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）是遗传漂变与自然选择随时间推移引发种群分化的演化过程。然而，物种形成所需的时长并无统一规律，即便极低水平的基因流也会阻碍种群分化，使得类群可能在演化长河中长期停留在物种形成的阈值阶段。本研究对线粒体DNA（mtDNA）与8个核内含子区域（nuDNA）进行测序，以估算欧亚绿翅鸭指名亚种（Anas crecca crecca）与北美绿翅鸭（A. c. carolinensis）之间的基因组分化水平与基因流强度。这两种鸭类在白令陆桥区域（亚洲东北部与北美洲西北部）存在分布交汇，且自约2.6 Ma的上新世-更新世过渡期以来，可能已周期性发生接触，彼时它们似乎已开始分化。二者雄性羽色存在可明确辨识的差异，线粒体DNA控制区序列分化率达6.9%；58份指名亚种样本中仅1份的单倍型与北美亚种聚为一类，86份北美亚种样本中则有2份出现此类情况。2个核基因座在二者间呈现强烈的遗传结构分化（ΦST > 0.35），但其余6个基因座未出现分化或仅表现出微弱的结构差异（ΦST = 0.0–0.06）。指名亚种与北美亚种间的线粒体基因流约为每个世代1个个体，双向均为此水平；但核基因区域的基因流呈现不对称性：分别约有1个与20个个体每世代迁入指名亚种与北美亚种种群。本研究表明，仅依靠单一标记开展物种界定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阻碍物种形成进程，使其无法完成最终的分化。