Random interbreeding between cryptic lineages of the Common Raven: evidence for speciation in reverse WILLIAM C. WEBB,* 1 JOHN M. MARZLUFF* and KEVIN E. OMLAND† *University of Washington, College of the Environment, Box 352100, Seattle, WA 98195-2100, USA, †Department of Biological Sciences, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA Abstract 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. Keywords: Common Raven, Corvus corax, cryptic species, despeciation, speciation in reverse Received 7 January 2011; revision received 25 February 2011, accepted 7 March 2011 Introduction Recent genetic studies show that some widespread spe- cies with conserved morphology harbour cryptic genetic variation, such as occurs in Blue Tits (Parus caeruleus) (Taberlet et al. 1992), Snow Geese (Chen caerulescens) (Quinn 1992) and Carolina Chickadees (Poecile carolinen- sis) (Gill et al. 1993, 1999). These so-called ‘cryptic mtDNA clades’ differ substantially in their DNA but are not detectable using traditional taxonomic tech- niques that rely on comparisons of phenotypic charac- ters. However, basing taxonomic decisions on any one type of evidence (e.g., mtDNA monophyly or ability to interbreed) can be problematic, so several authors have argued for the importance of basing species limits on multiple criteria (de Queiroz 1998, 2007; Helbig et al. 2002; Sites & Marshall 2004). The Common Raven (Corvus corax) breeds throughout the Northern Hemisphere in a wide variety of habitats (Boarman & Heinrich 1999; Dos Anjos et al. 2009), but exhibits very little morphological variation (Vaurie 1959). Several inconsistent subspecies delineations have been described (Willet 1941; Rea 1986; Ratcliffe 1997) based upon slight clinal variation but there is no evi- dence to suggest substantial population subdivisions. Despite phenotypic homogeneity, phylogeographic sur- veys show that Common Ravens exhibit substantial genetic variation. Mitochondrial sequence data (mtDNA) reveal two distinct mtDNA clades within the Common Raven designated as the ‘Holarctic’ and ‘Cali- fornia’ clades (Omland et al. 2000, 2006). These two clades are over 4% divergent in mtDNA coding genes; Correspondence: William C. Webb, Fax: 206 685 0790; E-mail: webb@u.washington.edu 1 Present address: 498 Vine Avenue, Sunnyvale, CA 94086-6353. Ó 2011 Blackwell Publishing Ltd Molecular Ecology (2011) doi: 10.1111/j.1365-294X.2011.05095.x