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Fig. 1 | BMC Biology

Fig. 1

From: Crossing fitness valleys via double substitutions within codons

Fig. 1

Conceptual scheme of double substitution analysis and the double fraction (DF) measure. a Point mutations are assumed to appear one at a time, such that observed double substitutions (b) occur through intermediate single substitution states. For each double substitution instance, there are two possible single substitution trajectories (a1 and a2). b Instances of single or double substitutions are inferred from the genomic data by construction of genomes triplets and relying on parsimony principle (see the “Methods” section). In brief, the parsimony principle implies that mutations occur along the thick branches in the trees shown in b. The double fraction is defined as the ratio between the number of double substitution instances b and the sum of relevant single (a1 + a2) and double (b) substitution instances

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