Random interbreeding between cryptic lineages of the Common Raven: evidence for speciation in reverse.

DNA sequence studies frequently reveal evidence of cryptic lineages in morphologically uniform species, many of which turn out to be evolutionarily distinct species. The Common Raven (Corvus corax) includes two deeply divergent mtDNA lineages: one lineage seems restricted to western North America and the other is Holarctic in distribution. These deep clades hint of the possibility of cryptic species in the western United States. We tested this hypothesis in a population consisting of an equal proportion of both mtDNA clades, by quantifying mating patterns and associated fitness consequences with respect to mtDNA. We also tested for morphological, behavioural and ecological correlates of sex and mtDNA clade membership. Mate pairings were random with respect to mtDNA clades, and there were no differences in reproductive success between assortatively and nonassortatively mated pairs. We found no differences in survival or resource use between clades. There were no differences in morphological or behavioural characters between mtDNA clades, except one clade trended towards greater mobility. These results suggest there are no barriers to gene flow between mtDNA clades and argue that the mtDNA clades have remerged in this population, likely due to a lack of ecological or signal differentiation between individuals in each lineage. Hence, in Common Ravens, phylogeographic structure in mtDNA is a reflection of likely past isolation rather than currently differentiated species.