The differences between Altiplano, Sierra, and U.S. populations raise a number of questions. For example, the apparently reduced potential for