Krosby Meade, Rohwer Sievert
Department of Biology and the Burke Museum, University of Washington, Seattle, WA 98195-1800, USA.
Proc Biol Sci. 2009 Feb 22;276(1657):615-21. doi: 10.1098/rspb.2008.1310.
Hybrid zones are natural experiments that expose the forces maintaining species differences. But for cases where a trait of one of the hybridizing pair appears shifted into the range of the other, the underlying mechanism can be difficult to infer. For example, hybridization between hermit warbler (Dendroica occidentalis) and Townsend's warbler (Dendroica townsendi) is restricted to narrow hybrid zones in Washington and Oregon, yet hermit mtDNA can be found in phenotypically pure Townsend's populations up to 2000 km north along the Pacific coast. This could reflect introgression of selectively favoured hermit mitochondria north across the hybrid zones, or a neutral genetic wake left behind following southern zone movement. Hermit mitochondrial haplotypes in populations of coastal Townsend's exhibit relatively high genetic diversity and significant divergence from those found in populations of hermit warblers. This contradicts the predictions of selective introgression, but is consistent with a northern population of hermits diverging in a glacial refugium before being replaced by Townsend's via aggressive hybridization. Previous field studies showing Townsend's males to be competitively superior to hermit males support this scenario, and suggest that the extreme hybrid zone movement evidenced by the hermit mitochondrial wake represents an extinction in progress.
杂交带是揭示维持物种差异的各种力量的自然实验。但对于杂交配对中的一方的某个性状似乎转移到另一方的范围内的情况，其潜在机制可能很难推断。例如，隐士莺（Dendroica occidentalis）和汤森氏莺（Dendroica townsendi）之间的杂交仅限于华盛顿和俄勒冈州的狭窄杂交带，然而在沿太平洋海岸向北达2000公里处的表型纯正的汤森氏莺种群中也能发现隐士莺的线粒体DNA。这可能反映了受到选择青睐的隐士莺线粒体跨越杂交带向北的基因渗入，或者是杂交带南移后留下的中性遗传痕迹。沿海汤森氏莺种群中的隐士莺线粒体单倍型表现出相对较高的遗传多样性，且与隐士莺种群中的单倍型有显著差异。这与选择性基因渗入的预测相矛盾，但与在冰川避难所中分化的北方隐士莺种群在被汤森氏莺通过激烈杂交取代之前的情况相符。先前的野外研究表明汤森氏莺雄鸟在竞争上优于隐士莺雄鸟，这支持了这种情况，并表明由隐士莺线粒体痕迹所证明的极端杂交带移动代表着一个正在发生的灭绝过程。
