Figure 2

The NNI adjacency graph. (a) The tree $\overline{G}$ is obtained from G by pruning and regrafting subtree G _v to the root of G. The vertex x ∈ V(G) is suppressed, and the new vertex above root in $\overline{G}$ is named x. (b) Two NNI operations NNI _G' (z') and NNI _G' (z) produce left-child G' _l and right-child G' _r of G' in $\mathcal{ X }$.