Nearest neighbor interchange (NNI) 


Nearest neighbor interchange (NNI) search for term

Nearest Neighbor Interchange is a tree topology search strategy which attempts to improve the likelihood of a given tree by performing the following operations on the tree. Each internal branch branch of a binary unrooted tree has 4 subtrees connected" to it (a subtree may consist of a single node). NNI exchanges those subtrees to obtain a new tree. There are only two exchanges which lead to new unrooted binary trees. The procedure is usually repeated for each internal branch, until no further likelihood improvements can be obtained.
A binary unrooted tree with N>2 leaves will have N-3 internal branches, thus a pass of the NNI algorithm\, which tries two trees per internal branch will examine 2(N-3) new trees. (http://www.hyphy.org/docs/analyses/methods/nni.html )