
636 CHAPTER 20. BIOLOGICAL NETWORKS
the real evolutionary development assuming that identical characteristics of di ffe r ent taxons
indicate a common ancestry.
A phylogenetic tree (in literature also called evolutionary tree) T = (V, E, δ) is a tree
consisting of nodes V (taxons) and edges E (links ). Leave n odes, i.e., nodes with exactly one
link, represent species, sequences , or similar entities; they are called operational taxonomical
units (and are repr es e nted by A ⊆ V ). Internal nodes represent (hypoth e tic al) ancestors
generated from phylogenetic analysis; they are called hypothetical taxonomic units. The
lengths of the edges δ : E →