Fig. 11

The order of terminal nodes is meaningless. One of the most common misconceptions about evolutionary trees is that the order of the terminal nodes provides information about their relatedness. Only branching order (i.e., the sequence of internal nodes) provides this information; because all internal nodes can be rotated without affecting the topology (Fig. 6), the order of the tips is meaningless. Nevertheless, there is a strong tendency for readers to take the tree in a as indicating that frogs are more closely related to fishes than humans are. They are not: both frogs and humans (and birds and lizards and cats) are equally closely related to fishes because as tetrapods they share a common ancestor to the exclusion of bony fishes. On the other hand, humans and cats are more closely related to each other than either is to any of the other species depicted because they share a recent common ancestor to the exclusion of the other species. The tree in b exhibits an identical topology to the one in a and is therefore equally valid. In this case, the same misinterpretation of “reading across the tips” would lead to the erroneous conclusion that birds are more closely related to fishes than cats are or that humans are more closely related to frogs than to lizards and birds. Because they share a common ancestor as amniotes, birds, cats, lizards, and humans are all equally related to frogs. It is good practice to rotate a few internal nodes mentally when first examining a tree to dispel misinterpretations based on reading the order of tips