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Studies of genetic variation across hybrid zones have demonstrated that the evolutionary dynamics within them are often complicated. Using DNA extracted from toe pads of 701 individuals collected by Sibley and Short (1964) about 50 years ago from across the Icterus bullockii-I. galbula hybrid zone, we calculated mitochondrial cline shape parameters and compared them with plumage-based inferences of the hybrid-zone structure. Genetic and hybrid index score clines, estimated from populations collected along the Platte River in Nebraska, were both concordant (equal widths) and coincident (same center). More generally, the proportion of I. bullockii haplotypes within a sampling locality was strongly and significantly correlated with mean hybrid index scores across Colorado, Kansas, Nebraska, Oklahoma, and South Dakota. The relatively narrow width of the mitochondria! cline (328 km) indicates selection against hybridization, which may be mediated through differences in either molt and migration schedules or thermoregulatory capabilities (or both) of the parental species. Our results provide the first robust historical genetic characterization of this avian hybrid zone, laying the foundation for more in-depth investigations of temporal patterns of gene flow and introgression. Received 6 July 2010, accepted 2 October 2010.

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Published as Matthew D. Carling, Lindsay G. Serene and Irby J. Lovette, The Auk, Vol. 128, No. 1 (January 2011), pp. 61-68. © 2011 by the American Ornithologists' Union. Copying and permissions notice: Authorization to copy this content beyond fair use (as specified in Sections 107 and 108 of the U. S. Copyright Law) for internal or personal use, or the internal or personal use of specific clients, is granted by the American Ornithologists' Union for libraries and other users, provided that they are registered with and pay the specified fee via Rightslink® on JSTOR or directly with the Copyright Clearance Center,

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