Long branch attraction
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Long branch attraction (LBA) causes species to seem more closely related in a phylogeny than they really are due to mutations or traits occurring independently (convergent evolution). These shared traits can be misinterpreted as being shared due to common ancestry. This occurs more often in long branches of a phylogeny. Until recently, long branch attraction was considered hypothetical due to insufficient evidence. However, today many different factors are taken into account in confirmation of the distance between two species (Bergsten 2005).
In phylogenetic and clustering analyses, LBA 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 exhibit the same states as those on another branch (homoplasy). A phylogenetic analysis will group these taxa together as a clade unless other synapomorphies outweigh the homoplastic features to group together true sister taxa.
These problems 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).
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.
The recognition of long-branch attraction implies that there is some other evidence that suggests that the phylogeny is incorrect. For example morphological data may suggest that taxa marked as closely related are not truly sister taxa. 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).
- 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.
- Bergsten J. (2005): "A review of long-branch attraction". Blackwell Publishing [cited 2014 Oct 1] 21(2):163-193. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1096-0031.2005.00059.x/pdf