Phylogeny Practice Test Practice

Neighbor-joining is a distance-based method for building a phylogenetic tree from a matrix of pairwise distances among taxa. At each step, it identifies a pair of taxa or groups that can be joined so that the overall length of the tree is minimized, then collapses that pair into a single node and recomputes distances to all other taxa. This process is repeated until only two nodes remain, yielding an unrooted tree.

The key assumption is that the observed distances are additive along a tree: the distance between any two taxa equals the sum of the branch lengths along the path connecting them. If the distance data are reasonably close to additive, neighbor-joining tends to recover a sensible topology and reasonable branch lengths, even when different lineages evolve at different rates. It does not require a molecular clock or a specific model of sequence evolution, unlike character-based maximum likelihood or Bayesian methods or parsimony approaches. Since the method produces an unrooted tree, rooting the final result requires additional information, such as an outgroup or a chosen rooting method.