Common Raven (Corvus corax) - HBW 14, p. 638

Taxonomy: Corvus Corax Linnaeus, 1758, Sweden.
Recent molecular-genetic investigations suggest that this species consists of at least three groups that are currently taking relatively independent evolutionary paths. Little, if any, genetic exchange occurs between populations of Old World and New World, allowing these two groups to maintain slight, but distinctive genetic signatures, in addition to which those in Canary Is (samples from Fuerteventura I only) are morphologically and genetically distinct from other Old World ravens (apparently having diverged c. 650,000 years ago). Thus, population in Canary Is, those elsewhere in Old World, and those in New World appear sufficiently isolated today to evolve independently. In Old World, a variety of described morphological forms may have been especially distinct 1 million years ago, but today they intergrade extensively and do not have unique mitochrondrial signatures. Similar process appears to be reducing genetic diversity in New World, where two clades recognized in current samples of mitochrondrial DNA: "California clade" found throughout W USA and represents most individuals in Mojave Desert, and "Holarctic clade" (which occurs throughout Old World) identified from samples in N North America (Alaska, Canada, Maine and Greenland); ravens from the two clades co-exist in roughly equal proportions in large area of North America (including Washington and Idaho), and where mating patterns and clade identity known (temperate rainforests of Olympic Peninsula, in Washington state) intercladal pairing is common, suggesting substantial gene flow between clades and a shared current evolutionary trajectory. Existence of both clades in W North America suggests that ravens may have colonized North America at least twice. Initially, "California clade" and "Holarctic clade" diverged from each other c. 2 million years ago, perhaps as glacial advances pushed original population into S refugia, and these distinct "California clade" ravens remained isolated from other ("Holarctic clade") ravens for c. 1 million years; during this time their sister-taxon, C. cryptoleucus, evolved, these taxa differing by 1.75% to 1.8% (suggesting that they diverged 1.1-0.6 million years ago, well after the two clades of present species diverged). "Holarctic clade" may have reinvaded North America across Bering Strait in last 15,000 years in the company of humans and grey wolves (Canis lupus). Many races described on basis of clines in morphology (decrease in size from N and high-elevation portions of range to S and desert parts) and plumage (decrease in bluish gloss and less pronounced lanceolate throat plumes from N to S); nominate race grades into laurencei in C Europe, and into kamtschaticus in W Siberia and W Russia and S to Black Sea, Caucasus, N Iran and Kazakhstan. Eleven subspecies currently recognized.