Role of minimization of chemical distances between amino acids in the evolution of the genetic code

The allocation of codons in the genetic code makes possible a moderate minimization of the chemical distances between pairs of neighboring amino acids in the code. However, the code is neither a global nor a local optimum with respect to distance minimization. These findings do not support the physicochemical postulate that distance minimization was a major factor shaping the evolution of the genetic code. They agree with the coevolution theory, which proposes that genetic code evolution was predominantly determined by the concession of codons from precursor to product amino acids in an expansion of the code to accommodate new varieties of amino acids, with distance minimization playing a subsidiary role in deciding the choice of codons to be acquired by the product amino acids from the codon domains of the precursor amino acids.