Long branch attraction
||This article provides insufficient context for those unfamiliar with the subject. (October 2009)|
Long branch attraction (LBA), a phenomenon in phylogenetic and clustering analyses, occurs if terminals or clades that are thought not to be one another's closest relatives cluster together due to misinterpretation of shared homoplasies, usually in DNA sequence data. It is a result of the way clustering algorithms work: terminals or taxa with many autapomorphies (character states unique to a single branch) may by chance (convergence) exhibit the same states as those on another branch. A phylogenetic analysis that does not down-weight these apparent similarities will group these taxa together as a clade if there are not other synapomorphies that outweigh these homoplastic features and group these taxa with their true sister taxa.
The result of LBA in evolutionary analyses is that rapidly evolving lineages may be inferred to be closely related, regardless of their true relationships. 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, parsimony 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.
These problem may be minimized by using methods that correct for multiple substitutions at the same site, by adding taxa related to those with the long branches that add additional true synapomorphies to the data, or by using alternative slower evolving traits (e. g., more conservative gene regions).
Of course, the recognition of an instance of long-branch attraction implies that there is some other evidence - often morphological - that suggests that the taxa so united are not, in fact, sister taxa. Otherwise, as per Hennig's Auxiliary Principle, putative synapomorphies should be viewed as de facto evidence of grouping (Hennig, 1966; Schuh and Brower, 2009).
- Bergsten, J. (2005): A review of long-branch attraction. Cladistics 21(2): 163-193. PDF fulltext
- Felsenstein, J. (2004): Inferring Phylogenies. Sinauer Associates, Sunderland, MA.
- Hennig, W. (1966): Phylogenetic Systematics. University of Illinois Press, Urbana, IL.
- Schuh, R. T. and Brower, A. V. Z. (2009): Biological Systematics: Principles and Applications, (2nd edn.) Cornell University Press, Ithaca, NY.