A parapatric propensity for breeding precludes the completion of speciation in common teal (Anas crecca, sensu lato).

Speciation is a process in which genetic drift and selection cause divergence over time. However, there is no rule dictating the time required for speciation, and even low levels of gene flow hinder divergence, so that taxa may be poised at the threshold of speciation for long periods of evolutionary time. We sequenced mitochondrial DNA (mtDNA) and eight nuclear introns (nuDNA) to estimate genomic levels of differentiation and gene flow between the Eurasian common teal (Anas crecca crecca) and the North American green-winged teal (Anas crecca carolinensis). These ducks come into contact in Beringia (north-eastern Asia and north-western North America) and have probably done so, perhaps cyclically, since the Pliocene-Pleistocene transition, ~2.6 Ma, when they apparently began diverging. They have diagnosable differences in male plumage and are 6.9% divergent in the mtDNA control region, with only 1 of 58 crecca and 2 of 86 carolinensis having haplotypes grouping with the other. Two nuclear loci were likewise strongly structured between these teal (Φ(st) ≥ 0.35), but six loci were undifferentiated or only weakly structured (Φ(st) = 0.0-0.06). Gene flow between crecca and carolinensis was ~1 individual per generation in both directions in mtDNA, but was asymmetrical in nuDNA, with ~1 and ~20 individuals per generation immigrating into crecca and carolinensis, respectively. This study illustrates that species delimitation using a single marker oversimplifies the complexity of the speciation process, and it suggests that even with divergent selection, moderate levels of gene flow may stall the speciation process short of completion.