True fruit flies in the Rhagoletis pomonella sibling species group are at the center of a long-standing debate concerning modes of speciation. The allopatric separation of populations is widely thought to be a prerequisite for speciation in sexually reproducing animals. However, speciation in the R. pomonella group appears to have occurred sympatrically as a consequence of these flies shifting and adapting to new host plants. The sympatric shift of R. pomonella from its native host hawthorn to introduced domestic apple, which occurred approximately 150 years ago, provides a test of whether host specialization is sufficient to allow populations to differentiate in the absence of geographic barriers to gene flow. We report the results of a geographic study of allozyme variation for hawthorn and apple infesting populations of R. pomonella across the eastern United States and Canada. Six loci consistently show significant allele frequency differences at paired apple and hawthorn sites. These six loci map to three different regions of the genome, and linkage disequilibrium exists between non-allelic genes within each of these regions. Allele frequencies for five of the six loci displaying host associated differences also co-vary significantly with latitude. Inter-host divergence is, therefore, superimposed on north-south clinal patterns of intra-host variation such that the magnitude of genetic divergence between hawthorn and apple flies is a function of latitude. The findings suggest that partially reproductively isolated "host races" can evolve in sympatry as a consequence of R. pomonella infesting new host plants. Host recognition and host associated developmental traits are discussed as important factors differentiating apple and hawthorn flies.
© 1990 The Society for the Study of Evolution.