History and dynamics of the most extensive hybrid zone on the Great Plains of North America

Hybridization affects the spatial and temporal patterns of morphological and genetic variation, shaping species evolution. Asclepias speciosa and A. syriaca provide an excellent system for uncovering how these patterns are generated. The ranges of A. speciosa and A. syriaca overlap on the North American Great Plains, and multiple intermediates are observed in this contact zone, consistent with hybridization. However, other processes, such as preservation of ancestral polymorphism or selective pressure imposed by environmental clines, could explain the presence of morphological intermediates. In this study, we characterized patterns of variation within and between A. speciosa and A. syriaca using morphological and genetic data to validate the hybrid origin of intermediates, evaluate the impact of hybridization on the parental species, and reconstruct the demographic history of hybridization events. We demonstrate that hybridization best explains the occurrence of large numbers of morphologically intermediate individuals in the contact zone and document bidirectional and asymmetric genetic introgression. We discover that the timing of secondary contact is relatively recent, coinciding with rapid range shifts during the Pleistocene. Our findings provide new insights into the dynamics of hybridization on the North American Great Plains, the setting for numerous contact zones between taxa affiliated with eastern and western biotas.