Data from: Age-dependent and lineage-dependent speciation and extinction in the imbalance of phylogenetic trees

It is known that phylogenetic trees are more imbalanced than expected from a birth–death model with constant rates of speciation and extinction, and also that imbalance can be better fit by allowing the rate of speciation to decrease as the age of the parent species increases. If imbalance is measured in more detail, at nodes within trees as a function of the number of species descended from the nodes, age-dependent models predict levels of imbalance comparable to real trees for small numbers of descendent species, but predicted imbalance approaches an asymptote not found in real trees as the number of descendent species becomes large. Age-dependence must therefore be complemented by another process such as inheritance of different rates along different lineages, which is known to predict insufficient imbalance at nodes with few descendent species, but can predict increasing imbalance with increasing numbers of descendent species.

已知系统发育树（phylogenetic trees）相较于恒定物种形成与灭绝速率的生灭模型（birth–death model）所预期的结果更为不平衡，且若允许物种形成速率随亲本物种的年龄增长而降低，则可更好地拟合该不平衡特性。若以更精细化的方式度量不平衡性——即依据树内各节点所衍生的后裔物种数量作为函数进行评估——年龄依赖模型（age-dependent models）在后裔物种数量较少时，可预测出与真实系统发育树相近的不平衡性水平；但当后裔物种数量增多时，其预测的不平衡性会趋近于一条真实树中不存在的渐近线。因此，年龄依赖模型需辅以另一类演化过程，例如不同谱系间速率差异的遗传（inheritance of different rates along different lineages）：这类过程已知会在后裔物种较少的节点处预测出不足的不平衡性，但可随后裔物种数量的增加，预测出不断升高的不平衡性。