Nason John D, Hamrick J L, Fleming Theodore H
Department of Botany, Iowa State University, Ames, Iowa 50011, USA.
Evolution. 2002 Nov;56(11):2214-26. doi: 10.1111/j.0014-3820.2002.tb00146.x.
Distinguishing the historical effects of gene migration and vicariance on contemporary genetic structure is problematic without testable biogeographic hypotheses based on preexisting geological and environmental evidence. The availability of such hypotheses for North America's Sonoran Desert has contributed to our understanding of the effect of historical vicariance and dispersal events on the diversification of this region's vertebrate biota but have not yet been applied to its flora. In this paper we describe a detailed allozyme analysis of the population genetic structure and phylogeography of the Sonoran Desert columnar cactus, Lophocereus schottii (senita). Inferred phylogroup distributions reflect two historical vicariance events: (1) a middle Pliocene northward transgression of the Sea of Cortéz that is reflected in well-supported Baja California peninsular and continental phylogroups but not in current taxonomic treatments of the species; and (2) a late Pliocene transpeninsular seaway across southern Baja that is reflected in tentative support for peninsular and southern Cape Region phylogroups corresponding to taxonomic varieties L. schottii var. schottii and L. schottii var. australis, respectively. A middle Pleistocene midpeninsular seaway hypothesized to explain congruent phylogroup distributions in several vertebrate taxa is not reflected in L. s. var. schottii, nor is the distinction of a third variety, L. s. var. tenuis, from continental populations of L. s. var. schottii. Linear regression of pairwise estimates of interpopulation differentiation (M and F(ST)/[1 - F(ST)]) on interpopulation geographic distance revealed significant evidence of isolation by distance within peninsular and continental phylogroups but not between them, consistent with historical vicariance between but not within these regions. We also found significant evidence of isolation by distance between putative L. s. var. schottii and L. s. var. australis phylogroups, suggesting that reproductive isolation between peninsular and Cape Region forms is incomplete. Within peninsular, but not continental, phylogroups, northward range expansion from southern Pleistocene refugia is reflected in significant declines in genetic variation with increasing latitude and in an area phenogram in which populations are progressively nested from south (ancestral) to north (descendant) along the Baja peninsula. Although the geographic concordance of phylogenetic topologies suggests that ancient vicariance events, and not dispersal, have primarily influenced the biogeographic distributions of Baja's vertebrate biota, the phylogeographic structure of L. schottii suggests that Sonoran Desert plant species may exhibit genetic signatures of postglacial range expansion and gene flow as well as vicariance.
如果没有基于既有地质和环境证据的可检验生物地理假说，区分基因迁移和地理隔离对当代遗传结构的历史影响是有问题的。北美索诺兰沙漠这类假说的可得性有助于我们理解历史上的地理隔离和扩散事件对该地区脊椎动物生物群多样化的影响，但尚未应用于其植物群。在本文中，我们描述了对索诺兰沙漠柱状仙人掌——肖特氏鸡冠柱（Senita）种群遗传结构和系统地理学的详细等位酶分析。推断的系统类群分布反映了两个历史地理隔离事件：（1）上新世中期科尔特斯海向北的海侵，这反映在得到充分支持的下加利福尼亚半岛和大陆系统类群中，但在该物种当前的分类处理中未体现；（2）上新世晚期横跨下加利福尼亚南部的半岛海峡，这反映在对分别对应于肖特氏鸡冠柱变种肖特氏鸡冠柱和肖特氏鸡冠柱变种南方鸡冠柱分类变种的半岛和南角地区系统类群的初步支持中。一个被假设用于解释几个脊椎动物类群中一致系统类群分布的更新世中期半岛中部海峡，在肖特氏鸡冠柱变种肖特氏鸡冠柱中未体现，肖特氏鸡冠柱变种细茎鸡冠柱与肖特氏鸡冠柱变种肖特氏鸡冠柱大陆种群的区分也未体现。种群间分化的成对估计值（M和F(ST)/[1 - F(ST)]）对种群间地理距离的线性回归显示，在半岛和大陆系统类群内有显著的距离隔离证据，但在它们之间没有，这与这些区域之间而非内部的历史地理隔离一致。我们还发现了在假定的肖特氏鸡冠柱变种和南方鸡冠柱变种系统类群之间存在距离隔离的显著证据，这表明半岛和角地区种群之间的生殖隔离并不完全。在半岛而非大陆系统类群内，从更新世晚期避难所向北的范围扩张反映在随着纬度增加遗传变异显著下降以及一个区域树形图中，在该图中种群沿着下加利福尼亚半岛从南（祖先）到北（后代）逐渐嵌套。尽管系统发育拓扑结构的地理一致性表明古代地理隔离事件而非扩散主要影响了下加利福尼亚脊椎动物生物群的生物地理分布，但肖特氏鸡冠柱的系统地理学结构表明，索诺兰沙漠植物物种可能展现出冰期后范围扩张、基因流动以及地理隔离的遗传特征。
