Ponniah Mark, Hughes Jane M
Co-operative Research Centre for Tropical Rainforest Ecology and Management, Australian School of Environmental Studies, Griffith University, Nathan, Queensland 4111, Australia.
Evolution. 2004 May;58(5):1073-85. doi: 10.1111/j.0014-3820.2004.tb00441.x.
The upland mesic rainforests of eastern Australia have been described as a "mesothermal archipelago" where a chain of cool mountain "islands" arise from a warm "sea" of tropical and subtropical lowlands. An endemic freshwater crayfish belonging to the genus Euastacus is found on each of these mountain "islands." The Euastacus are particularly suitable for the study of evolution because each mountain harbors a unique species, there are many taxa present providing replication within the group and, most importantly, their distribution is linear, extending along a south-north axis. This group could have evolved by "simultaneous vicariance" where there was one vicariant separation event of a widespread ancestor, or by "south to north stepping stone dispersal" where there were long distance dispersal events from neighboring mountain islands, starting in the south and proceeding north in a dispersal-colonization wave. We used pairwise genetic distances between nearest geographic neighbors as a novel way to test the two hypotheses. If diversification was due to "south to north stepping stone dispersal," then pairwise genetic distances between nearest geographic neighbors should decrease progressively the farther north the taxon pairs are found, reflecting the decreasing periods of isolation. In this case there should be a negative correlation between the south to north rank order of nearest neighbors and pairwise genetic distances. A Spearman's correlation on 16S mtDNA pairwise genetic distances and geographic rank order was not significant, indicating there was no support for the south to north stepping stone dispersal hypothesis. If simultaneous vicariance was responsible for diversification then all nearest geographic neighbor taxon pairs should have similar genetic distances and, therefore, the variance in nearest neighbor distances should be zero, or close to it. To test if the observed variance was tending towards zero we developed a randomization test where nearest neighbor taxon pairs were assigned random genetic distances and the variances calculated. The observed variance lay in the < 0.05 range of the simulated variances, providing support for the simultaneous vicariance hypothesis. The data also suggest there was simultaneous vicariance of at least two ancestral Queensland lineages. The timing of this vicariant event was probably in the Pliocene, which is consistent with the divergence times reported for other Australian mesic rainforest restricted taxa.
澳大利亚东部的高地中生雨林被描述为一个“中温群岛”，在这片区域中，一系列凉爽的山地“岛屿”从热带和亚热带低地的温暖“海洋”中崛起。在这些山地“岛屿”中的每一个上都发现了一种属于真螯虾属的特有淡水小龙虾。真螯虾特别适合用于进化研究，因为每座山上都有一个独特的物种，该群体中有许多分类单元可提供重复样本，而且最重要的是，它们的分布呈线性，沿南北轴延伸。这个群体可能是通过“同时隔离分化”进化而来，即一个广泛分布的祖先发生了一次隔离分化事件，或者是通过“从南到北的踏脚石式扩散”进化而来，即从相邻的山地岛屿发生了远距离扩散事件，从南部开始，以扩散 - 定殖波的形式向北推进。我们使用最近地理邻域之间的成对遗传距离作为一种新方法来检验这两种假设。如果物种分化是由于“从南到北的踏脚石式扩散”，那么最近地理邻域之间的成对遗传距离应该随着分类单元对向北分布得越远而逐渐减小，这反映了隔离期的缩短。在这种情况下，最近邻域的南北排名顺序与成对遗传距离之间应该存在负相关。对16S线粒体DNA成对遗传距离和地理排名顺序进行的斯皮尔曼相关性分析并不显著，这表明没有证据支持从南到北的踏脚石式扩散假设。如果同时隔离分化是物种分化的原因，那么所有最近地理邻域的分类单元对应该具有相似的遗传距离，因此，最近邻域距离的方差应该为零，或者接近零。为了检验观察到的方差是否趋于零，我们开发了一种随机化检验，为最近邻域的分类单元对分配随机遗传距离并计算方差。观察到的方差处于模拟方差的<0.05范围内，这为同时隔离分化假设提供了支持。数据还表明，至少有两个昆士兰祖先谱系发生了同时隔离分化。这次隔离事件的时间可能在上新世，这与其他澳大利亚中生雨林特有分类单元报告的分歧时间一致。
