Data from: Multiple morphological clocks and total-evidence tip-dating in mammals

Morphological integration predicts that correlated characters will coevolve; thus, each distinct suite of correlated characters might be expected to evolve according to a separate clock or ‘pacemaker’. Characters in a large morphological dataset for mammals were found to be evolving according to seven separate clocks, each distinct from the molecular clock. Total-evidence tip-dating using these multiple clocks inflated divergence time estimates, but potentially improved topological inference. In particular, single-clock analyses placed several meridiungulates and condylarths in a heterodox position as stem placentals, but multi-clock analyses retrieved a more plausible and orthodox position within crown placentals. Several shortcomings (including uneven character sampling) currently impact upon the accuracy of total-evidence dating, but this study suggests that when sufficiently large and appropriately constructed phenotypic datasets become more commonplace, multi-clock approaches are feasible and can affect both divergence dates and phylogenetic relationships.

形态整合（Morphological integration）理论指出，相关联的性状会发生协同演化；据此推测，每一组独特的相关性状组合或许都应按照独立的演化钟或‘节律器’完成演化。针对哺乳动物的大型形态学数据集的研究显示，其中的各类性状按照七套独立的演化钟进行演化，且每套演化钟均独立于分子钟（molecular clock）。采用这些多套演化钟的总证据末端定年（total-evidence tip-dating）分析方法，虽高估了类群分化时间的估算结果，但有望提升系统发育拓扑结构的推断精度。具体而言，单演化钟分析将若干南方有蹄类（meridiungulates）与踝节目类（condylarths）置于非主流的系统位置，即归类为胎盘类的干群；而多演化钟分析则将其恢复为冠群胎盘类中更合理且符合学界共识的演化位置。当前，总证据定年（total-evidence dating）方法的准确性仍受诸多缺陷制约，其中包括性状采样不均衡等问题。但本研究表明，当足够大型且构建规范的表型数据集愈发普及时，多演化钟分析方法将具备可行性，且能够同时对类群分化时间估算结果与系统发育关系产生影响。