I. Edge lengths are known. Distances between species can be immediately deduced: the distance between two species corresponds to the sum of the length of edges connecting them. The "bananaquit bird" dataset offers a good illustration of that case (section 3.1). II. Edge lengths are unknown (the GenBank taxonomy dataset: section 3.2). We then have to (arbitrary) choose the length of edges. Many solutions can be considered. One may assign the same length for all edges. However, such a choice is inappropriate when the tree depth depends on the level of refinement along branches. Such a situation is rather common in taxonomy: the Eukaryote classification is actually more refined than that of Bacteria and Achea (Fig. 2 ). If the length of edges is set constant along the tree, humans appear closer to some bacteria (which are microscopic prokaryotes) than to octopus (which is also an animal). The toy example presented in Figure 3 , left insert (a) provides a good illustration of this bias.