Nunn G B, Stanley S E
Department of Ornithology, American Museum of Natural History, New York, New York, USA.
Mol Biol Evol. 1998 Oct;15(10):1360-71. doi: 10.1093/oxfordjournals.molbev.a025864.
Variation in rates of molecular evolution now appears to be widespread. The demonstration that body size is correlated with rates of molecular evolution suggests that physiological and ecological factors may be involved in molecular rate variation, but large-scale comparative studies are still lacking. Here, we use complete cytochrome b sequences from 85 species of tube-nosed seabirds (order Procellariiformes) and 5 outgroup species of penguins (order Sphenisciformes) to test for an association between body mass and rates of molecular evolution within the former avian order. Cladistic analysis of the 90 sequences estimates a phylogeny largely consistent with the traditional taxonomy of the Procellariiformes. The Diomedeidae, Procellariidae, and Pelecanoididae are monophyletic, while the Hydrobatidae are basal and paraphyletic. However, the two subfamilies within the Hydrobatidae (Hydrobatinae and Oceanitinae) are monophyletic. A likelihood ratio test detects significant deviation from clocklike evolution in our data. Using a sign test for an association between body mass and branch length in the seabird phylogeny, we find that larger taxa tend to have shorter terminal branch lengths than smaller taxa. This observation suggests that rates of mitochondrial DNA evolution are slower for larger taxa. Rate calibrations based on the fossil record reveal concordant body size effects. We interpret these results as evidence for a metabolic rate effect, as the species in this order exhibit large differences in metabolic rates, which are known to be highly correlated with body mass in this group. Our results support previous findings of body size effects and show that this effect can be significant even within a single avian order. This suggests that even lineage-specific molecular clocks may not be tenable if calibrations involve taxa with different metabolic rates.
分子进化速率的差异如今看来十分普遍。体型与分子进化速率相关这一事实表明，生理和生态因素可能与分子速率变化有关，但仍缺乏大规模的比较研究。在此，我们使用了85种管鼻海鸟（鹱形目）和5种企鹅外类群物种（企鹅目）的完整细胞色素b序列，来检验前一个鸟类目中体重与分子进化速率之间的关联。对这90个序列进行的分支系统分析估计出的系统发育关系，在很大程度上与鹱形目的传统分类法一致。信天翁科、鹱科和水薙鸟科是单系的，而海燕科是基部类群且并系。然而，海燕科内的两个亚科（海燕亚科和大洋海燕亚科）是单系的。似然比检验在我们的数据中检测到显著偏离钟形进化的情况。通过对海鸟系统发育中体重与分支长度之间的关联进行符号检验，我们发现较大的分类单元往往比较小的分类单元具有更短的末端分支长度。这一观察结果表明，较大分类单元的线粒体DNA进化速率较慢。基于化石记录的速率校准揭示了一致的体型效应。我们将这些结果解释为代谢率效应的证据，因为这个目中的物种在代谢率上表现出很大差异，而在这个类群中已知代谢率与体重高度相关。我们的结果支持了先前关于体型效应的研究发现，并表明即使在单个鸟类目中这种效应也可能很显著。这表明，如果校准涉及具有不同代谢率 的分类单元，那么即使是谱系特异性分子钟也可能站不住脚。
