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Fig. 7 | Evolution: Education and Outreach

Fig. 7

From: Understanding Evolutionary Trees

Fig. 7

Unbalanced trees can be ladderized or nonladderized without changing the topology. Many evolutionary trees are “unbalanced,” meaning that not all of the sister groups contain the same number of species. This can be due to real differences in diversity among groups or to incomplete sampling in which not all contemporary species are included in the tree. Figure 3b shows a balanced tree, but the trees depicted here are unbalanced because the major branches do not contain an equal number of species (i.e., one branch from the root contains only one species, H, whereas the other branch includes species G, F, E, D, C, B, and A). In a and b, the trees are “ladderized,” which means that they are presented with the most diverse sister group on the same side of every internal node. c shows the same tree, with an identical topology, in a nonladderized format. This was done simply by rotating several nodes (Fig. 6) so that more diverse groups do not always appear to the right (a) or left (b) of the internal nodes. Although ladderized trees appear less cluttered, they are no more accurate than nonladderized ones, and in fact they may cause readers to falsely interpret the information provided in the tree (Figs. 11 and 14; see also Crisp and Cook 2005)

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