Figure 4

Changes in code cost function Φ when attempting to add the amino acids Leu, Thr, Glu and Arg to the four different positions illustrated in Figure 3. Current, Intermediate and New denote the value of Φ before reassignment, immediately after reassignment, and considerably after reassignment when the genes have adapted to the new code. The difference δ Φ = Φ_int-Φ_cur is the selective barrier against addition of the new amino acid. The figure shows that it is favourable to add Leu in column 1, Thr in column 2, and Glu in column 3, but it is not favourable to add Arg in any of the columns.