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In phylogenetics, maximum parsimony is an optimality criterion under which the phylogenetic tree that minimizes the total number of character-state changes is to be preferred. Under the maximum-parsimony criterion, the optimal tree will minimize the amount of homoplasy (i.e., convergent evolution, parallel evolution, and evolutionary reversals). In other words, under this criterion, the shortest possible tree that explains the data is considered best. The principle is akin to Occam's razor, which states that—all else being equal—the simplest hypothesis that explains the data should be selected. Some of the basic ideas behind maximum parsimony were presented by James S. Farris 〔Farris, J. S. (1970). Methods for computing Wagner trees. ''Systematic Zoology'' 19, 83-92.〕 in 1970 and Walter M. Fitch in 1971.〔Fitch, W. M. (1971). Toward defining the course of evolution: minimum change for a specified tree topology. ''Systematic Zoology'' 20 (4), 406-416〕 Maximum parsimony is an intuitive and simple criterion, and it is popular for this reason. However, although it is easy to ''score'' a phylogenetic tree (by counting the number of character-state changes), there is no algorithm to quickly ''generate'' the most-parsimonious tree. Instead, the most-parsimonious tree must be found in "tree space" (i.e., amongst all possible trees). For a small number of taxa (i.e., fewer than nine) it is possible to do an exhaustive search, in which every possible tree is scored, and the best one is selected. For nine to twenty taxa, it will generally be preferable to use branch-and-bound, which is also guaranteed to return the best tree. For greater numbers of taxa, a heuristic search must be performed. Because the most-parsimonious tree is always the shortest possible tree, this means that—in comparison to the "true" tree that actually describes the evolutionary history of the organisms under study—the "best" tree according to the maximum-parsimony criterion will often underestimate the actual evolutionary change that has occurred. In addition, maximum parsimony is not statistically consistent. That is, it is not guaranteed to produce the true tree with high probability, given sufficient data. As demonstrated in 1978 by Joe Felsenstein, maximum parsimony can be inconsistent under certain conditions, such as long-branch attraction. ==Alternate characterization and rationale== The maximization of parsimony (preferring the simpler of two otherwise equally adequate theorizations) has proven useful in many fields. Occam's razor, a principle of theoretical parsimony suggested by William of Ockham in the 1320s, asserted that it is vain to give an explanation which involves more assumptions than necessary. Alternatively, phylogenetic parsimony can be characterized as favoring the trees that maximize explanatory power by minimizing the number of observed similarities that cannot be explained by inheritance and common descent.〔Farris, J. S. (1983). The logical basis of phylogenetic analysis. In ''Advances in Cladistics Vol. 2'' (eds. N. I. Platnick, and V. A. Funk), pp. 7-36. Columbia University Press, New York, New York.〕〔Farris, J. S. (2008). Parsimony and explanatory power. ''Cladistics'' 24, 1-23.〕 Minimization of required evolutionary change on the one hand and maximization of observed similarities that can be explained as homology on the other may result in different preferred trees when some observed features are not applicable in some groups that are included in the tree, and the latter can be seen as the more general approach.〔De Laet J. (2005). Parsimony and the problem of inapplicables in sequence data. Pp. 81-116 in Albert, V.A. (ed.) ''Parsimony, phylogeny and genomics.'' Oxford University Press, ISBN 0-19-856493-7〕 While evolution is not an inherently parsimonious process, centuries of scientific experience lend support to the aforementioned principle of parsimony (Occam's razor). Namely, the supposition of a simpler, more parsimonious chain of events is preferable to the supposition of a more complicated, less parsimonious chain of events. Hence, parsimony (''sensu lato'') is typically sought in constructing phylogenetic trees, and in scientific explanation generally.〔Jaynes, E.T. (2003) in Bretthorst, G.L. (Ed.), ''Probability theory: the logic of science.'' Cambridge, UK: Cambridge University Press.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Maximum parsimony (phylogenetics)」の詳細全文を読む スポンサード リンク
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