Cryptic and extensive hybridization between ancient lineages of American crows

Most species and therefore most hybrid zones have historically been defined using phenotypic characters. However, both speciation and hybridization can occur with negligible morphological differentiation. Recently developed genomic tools provide the means to better understand cryptic speciation and hybridization. The Northwestern Crow (Corvus caurinus) and American Crow (Corvus brachyrhynchos) are continuously distributed sister taxa that lack reliable traditional characters for identification. In this study, our objective was to use genomic data and geographically robust sampling of American and Northwestern crows to better understand the evolutionary history of these presumptive species that lack well-defined phenotypic characters. Specifically, we set out to 1) assess whether Northwestern and American crows represent independently evolving evolutionary lineages, and, if so, 2) determine the extent to which they might hybridize or exhibit reproductive isolation, 3) test the role of specific geographic barriers in potentially structuring gene flow, and 4) better understand their evolutionary and biogeographic history.