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Summary
Ever since the carrion crow (Corvus corone) and the hooded crow (Corvus cornix) were described by Linnaeus as two species in 1758 (1), their taxonomic status has been debated. These two phenotypically distinct crow taxa have a Palearctic breeding distribution with two stable zones of hybridization, one of which runs roughly north to south through central Europe (2) (see the figure). Primarily based on lack of complete reproductive isolation and because of genomewide genetic homogeneity, they are often considered to represent two subspecies of the carrion crow (3, 4). Some researchers, however, elevated these two taxa to full species in 2003 (5), a proposal supported by apparent nonrandom mating and reduced hybrid fitness. Earlier work (4) suggested that differences in gene expression, despite the lack of genomic nucleotide divergence, could serve as a sensitive indicator of speciation, although the exact mechanisms driving this process remained unknown. On page 1410 of this issue, Poelstra et al. (6) show, using a speciation genomics approach (7), how differential gene expression of only a small proportion (<0.28%) of the crow genome helps the carrion crow and hooded crow to remain phenotypically distinct.
