Data from: Within- and among-genus components of size evolution during mass extinction, recovery, and background intervals: a case study of Late Permian through Late Triassic foraminifera

One of the best-recognized patterns in the evolution of organismal size is the tendency for mean and maximum size within a clade to decrease following a major extinction event and to increase during the subsequent recovery interval. Because larger organisms are typically thought to be at higher extinction risk than their smaller relatives, it has commonly been assumed that size reduction mostly reflects the selective extinction of larger species. However, to our knowledge the relative importance of within- and among-lineage processes in driving overall trends in body size has never been compared quantitatively. In this study, we use a global, specimen-level database of foraminifera to study size evolution from the Late Permian through Late Triassic. We explicitly decompose size evolution into within- and among-genus components. We find that size reduction following the end- Permian mass extinction was driven more by size reduction within surviving species and genera than by the selective extinction of larger taxa. Similarly, we find that increase in mean size across taxa during Early Triassic biotic recovery was a product primarily of size increase within survivors and the extinction of unusually small taxa, rather than the origination of new, larger taxa. During background intervals we find no strong or consistent tendency for extinction, origination, or within- lineage change to move the overall size distribution toward larger or smaller sizes. Thus, size stasis during background intervals appears to result from small and inconsistent effects of within- and among-lineage processes rather than from large but offsetting effects of within- and among-taxon components. These observations are compatible with existing data for other taxa and extinction events, implying that mass extinctions do not influence size evolution by simply selecting against larger organisms. Instead, they appear to create conditions favorable to smaller organisms.生物体型演化领域最具代表性的模式之一，是某一演化支（clade）内的平均体型与最大体型往往在重大灭绝事件后出现下降，并在后续的生物复苏期内逐步回升。由于学界普遍认为体型较大的生物相较于小型同类面临更高的灭绝风险，因此此前广泛假设，体型缩减主要源于大型物种的选择性灭绝。然而据我们所知，支系内部与支系间的过程在驱动整体体型变化趋势中的相对重要性，此前从未得到定量对比。本研究依托涵盖全球范围的有孔虫（Foraminifera）标本级数据库，探究晚二叠世（Late Permian）至晚三叠世（Late Triassic）的体型演化历程。我们明确将体型演化拆解为属内与属间两个组分。研究结果显示，二叠纪末大灭绝（end-Permian mass extinction）后的体型缩减，更多是由幸存物种与属内的体型减小所驱动，而非大型类群的选择性灭绝。同理，早三叠世（Early Triassic）生物复苏期间，各类群平均体型的提升主要源于幸存者的体型增大以及异常小型类群的灭绝，而非新诞生的大型新类群。在背景灭绝间期，我们未观测到灭绝、类群起源或支系内变化会持续推动整体体型分布向更大或更小体型方向发展的显著趋势。因此，背景间期的体型停滞现象，似乎源于支系内与支系间过程的微弱且不一致的影响，而非类群内与类群间组分存在强大但相互抵消的效应。上述观测结果与其他类群及灭绝事件的现有数据相符，这表明大灭绝并非仅通过对大型生物的选择来作用于体型演化。相反，大灭绝似乎为小型生物创造了更适宜的生存条件。