Previous estimates of nucleotide substitution rates are routinely applied as secondary or “universal” molecular clock calibrations for estimating evolutionary timescales in groups that lack independent timing information. A major limitation of this approach is that rates can vary considerably among taxonomic groups, but the assumption of rate constancy is rarely evaluated prior to using secondary rate calibrations. Here I evaluate whether an insect mitochondrial DNA clock is appropriate for estimating timescales in Collembola—a group of insect-like arthropods characterized by high levels of cryptic diversity. Relative rates of substitution in cytochrome oxidase subunit 1 (COI) were inferred via Bayesian analysis across a topologically constrained Hexapod phylogeny using a relaxed molecular clock model. Rates for Collembola did not differ significantly from the average rate or from the rates estimated for most other groups (25 of 30), suggesting that (1) their apparent cryptic diversity cannot be explained by accelerated rates of molecular evolution and (2) clocks calibrated using “universal” insect rates may be appropriate for estimating evolutionary timescales in this group. However, of the 31 groups investigated, 10 had rates that deviated significantly from the average (6 higher, 4 lower), underscoring the need for caution and careful consideration when applying secondary insect rate calibrations. Lastly, this study exemplifies a relatively simple approach for evaluating rate constancy within a taxonomic group to determine whether the use of secondary rates are appropriate for molecular clock calibrations.

中文翻译：

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在没有独立时间信息的情况下推断进化时间尺度：评估“通用”昆虫率以校准弹尾目（六足动物）分子钟

以前对核苷酸取代率的估计通常用作二级或“通用”分子钟校准，用于估计缺乏独立时间信息的群体的进化时间尺度。这种方法的一个主要限制是分类群之间的比率可能会有很大差异，但在使用二级比率校准之前很少评估比率恒定性的假设。在这里，我评估昆虫线粒体 DNA 时钟是否适合估计弹尾虫的时间尺度——弹尾目是一组以高度隐秘多样性为特征的类昆虫节肢动物。细胞色素氧化酶亚基 1 (COI) 的相对取代率是通过贝叶斯分析推断的，使用松弛的分子钟模型对拓扑约束的六足动物系统发育进行了推断。Collembola 的比率与平均比率或大多数其他群体的估计比率（30 个中的 25 个）没有显着差异，这表明 (1) 它们明显的神秘多样性不能用加速的分子进化速度来解释和 (2) 校准时钟使用“通用”昆虫率可能适合估计该组的进化时间尺度。然而，在所调查的 31 个组中，有 10 个组的比率显着偏离平均值（6 个较高，4 个较低），强调在应用二级昆虫比率校准时需要谨慎和仔细考虑。最后，这项研究举例说明了一种相对简单的方法，用于评估分类群内的速率稳定性，以确定二级速率的使用是否适合分子钟校准。