Figure 2.

(a) A rooted, bifurcating phylogenetic tree T built on five taxa labeled with a, b, c, d and e. The internal nodes are shown with x₀, x₁, x₂ and x₃. (b) A clade of T rooted at x₁. (b) and (c) Two subtrees of T by contracting the taxa sets {d, e} and {a, e}.