Marko Peter B
Section of Evolution and Ecology and Center for Population Biology, University of California, Davis, California, 95616.
Evolution. 1998 Jun;52(3):757-774. doi: 10.1111/j.1558-5646.1998.tb03700.x.
Two recently diverged northeastern Pacific sibling snail species, Nucella ostrina and N. emarginata, currently inhabit adjacent zoogeographic provinces. Their distributions overlap in central California to the north of a major faunal boundary at Point Conception, California (PC). To test the hypothesis that modern sympatry is due to a recent northward range expansion by N. emarginata, I analyzed the population structures of both species with nuclear (allozyme) and mitochondrial DNA (mtDNA) markers. Populations of N. emarginata in the region of overlap exhibit significantly lower heterozygosity and allelic diversity than either populations to the south of PC or populations of N. ostrina. A single mtDNA haplotype characterizes all but one population of N. emarginata sampled in this region, but no haplotype to the south of PC is found at more than one locality. MtDNA haplotypes and allozyme allele frequencies also indicate monophyly of central California populations of N. emarginata. Sharp differences in allelic diversity over small geographic distances may reflect the action of natural selection, but because both nuclear and mtDNA markers display concordant patterns, a range expansion across PC best explains patterns of genetic variation in N. emarginata. Allozymes and mtDNA also reveal that the geologically older N. ostrina is paraphyletic with respect to N. emarginata. This pattern is consistent with, but not indicative of, a peripheral isolation model of speciation. Low genetic diversity is also expected if a significant bottleneck occurred at speciation. However, low allelic diversity is not universal throughout the geographic range of N. emarginata; high allelic diversity at the southern end of the distribution of N. emarginata suggests that in the past N. emarginata has been geographically restricted much further south than PC. A northward range expansion across PC by N. emarginata may thus represent only the most recent postglacial movement by the species. The thermal and oceanographic discontinuities found at PC may not have been directly involved in geographic isolation if N. emarginata originated much further south of this modern boundary. Despite uncertainty regarding the exact spatial distribution of populations at speciation, genetic data indicate that even though N. ostrina and N. emarginata currently exhibit a broad range of geographic overlap, speciation was likely allopatric and was initiated by physical isolation of populations in different zoogeographic provinces.
最近分化的两个东北太平洋姊妹蜗牛物种，斯特里纳核螺（Nucella ostrina）和凹缘核螺（N. emarginata），目前栖息在相邻的动物地理省份。它们的分布在加利福尼亚中部重叠，位于加利福尼亚州概念点（PC）的一个主要动物区系边界以北。为了检验现代同域分布是由于凹缘核螺最近向北扩展分布范围这一假设，我用核（等位酶）和线粒体DNA（mtDNA）标记分析了这两个物种的种群结构。重叠区域的凹缘核螺种群的杂合度和等位基因多样性明显低于PC以南的种群或斯特里纳核螺的种群。在该区域采样的凹缘核螺种群中，除了一个种群外，所有种群都由单一的mtDNA单倍型表征，但在PC以南没有一个单倍型在多个地点被发现。mtDNA单倍型和等位酶等位基因频率也表明加利福尼亚中部的凹缘核螺种群是单系的。在小地理距离上的等位基因多样性的显著差异可能反映了自然选择的作用，但由于核标记和mtDNA标记都显示出一致的模式，跨越PC的分布范围扩展最能解释凹缘核螺的遗传变异模式。等位酶和mtDNA还表明，地质年代较久的斯特里纳核螺相对于凹缘核螺是并系的。这种模式与物种形成的边缘隔离模型一致，但并不表明就是这种模型。如果在物种形成时发生了重大瓶颈，也会预期出现低遗传多样性。然而，低等位基因多样性在凹缘核螺的整个地理分布范围内并不普遍；凹缘核螺分布南端的高等位基因多样性表明，过去凹缘核螺在地理上的分布比PC更靠南。因此，凹缘核螺跨越PC向北的分布范围扩展可能仅代表该物种最近的冰期后移动。如果凹缘核螺起源于这个现代边界以南更远的地方，在PC发现的热和海洋学不连续性可能没有直接参与地理隔离。尽管关于物种形成时种群的确切空间分布存在不确定性，但遗传数据表明，即使斯特里纳核螺和凹缘核螺目前表现出广泛的地理重叠，物种形成很可能是异域性的，并且是由不同动物地理省份的种群的物理隔离引发的。
