The rate of substitution of nucleotides in the DNA sequence in equilibrium under the mutational pressure described by: A – the "real BbTS" and, C – the "artificial BbTS". Bold lines show the fractions of nucleotides which have not been substituted yet, light lines indicate the fractions of nucleotides which appeared when substituting other nucleotides. Plots B and D represent the relations between half time of substitutions and the sizes of nucleotide fractions for sequences in equilibrium under the mutational pressure of "real BbTS" and "artificial BbTS" respectively. See also text and description for Fig. 1.