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.

生命之树各谱系间的物种丰富度差异显著，某些情况下甚至超过百万倍（例如，扁形动物与节肢动物）。关于谱系间丰富度差异的两大主要解释为谱系年龄假说（即物种丰富的谱系更为古老）与物种分化率假说（即物种丰富的谱系分化速度更快，其中物种分化率是物种形成与灭绝在时间上的净平衡）。本研究旨在探究生命之树中物种分化率的变化模式。我们探讨了分化率在不同高级分类群中的变化，以及高级分类群内部的分化率是否与内部的亚谱系相关联，以及谱系的分化率与物种丰富度之间的关系。研究发现，分化率存在显著差异，植物中的分化率几乎是动物的近两倍，而真核生物的分化率约为原核生物的十倍。每个界级谱系的分化率与其内部的亚谱系之间存在显著相关性。尽管在解读分化率与丰富度之间的关系时需谨慎，但两者之间的正向关系并非必然。我们发现，分化率的变异性似乎解释了生命之树中各谱系间丰富度差异的大部分，这与先前研究的结论形成对比。