Editing Distance matrix (section)

===== UPGMA =====
The basic principle of UPGMA (Unweighted Pair Group Method with Arithmetic Mean) is that similar species should be closer in the phylogenetic tree. Hence, it builds the tree by clustering similar sequences iteratively. The method works by building the phylogenetic tree bottom up from its leaves. Initially, we have {{Math|''n''}} leaves (or {{Math|''n''}} singleton trees), each representing a species in {{Math|''S''}}. Those {{Math|''n''}} leaves are referred as {{Math|''n''}} clusters. Then, we perform {{Math|''n''-1}} iterations. In each iteration, we identify two clusters {{Math|''C''<sub>1</sub>}} and {{Math|''C''<sub>2</sub>}} with the smallest average distance and merge them to form a bigger cluster {{Math|''C''}}. If we suppose {{Math|M}} is ultrametric, for any cluster {{Math|''C''}} created by the UPGMA algorithm, {{Math|''C''}} is a valid ultrametric tree.