title: Heterotachy and long-branch attraction in phylogenetics.
abstract: BACKGROUND
  Probabilistic methods have progressively supplanted the Maximum Parsimony (MP) method for inferring phylogenetic trees. One of the major reasons for this shift was that MP is much more sensitive to the Long Branch Attraction (LBA) artefact than is Maximum Likelihood (ML). However, recent work by Kolaczkowski and Thornton suggested, on the basis of simulations, that MP is less sensitive than ML to tree reconstruction artefacts generated by heterotachy, a phenomenon that corresponds to shifts in site-specific evolutionary rates over time. These results led these authors to recommend that the results of ML and MP analyses should be both reported and interpreted with the same caution. This specific conclusion revived the debate on the choice of the most accurate phylogenetic method for analysing real data in which various types of heterogeneities occur. However, variation of evolutionary rates across species was not explicitly incorporated in the original study of Kolaczkowski and Thornton, and in most of the subsequent heterotachous simulations published to date, where all terminal branch lengths were kept equal, an assumption that is biologically unrealistic.
  
  
  RESULTS
  In this report, we performed more realistic simulations to evaluate the relative performance of MP and ML methods when two kinds of heterogeneities are considered: (i) within-site rate variation (heterotachy), and (ii) rate variation across lineages. Using a similar protocol as Kolaczkowski and Thornton to generate heterotachous datasets, we found that heterotachy, which constitutes a serious violation of existing models, decreases the accuracy of ML whatever the level of rate variation across lineages. In contrast, the accuracy of MP can either increase or decrease when the level of heterotachy increases, depending on the relative branch lengths. This result demonstrates that MP is not insensitive to heterotachy, contrary to the report of Kolaczkowski and Thornton. Finally, in the case of LBA (i.e. when two non-sister lineages evolved faster than the others), ML outperforms MP over a wide range of conditions, except for unrealistic levels of heterotachy.
  
  
  CONCLUSIONS
  For realistic combinations of both heterotachy and variation of evolutionary rates across lineages, ML is always more accurate than MP. Therefore, ML should be preferred over MP for analysing real data, all the more so since parametric methods also allow one to handle other types of biological heterogeneities much better, such as among sites rate variation. The confounding effects of heterotachy on tree reconstruction methods do exist, but can be eschewed by the development of mixture models in a probabilistic framework, as proposed by Kolaczkowski and Thornton themselves.
authors.affiliation: Canadian Institute for Advanced Research, Centre Robert-Cedergren, Département de Biochimie, Université de Montréal, Succursale Centre-Ville, Montréal, Québec H3C3J7, Canada. herve.philippe@umontreal.ca
authors.forename: Hervé
authors.initials: H
authors.lastname: Philippe
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhou
authors.forename: Henner
authors.initials: H
authors.lastname: Brinkmann
authors.forename: Nicolas
authors.initials: N
authors.lastname: Rodrigue
authors.forename: Frédéric
authors.initials: F
authors.lastname: Delsuc
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D019295:Computational Biology; D003198:Computer Simulation; D004272:DNA, Mitochondrial; D019143:Evolution, Molecular; D005821:Genetic Techniques; D014644:Genetic Variation; D023281:Genomics / Q000379:methods*; D016013:Likelihood Functions; D008962:Models, Theoretical; D010802:Phylogeny; D011336:Probability
chemical_list: D004272:DNA, Mitochondrial