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 first population genomic study of Northwestern and American crows, we use genomic SNPs (nuDNA) and mtDNA to investigate the degree of genetic differentiation between these crows and the extent to which they may hybridize. Our results indicate that American and Northwestern crows have distinct evolutionary histories, supported by two nuDNA ancestry clusters and two 1.1%-divergent mtDNA clades dating to the late Pleistocene, when glacial advances may have isolated crow populations in separate refugia. We document extensive hybridization, with geographic overlap of mtDNA clades and admixture of nuDNA across >900 km of western Washington and western British Columbia. This broad hybrid zone consists of late-generation hybrids and backcrosses, but not recent (e.g., F1) hybrids. Nuclear DNA and mtDNA clines had concordant widths and were both centred in southwestern British Columbia, farther north than previously postulated. Overall, our results suggest a history of reticulate evolution in American and Northwestern crows, perhaps due to recurring neutral expansion(s) from Pleistocene glacial refugia followed by lineage fusion(s). However, we do not rule out a contributing role for more recent potential drivers of hybridization, such as expansion into human-modified habitats.

中文翻译：

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美国乌鸦古代血统之间的隐秘和广泛杂交。

历史上大多数物种，因此大多数杂种区是使用表型特征定义的。但是，形态形成和杂交都可以在形态分化可忽略不计的情况下发生。最近开发的基因组工具提供了更好地了解隐性物种形成和杂交的手段。西北乌鸦（Corvus caurinus）和美洲乌鸦（Corvus brachyrhynchos）是连续分布的姐妹类群，缺乏可靠的传统特征来进行识别。在西北和美洲乌鸦的首次人口基因组研究中，我们使用基因组SNP（nuDNA）和mtDNA研究这些乌鸦之间的遗传分化程度以及它们杂交的程度。我们的结果表明，美国和西北地区的乌鸦有着独特的进化史，当冰川发展可能在单独的避难所中分离出了乌鸦种群时，有两个nuDNA祖先集群和两个1.1％差异的mtDNA进化枝支持，这些进化枝可以追溯到更新世晚期。我们记录了广泛的杂交，在超过900公里的华盛顿西部和不列颠哥伦比亚省西部，mtDNA进化枝和nuDNA的地理重叠。这个宽广的杂种区由后代杂种和回交组成，但不包括最近的杂种（例如F1）。核DNA和mtDNA谱系的宽度一致，都位于不列颠哥伦比亚省西南部，比以前假定的更北。总体而言，我们的研究结果表明，美国和西北乌鸦有网状进化史，这可能是由于更新世冰川避难所反复出现中性扩张并沿袭融合所致。然而，