 The final simplifying assumption treats lineage splits as singular events that wholly separate a lineage into two distinct new branches that we can cleanly track. When we think about the process of speciation, it is clear that speciation and lineage separation more generally is often not instantaneous (de Queiroz 1998; Harrison 1998). What is less clear is how this matters to phylogenetics. Thinking carefully about lineage splits and the internal structure of lineages forces us to examine the nature and relationships of the multilevel organization of the lineages we track (Haber 2012, 2019). More inclusive lineages contain less-inclusive lineages as parts, reflecting the hierarchical organization found in biology–a feature of lineages well appreciated by early phylogeneticists, e.g., Hennig 1966; Griffiths 1974). Species trees, for example, contain gene trees within their branches (Maddison 1997).