Data from: 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 tSpeciation 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 (A. c. carolinensis). These ducks come into contact in Beringia (northeastern Asia and northwestern North America) and have likely 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 (ΦST > 0.35), but six loci were undifferentiated or only weakly structured (ΦST = 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.物种形成（Speciation）是指遗传漂变（genetic drift）与选择作用随时间推移引发种群分化的过程。然而，物种形成所需的进化时长并无统一规律，即便低水平的基因流（gene flow）也会阻碍种群分化，致使类群在漫长的进化历程中长期停滞于物种形成的阈值阶段。本研究对线粒体DNA（mtDNA）与8个核内含子（nuDNA）进行测序，以估算欧亚普通凫（Anas crecca crecca）与北美绿翅凫（A. c. carolinensis）之间的基因组分化水平与基因流情况。这两种鸭在白令陆桥（Beringia，涵盖东北亚与西北美洲区域）存在分布重叠，自约260万年前的上新世-更新世过渡期以来，二者可能周期性发生接触，彼时它们似乎已开始出现分化。二者在雄性羽色上具备可辨识的差异，线粒体DNA控制区的序列分化率达6.9%；在58份欧亚普通凫样本中，仅1份的单倍型（haplotype）聚类于北美绿翅凫类群，而86份北美绿翅凫样本中仅2份出现此类单倍型。两个核基因座（nuclear loci）在这两种凫之间呈现显著的遗传结构分化（ΦST > 0.35），但剩余6个基因座未出现分化或仅呈现弱结构分化（ΦST = 0.0–0.06）。欧亚普通凫与北美绿翅凫之间的线粒体基因流约为每代双向各1个个体，但核基因层面的基因流呈现不对称性：每代分别约有1个和20个个体迁入欧亚普通凫与北美绿翅凫种群。本研究表明，仅依靠单一标记开展物种界定（species delimitation）会过度简化物种形成过程的复杂性，同时提示即便存在趋异选择，中等水平的基因流仍可能使物种形成过程在完成前陷入停滞。