Beerli Peter, Hotz Hansjürg, Uzzell Thomas
Zoologisches Museum, Universität Zürich, Switzerland.
Department of Ecology, Ethology, and Evolution, University of Illinois, Urbana, Illinois, 61821.
Evolution. 1996 Aug;50(4):1676-1687. doi: 10.1111/j.1558-5646.1996.tb03939.x.
Reliable estimates of phylogenetic relationships and divergence times are a crucial requirement for many evolutionary studies, but are usually difficult because fossils are scarce and their interpretation is often uncertain. Frogs are fresh water animals that generally are unable to cross salt water barriers (their skin is readily permeable to both salt and water). The geologically determined ages of salt water barriers that isolate related frog populations thus provide an independent measure of the minimum date of genetic divergence between pairs of such populations. For the genetically well-studied western Palearctic water frogs (Rana esculenta group), the Aegean region provides an ideal area for determining the relationship between genetic divergence and time of spatial isolation, using a nested set of geologically determined isolation times (12,000 yr, 200,000 yr, 1.8 Myr, 2-3 Myr, and 5.2 Myr). Using 31 electrophoretic loci for 33 pairs of neighboring frog populations, a linear relationship between geologically determined isolation time and Hillis' modified Nei genetic distance was found: D = (0.04 ± 0.01) + (0.10 ± 0.01) isolation time [Myr] corresponding to an average divergence rate ("molecular clock" pace) of 0.10 D /Myr (0.10 D /Myr). This rate is in the range of previous estimates reported for protein electrophoretic data; the value is conservative because relatively few of the loci used are "fast evolvers" (13%; sAAT, ALB, EST-5, MPI). Removing these fast evolvers from the analysis results in 0.08 D /Myr (0.08 D /Myr). The confidence limits for estimation of the divergence time given the genetic distance are large, but unusually narrow for this kind of study; they permit us to estimate divergence times during the Pliocene and Miocene. Few previous studies, including sequence analyses, have provided reasonable estimates of divergence time for the Pliocene. A test using the outgroup taxa Rana perezi and Rana saharica (also isolated for 5.2 Myr by the Strait of Gibraltar) fits the calibration well: observed genetic Nei distance D = 0.55, expected D = 0.56. The calculated divergence times, based on this absolute molecular clock, suggest a series of speciation events after the Messinian (5.2 Myr), possibly triggered by the rapid ecological changes accompanying the desiccation and refilling of the Mediterranean Basin.
可靠的系统发育关系和分歧时间估计是许多进化研究的关键要求，但通常很困难，因为化石稀少且其解释往往不确定。青蛙是淡水动物，通常无法跨越盐水屏障（它们的皮肤对盐和水都很容易渗透）。隔离相关青蛙种群的盐水屏障的地质确定年龄因此提供了对这些种群对之间遗传分歧最小日期的独立衡量。对于经过充分基因研究的西古北区水蛙（食用蛙组），爱琴海地区提供了一个理想区域，利用一组嵌套的地质确定隔离时间（12000年、200000年、180万年、200 - 300万年和520万年）来确定遗传分歧与空间隔离时间之间的关系。使用针对33对相邻青蛙种群的31个电泳位点，发现地质确定的隔离时间与希利斯修正的内氏遗传距离之间存在线性关系：D = (0.04 ± 0.01) + (0.10 ± 0.01)隔离时间[百万年]，对应平均分歧率（“分子钟”速度）为0.10 D /百万年（0.10 D /百万年）。这个速率在先前报道的蛋白质电泳数据估计范围内；该值较为保守，因为所使用的位点中相对较少是“快速进化者”（13%；sAAT、ALB、EST - 5、MPI）。从分析中去除这些快速进化者后得到0.08 D /百万年（0.08 D /百万年）。给定遗传距离时分歧时间估计的置信限很大，但对于此类研究来说异常狭窄；它们使我们能够估计上新世和中新世期间的分歧时间。包括序列分析在内，以前很少有研究能对上新世的分歧时间提供合理估计。使用外类群物种佩氏蛙和撒哈拉蛙（也被直布罗陀海峡隔离了520万年）进行的测试与校准非常吻合：观察到的遗传内氏距离D = 0.55，预期D = 0.56。基于这个绝对分子钟计算出的分歧时间表明，在墨西拿期（520万年）之后发生了一系列物种形成事件，可能是由地中海盆地干涸和重新蓄水伴随的快速生态变化引发的。
