Heads Michael
Biology Department, University of the South Pacific, PO Box 1168, Suva, Fiji Islands.
Cladistics. 2005 Feb;21(1):62-78. doi: 10.1111/j.1096-0031.2005.00052.x.
Taxa have been dated using three methods: equating their age with the age of the oldest known fossil, with the age of strata the taxa are endemic to, and with the age of paleogeographic events. All three methods have been adopted as methods of dating nodes in molecular phylogenies. The first method has been the most popular, but both this and the second method involve serious difficulties. Studies often, correctly, introduce oldest known fossils as providing minimum ages for divergences. However, in the actual analyses these ages, and ages derived from them, are often treated as absolute ages and earlier geological events are deemed irrelevant to the phylogeny. In fact, only younger geological events can be irrelevant. Studies correlating the age of nodes with age of volcanic islands often overlook the fact that these islands have been produced at subduction zones or hot spots where small, individually ephemeral islands are constantly being produced and disappearing, and a metapopulation can survive indefinitely. Correlating the age of taxa with that of associated paleogeographic events is probably the most promising method but has often been used in a simplistic way, for example in assuming that all divergence across the Isthmus of Panama dates to its final rise. Most workers now agree that a global molecular clock does not exist, and that rates can change between lineages and within a lineage over time. New methods of estimating branch lengths do not assume a strict clock, but the number of models for molecular evolution is then effectively infinite. Problems with calibrating the nodes, as well as with substitution models, mean that phylogeography's claim to be able to test between vicariance and dispersal is not justified.
将其年龄与已知最古老化石的年龄、该分类群所特有的地层年龄以及古地理事件的年龄等同起来。这三种方法都已被用作分子系统发育中节点的年代测定方法。第一种方法最为流行，但这两种方法都存在严重困难。研究通常正确地引入已知最古老的化石来为分歧提供最小年龄。然而，在实际分析中，这些年龄以及从中得出的年龄往往被视为绝对年龄，而早期地质事件被认为与系统发育无关。实际上，只有较年轻的地质事件才可能无关。将节点年龄与火山岛年龄相关联的研究常常忽略了这样一个事实，即这些岛屿是在俯冲带或热点形成的，在那里小的、单个短暂存在的岛屿不断产生和消失，而一个复合种群可以无限期地生存。将分类群的年龄与相关古地理事件的年龄相关联可能是最有前途的方法，但常常被简单化地使用，例如假设巴拿马地峡两侧的所有分歧都可追溯到其最终隆起之时。现在大多数研究人员都认为不存在全球分子钟，而且进化速率在不同谱系之间以及同一谱系内部都会随时间变化。估计分支长度的新方法并不假定有严格的分子钟，但分子进化的模型数量实际上是无限的。校准节点以及替代模型存在的问题意味着系统地理学声称能够在隔离分化和扩散之间进行检验是没有依据的。
