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depths separating Mexican and U.S. haplotypes relative to an outgroup sequence than nine genes residing on chromosomes 1–3. We discuss the implications of secondary contact and differential introgression with respect to sympatric host race formation and speciation in Rhagoletis, reconciling some of the seemingly dichotomous views of Mayr, Dobzhansky, and Bush concerning modes of divergence.

Ernst Mayr helped to transform speciation into aholistic science. With his influential book Systematics and the Origins of Species, Mayr (1942) integrated and synthesized information from genetics, natural history, biogeography, and phylogenetics into a coherent concept of a biological species and a theory for allopatric speciation. Mayr stressed the critical importance of biogeography and systematics as cornerstones for understanding speciation. Divorced from time, space, and phylogenetic relationship, the analysis of reproductive isolation (the defining characteristic of biological species) loses evolutionary context and meaning. The proper chronological ordering of taxa at various stages of divergence also becomes untenable, prohibiting evaluation of the type, sequence, and importance of ecological, demographic, and genetic factors leading to speciation. Therefore, Mayr (1942, 1963) presented a cogent strategy for studying speciation, clarifying the nature of the question and the critical parameters for investigating the process.

Despite widespread acceptance of Mayr’s general framework for studying speciation, several seemingly dichotomous views and personalities nevertheless have shaped and still greatly influence our understanding of the process. Mayr (1942, 1963) made a forceful argument that geographic isolation (allopatry) is a requisite first step for facilitating divergence in animals. He stressed the coadapted nature of the genome and gene pools, as well as the need for allopatry to break the cohesive chains of gene flow to permit populations to diverge independently. In contrast, Guy Bush (1966, 1969) championed the importance of ecological adaptation in speciation. This view was epitomized in his arguments that certain phytophagous insect specialists speciate sympatrically in the process of shifting and adapting to new host plants. Theodosius Dobzhansky (1937, 1981) pioneered the genetic study of speciation, mapping genetic factors responsible for hybrid sterility and inviability and surveying natural populations to assess levels of genetic variation. He crystallized the view that speciation represents the transformation of within-population variation into between-taxa differences through the evolution of inherent reproductive isolating barriers. Dobzhansky (1981) also was a strong advocate of genetic coadaptation, especially with regard to balanced

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