Wake D B
Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, CA 94720-3160, USA.
Proc Natl Acad Sci U S A. 1997 Jul 22;94(15):7761-7. doi: 10.1073/pnas.94.15.7761.
The Ensatina eschscholtzii complex of plethodontid salamanders, a well-known "ring species," is thought to illustrate stages in the speciation process. Early research, based on morphology and coloration, has been extended by the incorporation of studies of protein variation and mitochondrial DNA sequences. The new data show that the complex includes a number of geographically and genetically distinct components that are at or near the species level. The complex is old and apparently has undergone instances of range contraction, isolation, differentiation, and then expansion and secondary contact. While the hypothesis that speciation is retarded by gene flow around the ring is not supported by molecular data, the general biogeographical hypothesis is supported. There is evidence of a north to south range expansion along two axes, with secondary contact and completion of the ring in southern California. Current research targets regions once thought to show primary intergradation, but which molecular markers reveal to be zones of secondary contact. Here emphasis is on the subspecies E. e. xanthoptica, which is involved in four distinct secondary contacts in central California. There is evidence of renewed genetic interactions upon recontact, with greater genetic differentiation within xanthoptica than between it and some of the interacting populations. The complex presents a full array of intermediate conditions between well-marked species and geographically variable populations. Geographically differentiated segments represent a diversity of depths of time of isolation and admixture, reflecting the complicated geomorphological history of California. Ensatina illustrates the continuing difficulty in making taxonomic assignments in complexes studied during species formation.
无肺螈科蝾螈的埃氏剑螈复合体是一种著名的“环物种”，被认为阐释了物种形成过程中的各个阶段。早期基于形态学和颜色的研究，通过纳入蛋白质变异和线粒体DNA序列的研究得以扩展。新数据表明，该复合体包含一些在地理和遗传上不同的组分，这些组分处于物种水平或接近物种水平。该复合体年代久远，显然经历过分布范围收缩、隔离、分化，然后是扩张和再次接触的情况。虽然物种形成受环周围基因流阻碍的假说未得到分子数据的支持，但一般生物地理学假说得到了支持。有证据表明沿着两个轴存在从北到南的分布范围扩张，在南加利福尼亚有再次接触和环的形成。当前的研究目标是那些曾被认为显示初级渐变群的区域，但分子标记显示这些区域是再次接触带。这里重点关注亚种E. e. xanthoptica，它在加利福尼亚中部涉及四次不同的再次接触。有证据表明再次接触时会重新发生基因相互作用，xanthoptica内部的遗传分化大于它与一些相互作用种群之间的遗传分化。该复合体呈现出一系列介于界限分明的物种和地理上可变种群之间的中间状态。地理上有差异的部分代表了不同深度的隔离和混合时间，反映了加利福尼亚复杂的地貌历史。埃氏剑螈说明了在物种形成过程中对复合体进行分类学划分时持续存在的困难。
