Schneider C J
Museum of Vertebrate Zoology, University of California, Berkeley, 94720, USA,
Mol Ecol. 1996 Apr;5(2):239-49.
Distinguishing between primary and secondary intergradation among differentiated populations, and the relative importance of drift and selection, are persistent problems in evolutionary biology. An historical perspective on population interactions can provide insight into the nature of contacts, and thus help resolve these questions. Continuously distributed populations of Anolis marmoratus from the island of Basse Terre in the Guadeloupean archipelago of the Lesser Antilles show a striking degree of geographic variation in morphology. Initial surveys of mtDNA variation from throughout the Guadeloupean Archipelago revealed one case where levels of sequence difference and phylogenetic relationships of alleles from morphologically differentiated populations from the east coast of Basse Terre were consistent with primary intergradation. In this paper, I examine the genetic population structure of a series of populations spanning this north-south cline in morphological variation to test the hypothesis of primary intergradation. Sequences of the mitochondrial cytochrome-b gene from 50 individuals representing five populations spanning the cline were obtained and fourteen unique haplotypes (differing by 2% or less) were detected. Patterns of nucleotide substitution among haplotypes do not deviate from neutral expectation indicating no effect of selection at the level of mtDNA sequences. Estimates of population structure and gene flow were made using both summary statistics for nucleotide diversity (Nst) and cladistic methods. The results are sensitive to the choice of gene flow model, and this is discussed in detail. Mitochondrial variation in the northern populations may not be at equilibrium, and the phylogeny of alleles is consistent with a recent increase in effective population size. Estimates of nucleotide diversity, gene flow, and the phylogenetic relationships of haplotypes indicate that the southern-most population (representing the extreme of morphological variation along this cline) has been relatively isolated from populations to the north and has experienced a reduced effective population size. The apparent clinal variation between the southern population and the others may therefore reflect secondary contact and introgression rather than primary intergradation.
区分分化种群中的初级渐变和次级渐变，以及漂变和选择的相对重要性，是进化生物学中一直存在的问题。从历史角度看待种群间相互作用，有助于深入了解接触的本质，从而帮助解决这些问题。小安的列斯群岛瓜德罗普群岛巴斯特尔岛上连续分布的马氏安乐蜥种群，在形态上呈现出显著的地理变异。对整个瓜德罗普群岛线粒体DNA变异的初步调查发现，在一个案例中，巴斯特尔岛东海岸形态分化种群的序列差异水平和等位基因的系统发育关系与初级渐变一致。在本文中，我研究了一系列跨越这一形态变异南北渐变群的种群的遗传种群结构，以检验初级渐变的假设。获得了代表跨越渐变群的五个种群的50个个体的线粒体细胞色素b基因序列，并检测到14个独特的单倍型（差异在2%或以下）。单倍型之间的核苷酸替换模式并未偏离中性预期，这表明在mtDNA序列水平上没有选择作用。使用核苷酸多样性的汇总统计量（Nst）和分支方法对种群结构和基因流进行了估计。结果对基因流模型的选择很敏感，并对此进行了详细讨论。北方种群的线粒体变异可能尚未达到平衡，等位基因的系统发育与有效种群大小最近的增加一致。核苷酸多样性、基因流的估计以及单倍型的系统发育关系表明，最南端的种群（代表沿此渐变群形态变异的极端情况）与北方种群相对隔离，有效种群大小有所减少。因此，南方种群与其他种群之间明显的渐变变异可能反映的是次级接触和渐渗，而非初级渐变。
