combined ESM

Species richness varies dramatically among clades across the Tree of Life, by over a million-fold in some cases (e.g. placozoans versus arthropods). Two major explanations for differences in richness among clades are the clade-age hypothesis (i.e. species-rich clades are older) and the diversification-rate hypothesis (i.e. species-rich clades diversify more rapidly, where diversification rate is the net balance of speciation and extinction over time). Here, we examine patterns of variation in diversification rates across the Tree of Life. We address how rates vary across higher taxa, whether rates within higher taxa are related to the subclades within them, and how diversification rates of clades are related to their species richness. We find substantial variation in diversification rates, with rates in plants nearly twice as high as in animals, and rates in eukaryotes approximately 10-fold faster than prokaryotes. Rates for each kingdom-level clade are then significantly related to the subclades within them. Although caution is needed when interpreting relationships between diversification rates and richness, a positive relationship between the two is not inevitable. We find that variation in diversification rates seems to explain most variation in richness among clades across the Tree of Life, in contrast with the conclusions of previous studies.生命之树（Tree of Life）上各演化支（clade）的物种丰富度差异极大，部分类群间的差异幅度可超过百万倍，例如扁盘动物门（placozoans）与节肢动物门（arthropods）的对比。针对演化支间物种丰富度的差异，学界提出两种主流解释：支系年龄假说（clade-age hypothesis）与分化速率假说（diversification-rate hypothesis）；前者认为物种丰富的演化支形成时间更久，后者则指出物种丰富的演化支分化速率（diversification rate）更快——其中分化速率指物种形成与灭绝随时间推移的净平衡值。本研究旨在剖析生命之树各分支的分化速率变异模式，探讨三个核心问题：分化速率在高级分类单元间的变化规律、高级分类单元内的分化速率与其内部亚支系（subclades）的相关性，以及演化支的分化速率与其物种丰富度的关联方式。研究结果显示，分化速率存在显著变异：植物的分化速率近乎为动物的两倍，真核生物的分化速率约为原核生物的10倍。各界级演化支的分化速率均与其内部的亚支系存在显著相关性。尽管在解读分化速率与物种丰富度的关联时需保持谨慎，但二者并非必然呈现正相关关系。本研究发现，分化速率的变异似乎可以解释生命之树中各演化支间物种丰富度的绝大多数差异，这与既往研究的结论相悖。