Bensch Staffan, Andersson Tord, Åkesson Susanne
Department of Animal Ecology, Ecology Building, Lund University, S-223 62, Lund, Sweden.
Evolution. 1999 Dec;53(6):1925-1935. doi: 10.1111/j.1558-5646.1999.tb04573.x.
A migratory divide is a narrow region in which two populations showing different migratory directions meet arid presumably also mate and hybridize. Banding of willow warblers, Phylloscopus trochilus, in Europe has demonstrated a migratory divide latitudinally across central Scandinavia. In autumn, southern birds migrate southwest to tropical West Africa, whereas northern birds migrate southeast to East and South Africa. The migratory divide is associated with concordant differences in size and plumage coloration. Based on morphology, we estimate the width of the transition zone between northern and southern willow warblers to be less than 350 km. We found indication of linkage disequilibria around the migratory divide, in that measures of body size were correlated with plumage coloration within the contact zone, but uncorrelated within the populations south or north of the contact zone. The presence of linkage disequilibria and the fact that several morphological clines occur together suggest that the hybrid zone is a result of secondary contact between populations that have differentiated in allopatry. This interpretation is in accord with the knowledge of the recolonization pattern of the Scandinavian peninsula after the last glaciation; animals and plants appeared to have colonized either from the south or from the north around the northern bay of the Baltic Sea. If northern and southern willow warblers resided in allopatric populations during late Pleistocene glaciations and the hybrid zone is a result of postglacial range expansions, we would expect some degree of genetic differentiation accumulated during the period in isolation. In contrast, northern and southern willow warblers are near panmictic in the frequencies of alleles of mitochondrial DNA and at two microsatellite loci. The observed pattern, clear morphological and behavioral differentiation without genetic differentiation at neutral loci, suggests either that the differences are maintained by strong selection on the expressed genes in combination with high levels of current gene flow or, in the case of weak gene flow, that the divergence in morphology and behavior is very recent.
迁徙分界线是一个狭窄区域，在此区域内，两个表现出不同迁徙方向的种群相遇，并且大概也会进行交配和杂交。对欧洲柳莺（Phylloscopus trochilus）的环志研究表明，在斯堪的纳维亚半岛中部存在一条纬度上的迁徙分界线。秋季时，南部的柳莺向西南方向迁徙至热带西非，而北部的柳莺则向东南方向迁徙至东非和南非。这条迁徙分界线与柳莺在体型和羽毛颜色上的一致性差异相关。基于形态学，我们估计北部和南部柳莺之间过渡带的宽度小于350千米。我们发现在迁徙分界线周围存在连锁不平衡的迹象，即体型测量值与接触区内的羽毛颜色相关，但在接触区以南或以北的种群内不相关。连锁不平衡的存在以及几个形态渐变群共同出现的事实表明，杂交区是在异域分化的种群之间二次接触的结果。这种解释与末次冰期后斯堪的纳维亚半岛重新殖民化模式的知识相符；动植物似乎是从波罗的海北部海湾周围的南部或北部重新殖民的。如果在更新世晚期冰期期间，北部和南部柳莺处于异域种群状态，且杂交区是冰期后范围扩张的结果，那么我们预计在隔离期间会积累一定程度的遗传分化。然而，北部和南部柳莺在线粒体DNA等位基因频率以及两个微卫星位点上几乎是随机交配的。观察到的模式，即明显的形态和行为分化但在中性位点没有遗传分化，表明要么这些差异是由对表达基因的强烈选择与当前高水平的基因流共同维持的，要么在基因流较弱的情况下，形态和行为的差异是非常近期才出现的。
