AbstractNorth AmericanPapilio canadensisandP. glaucus(Lepidoptera: Papilionidae, thesePapilio = Pterourus) have previously been described as having allopatric distributions separated by a narrow hybrid zone running from Minnesota to southern New England, and southward in the Appalachian Mountains (possibly to northern Georgia). Recent patterns of hybridization and introgression suggest a more complex interaction between the two, possibly even resulting in the formation of a new species (Pterourus appalachiensisPavulaan & Wright, 2002). Recently, extensive northward interspecific introgression ofP. glaucus‐diagnostic traits has been observed in the hybrid zone. These include wing bands and other color patterns, the ability to feed on tulip tree leaves, andHk‐100allozymes; all are autosomally encoded. However, there has been little northward introgression of certain otherP. glaucustraits (such as facultative diapause and bivoltinism, andLdh‐100allozymes, both X‐linked; and the Y‐linked melanic mimicry gene in females). Interspecific recombination of the X‐chromosome has evidently occurred, as shown by discordant patterns of X‐linked markers. TheP. glaucusX‐linkedPgd‐100andPgd‐50alleles have introgressed 200–400 km north of the historical hybrid zone, yet theP. glaucusX‐linkedLdh‐100allele has not. The allele frequency shift for both genes is more closely related to the ‘thermal landscape’ (i.e., accumulated degree‐days above a developmental base threshold of 50 °F (=10 °C)) than to latitude. Delayed post‐diapause eclosion of cohorts within the hybrid zone, e.g., the New York/Vermont border area, has produced a natural ‘false‐second generation’ flight (a hybrid swarm of synchronous males and females, where 2300–2700 °F degree‐days have accumulated each year since 1998) that is reproductively isolated from flights of both parental species. Moreover, the newly describedP. appalachiensisexhibits a unique combination of traits. These include obligate diapause, a univoltine habit, and theLdh‐80orLdh‐40alleles (as forP. canadensis), thePgd‐100orPgd‐50alleles (as forP. glaucus), and a delayed ‘false‐second generation’ reproductive flight period (as observed in the hybrid zone). Since 2001, a rare allele or ‘hybrizyme’ (Ldh‐20) has appeared in this false second generation at high frequencies (40–50%). We hypothesize that strong selection against the facultative diapause (od‐)trait (and the linkedLdh‐100allele) in regions with 2800 °F degree‐days or less, and divergent selection in favor ofPgd‐100(or a closely linked trait) combined with allochronic reproductive isolation, has resulted in recombinational, parapatric, hybrid speciation. There is no evidence at present that host‐plant shifts or changes in sex pheromones have driven this process, in contrast to many other speciation events in the Lepidoptera.