Hedges S B, Parker P H, Sibley C G, Kumar S
Department of Biology and Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park, 16802, USA.
Nature. 1996 May 16;381(6579):226-9. doi: 10.1038/381226a0.
The classical hypothesis for the diversification of birds and mammals proposes that most of the orders diverged rapidly in adaptive radiations after the Cretaceous/Tertiary (K/T) extinction event 65 million years ago. Evidence is provided by the near-absence of fossils representing modern orders before the K/T boundary. However, fossil-based estimates of divergence time are known to be conservative because of sampling biases, and some molecular/time estimates point to earlier divergences among orders. In an attempt to resolve this controversy, we have estimated times of divergence among avian and mammalian orders with a comprehensive set of genes that exhibit a constant rate of substitution. Here we report molecular estimates of divergence times that average about 50-90% earlier than those predicted by the classical hypothesis, and show that the timing of these divergences coincides with the Mesozoic fragmentation of emergent land areas. This suggests that continental breakup may have been an important mechanism in the ordinal diversification of birds and mammals.
关于鸟类和哺乳动物多样化的经典假说提出，在6500万年前的白垩纪/第三纪（K/T）灭绝事件之后，大多数目在适应性辐射中迅速分化。K/T边界之前几乎没有代表现代目的化石，这为该假说提供了证据。然而，由于采样偏差，基于化石的分化时间估计已知是保守的，并且一些分子/时间估计表明目之间的分化更早。为了解决这一争议，我们用一组表现出恒定替代率的综合基因估计了鸟类和哺乳动物目之间的分化时间。在此，我们报告了分化时间的分子估计，其平均比经典假说预测的时间早约50 - 90%，并表明这些分化的时间与中生代新兴陆地区域的分裂相吻合。这表明大陆分裂可能是鸟类和哺乳动物目级多样化的一个重要机制。
