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In biology, a substitution model describes the process from which a sequence of characters changes into another set of traits. For example, in cladistics, each position in the sequence might correspond to a property of a species which can either be present or absent. The alphabet could then consist of "0" for absence and "1" for presence. Then the sequence 00110 could mean, for example, that a species does not have feathers or lay eggs, does have fur, is warm-blooded, and cannot breathe underwater. Another sequence 11010 would mean that a species has feathers, lays eggs, does not have fur, is warm-blooded, and cannot breathe underwater. In phylogenetics, sequences are often obtained by firstly obtaining a nucleotide or protein sequence alignment, and then taking the bases or amino acids at corresponding positions in the alignment as the characters. Sequences achieved by this might look like AGCGGAGCTTA and GCCGTAGACGC. Substitution models are used for a number of things: # Constructing evolutionary trees in phylogenetics or cladistics. # Simulating sequences to test other methods and algorithms. == Neutral, independent, finite sites models == Most substitution models used to date are neutral, independent, finite sites models. ; Neutral : Selection does not operate on the substitutions, and so they are unconstrained. ; Independent : Changes in one site do not affect the probability of changes in another site. ; Finite Sites : There are finitely many sites, and so over evolution, a single site can be changed multiple times. This means that, for example, if a character has value 0 at time 0 and at time ''t'', it could be that no changes occurred, or that it changed to a 1 and back to a 0, or that it changed to a 1 and back to a 0 and then to a 1 and then back to a 0, and so on. 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「substitution model」の詳細全文を読む スポンサード リンク
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