#### 12.3.1 Neighbor-joining

In this method, neighbors are deﬁned as a pair of leaves with one node connecting them. The principle of this method is to ﬁnd pairs of leaves that minimize the total branch length at each stage of clustering, starting with a star-like tree. The branch lengths and an unrooted tree topology can quickly be obtained by using this method without assuming a molecular clock (see Saitou & Nei 1987).