 In the previous sections we treated phylogenetic trees as straight-forwardly bifurcating models, regularly referring back to  Figure 1.  Branches represent ancestral lineages, and nodes represent lineage splits. Yet, this view of phylogenetics is also highly idealized, reflecting at least three simplifying assumptions: (1) that lineages split but do not fuse; (2) that changes on one lineage do not affect other lineages, and (3) that splits are singular, simple events. Each of these assumptions turn out to be false.