Allochronic isolation and incipient hybrid speciation in tiger swallowtail butterflies

Hybridization leading to reproductively isolated, novel genotypes is poorly understood as a means of speciation and few empirical examples have been studied. In 1999, a previously non-existent delayed flight of what appeared to be the Canadian tiger swallowtail butterfly, Papilio canadensis, was observed in the Battenkill River Valley, USA. Allozyme frequencies and morphology suggest that this delayed flight was the product of hybridization between Papilio canadensis and its sibling species Papilio glaucus. The mitochondrial DNA (mtDNA) restriction fragment length polymorphisms presented here indicate that only P. canadensis-like mtDNA occurs in this population, suggesting that introgression likely occurred from hybrid males mating with P. canadensis females. Preliminary studies of this population indicated that delayed post-diapause pupal emergence in this hybrid genotype was the root cause behind the observed delayed flight, which suggests a potential empirical example of a mechanism leading to reproductive isolation. Here we provide further evidence of the role of adult pupal emergence as a reproductive barrier likely leading to reproductive isolation. In particular, we present results from pupal emergence studies using four different spring and two different winter temperature treatments. The results indicate a clear separation of adult emergences between the hybrid population and both parental species. However, our results indicate that exceptionally hot springs are likely to lead to greater potential for overlap between the local parental species, P. canadensis, and this delayed population with hybrid origins. Conversely, our results also show that warmer winters are likely to increase the temporal separation of the hybrid population and the parental species. Finally, we report recently collected evidence that this hybrid population remains morphologically distinct.