Contact zones between recently diverged taxa provide opportunities to examine the causes of reproductive isolation and the processes that determine whether two species can coexist over a broad region. The Pacific wren (Troglodytes pacificus) and winter wren (Troglodytes hiemalis) are two morphologically similar songbirds that started diverging about 4 million years ago, older than most sister species pairs of temperate songbirds. The ranges of these species come into narrow contact in western Canada, where the two species remain distinct. To assess evidence for differentiation, hybridization and introgression in this system, we examined variation in over 250,000 single nucleotide polymorphism markers distributed across the genome. The two species formed highly divergent genetic clusters, consistent with long-term differentiation. In a set of 75 individuals, two first-generation hybrids (i.e., F₁’s) were detected, indicating only moderate levels of assortative mating between these taxa. We found no recent backcrosses or other evidence of recent breeding success of F₁’s, indicating very low or zero fitness of F₁ hybrids. Examination of genomic variation shows evidence for only a single backcrossing event many generations ago. The moderate rate of hybridization combined with very low F₁ hybrid fitness is expected to result in a population sink in the contact zone, largely explaining the narrow overlap of the two species. If such dynamics are common in nature, they could explain the narrow range overlap often observed between pairs of closely related species.

中文翻译：

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低适应度杂交种的持续生产限制了不同神秘物种之间的范围重叠

最近分化的分类群之间的接触区提供了检查生殖隔离的原因以及确定两个物种是否可以在广阔区域共存的过程的机会。太平洋鹪鹩 ( Troglodytes pacificus ) 和冬鹪鹩 ( Troglodytes hiemalis) 是两种形态相似的鸣禽，它们在大约 400 万年前开始分化，比大多数温带鸣禽的姐妹物种对更古老。这些物种的分布范围在加拿大西部有狭窄的接触，这两个物种仍然不同。为了评估该系统中分化、杂交和基因渗入的证据，我们检查了分布在整个基因组中的超过 250,000 个单核苷酸多态性标记的变异。这两个物种形成了高度不同的遗传簇，与长期分化一致。在一组 75 个人中，检测到两个第一代杂种（即 F ₁），表明这些分类群之间只有中等水平的分类交配。_{我们没有发现最近的回交或其他 F 1}最近育种成功的证据，表明 F ₁ 杂种的适应度非常低或为零。对基因组变异的检查显示了许多世代前只有一个回交事件的证据。中等的杂交率加上非常低的 F ₁ 杂交适应度预计会导致接触区的种群下降，这在很大程度上解释了这两个物种的狭窄重叠。如果这种动态在自然界中很常见，它们可以解释在密切相关的物种之间经常观察到的窄范围重叠。