  UPGMA (Unweighted Pair Group Method with Arithmetic Mean) is a widely used
clustering method. Here we show that UPGMA is a greedy heuristic for the
normalized equidistant minimum evolution (NEME) problem, that is, finding a
rooted tree that minimizes the minimum evolution score relative to the
dissimilarity matrix among all rooted trees with the same leaf-set in which all
leaves have the same distance to the root. We prove that the NEME problem is
NP-hard. In addition, we present some heuristic and approximation algorithms
for solving the NEME problem, including a polynomial time algorithm that yields
a binary, rooted tree whose NEME score is within O(log^2 n) of the optimum. We
expect that these results to eventually provide further insights into the
behavior of the UPGMA algorithm.
