For example, in DNA sequence-based analyses, the problem arises when sequences from two (or more) lineages evolve rapidly. There are only four possible nucleotides and when DNA substitution rates are high, the probability that two lineages will evolve the same nucleotide at the same site increases. When this happens, a phylogenetic analysis may erroneously interpret this homoplasy as a synapomorphy (i.e., evolving once in the common ancestor of the two lineages). The opposite effect may also be observed, in that if two (or more) branches exhibit particularly slow evolution among a wider, fast evolving group, those branches may be misinterpreted as closely related. As such, "long branch attraction" can in some ways be better expressed as "branch length attraction". However, it is typically long branches that exhibit attraction. The recognition of long-branch attraction implies that there is some other evidence that suggests that the phylogeny is incorrect. For example, two different sources of data (i.e. molecular and morphological) or even different methods or partition schemes might support different placement for the long-branched groups. Hennig's Auxiliary Principle suggests that synapomorphies should be viewed as de facto evidence of grouping unless there is specific contrary evidence (Hennig, 1966; Schuh and Brower, 2009). A simple and effective method for determining whether or not long branch attraction is affecting tree topology is the SAW method, named for Siddal and Whiting. If long branch attraction is suspected between a pair of taxa (A and B), simply remove taxon A ("saw" off the branch) and re-run the analysis. Then remove B and replace A, running the analysis again. If either of the taxa appears at a different branch point in the absence of the other, there is evidence of long branch attraction. Since long branches can't possibly attract one another when only one is in the analysis, consistent taxon placement between treatments would indicate long branch attraction is not a problem. Assume for simplicity that we are considering a single binary character (it can either be + or -) distributed on the unrooted "true tree" with branch lengths proportional to amount of character state change, shown in the figure. Because the evolutionary distance from B to D is small, we assume that in the vast majority of all cases, B and D will exhibit the same character state. Here, we will assume that they are both + (+ and - are assigned arbitrarily and swapping them is only a matter of definition).