Ongoing production of low-fitness hybrids limits range overlap between divergent cryptic species

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.

新近分化类群间的接触带，为探究生殖隔离的成因，以及决定两个物种能否在大范围区域内共存的过程，提供了绝佳的研究契机。太平洋鹪鹩（Pacific Wren，学名Troglodytes pacificus）与冬鹪鹩（Winter Wren，学名Troglodytes hiemalis）是两种形态相似的鸣禽，其分化始于约400万年前，早于多数温带鸣禽的姊妹物种类群分化时间。二者的分布范围在加拿大西部形成狭窄接触带，且在此区域内始终保持物种分化独立性。为评估该研究系统中物种分化、杂交与基因渐渗的相关证据，我们对分布于全基因组的超过25万个单核苷酸多态性（single nucleotide polymorphism, SNP）标记的变异情况进行了分析。两个物种形成了高度分化的遗传聚类，与二者长期分化的演化历程相符。在75个个体的样本中，共检测到2只第一代杂交个体（即F1代），表明二者间的选型交配仅处于中等水平。未检测到近期回交事件，也未发现F1代个体近期繁殖成功的其他证据，说明F1杂交个体的适合度极低甚至为零。对基因组变异的分析显示，仅存在一次发生于多代之前的回交事件痕迹。中等水平的杂交发生率与极低的F1杂交适合度相结合，预计会在接触带内形成种群汇，这在很大程度上解释了两个物种为何仅存在狭窄的分布重叠区域。倘若这类演化动态在自然界中普遍存在，则可解释为何近缘物种类群间常出现狭窄的分布重叠现象。