Department of Biology and Burke Museum, University of Washington, Seattle, Washington, United States of America.
PLoS One. 2010 Nov 30;5(11):e14164. doi: 10.1371/journal.pone.0014164.
Movements of hybrid zones - areas of overlap and interbreeding between species - are difficult to document empirically. This is true because moving hybrid zones are expected to be rare, and because movement may proceed too slowly to be measured directly. Townsend's warblers (Dendroica townsendi) hybridize with hermit warblers (D. occidentalis) where their ranges overlap in Washington and Oregon. Previous morphological, behavioral, and genetic studies of this hybrid zone suggest that it has been steadily moving into the geographical range of hermit warblers, with the more aggressive Townsend's warblers replacing hermit warblers along ∼2000 km of the Pacific coast of Canada and Alaska. Ongoing movement of the zone, however, has yet to be empirically demonstrated.
METHODOLOGY/PRINCIPAL FINDINGS: We compared recently sampled hybrid zone specimens to those collected 10-20 years earlier, to test directly the long-standing hypothesis of hybrid zone movement between these species. Newly sampled specimens were more Townsend's-like than historical specimens, consistent with ongoing movement of the zone into the geographical range of hermit warblers.
CONCLUSIONS/SIGNIFICANCE: While movement of a hybrid zone may be explained by several possible mechanisms, in this case a wealth of existing evidence suggests that movement is being driven by the competitive displacement of hermit warblers by Townsend's warblers. That no ecological differences have been found between these species, and that replacement of hermit warblers by Townsend's warblers is proceeding downward in latitude and elevation - opposite the directions of range shifts predicted by recent climate change - further support that this movement is not being driven by alternative environmental factors. If the mechanism of competitive displacement is correct, whether this process will ultimately lead to the extinction of hermit warblers will depend on the continued maintenance of the dramatic competitive asymmetry observed between the species.
杂种区的移动——物种之间重叠和杂交的区域——很难通过经验来证明。这是因为移动的杂种区预计是罕见的，而且由于移动可能进展缓慢而无法直接测量。在华盛顿州和俄勒冈州，迁徙莺（Dendroica townsendi）与隐士莺（D. occidentalis）杂交，它们的分布范围重叠。此前对该杂种区的形态学、行为学和遗传学研究表明，它一直在稳步向隐士莺的地理分布区移动，更具攻击性的迁徙莺在加拿大和阿拉斯加太平洋沿岸约 2000 公里的范围内取代了隐士莺。然而，该区域的持续移动尚未得到经验证明。
方法/主要发现：我们将最近采集的杂种区标本与 10-20 年前采集的标本进行了比较，直接检验了这些物种之间杂种区移动的长期假设。新采集的标本比历史标本更像迁徙莺，这与该区域向隐士莺地理分布区的持续移动相一致。
结论/意义：虽然杂种区的移动可能有几种可能的机制来解释，但在这种情况下，大量现有的证据表明，这种移动是由迁徙莺对隐士莺的竞争取代所驱动的。这两种物种之间没有发现生态差异，而且迁徙莺对隐士莺的取代正在沿着纬度和海拔向下进行——与最近气候变化预测的范围变化方向相反——进一步支持了这种移动不是由替代环境因素驱动的。如果竞争取代的机制是正确的，那么这个过程是否会最终导致隐士莺灭绝，将取决于物种之间观察到的巨大竞争不对称性是否能持续保持。
