SPR operation on a rooted phylogenetic tree. The tree in panel (a) is subjected to an SPR operation, with participants and direction indicated with the dashed arrow. Edge E4 is the donor edge, which is split by acquisition of the recipient edge E3. Since they are no longer split by E3, edges E0 and E2 are consolidated into a single edge, which implies the same split of taxa as E2. Splitting E4 yields a new parent edge E(3+4), and a child edge that implies the same split as E4. Finally, the bipartitions implied by every other edge up to the common ancestor of the donor and recipient edges (in this case, the root R) are modified by the swapping of subtree t from one partition to the other. Thus, in this case E1 now implies a different set of taxa and is renamed E(1+3). Edges that are not part of the donor/recipient pair or ancestral to these edges are not affected by the inferred transfer event.
Beiko and Hamilton BMC Evolutionary Biology 2006 6:15 doi:10.1186/1471-2148-6-15