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Long branch attraction
In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lineage is sufficient to cause that lineage to appear similar (thus closely related) to another long-branched lineage, solely because they have both undergone a large amount of change, rather than because they are related by descent. Such bias is more common when the overall divergence of some taxa results in long branches within a phylogeny. Long-branches are often attracted to the base of a phylogenetic tree, because the lineage included to represent an outgroup is often also long-branched. The frequency of true LBA is unclear and often debated.〔Anderson, F. E., & Swofford, D. L. (2004). Should we be worried about long-branch attraction in real data sets? Investigations using metazoan 18S rDNA. Molecular phylogenetics and evolution, 33(2), 440-451.〕〔Huelsenbeck, J. P. (1997). Is the Felsenstein zone a fly trap?. Systematic Biology, 46(1), 69-74.〕 Although often viewed as a failing of parsimony-based methodology, LBA can result from a variety of scenarios and be inferred under multiple analysis paradigms.
==Causes==
LBA was first recognized as problematic when analyzing discrete morpological character sets under parsimony criteria, however Maximum Likelihood analyses of DNA or protein sequences are also susceptible. A simple hypothetical example can be found in Felsenstein 1978 where it is demonstrated that for certain unknown "true" trees, some methods can show bias for grouping long branches, ultimately resulting in the inference of a false sister relationship.〔Felsenstein, J. (1978). Cases in which parsimony or compatibility methods will be positively misleading. Systematic Biology, 27(4), 401-410.〕 Often this is because convergent evolution of one or more characters included in the analysis has occurred in multiple taxa. Although they were derived independently, these shared traits can be misinterpreted in the analysis as being shared due to common ancestry.
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).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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