Freese and Sueoka propose neutral mutations to explain the
difference between frequencies of amino acids and nucleotides.
E. Freese. 9162. On the evolution of basse composition of
DNA. Journal of Theoretical Biology 3: 82-101.
N. Sueoka. 1962. On the genetic basis of variation and
heterogeneity of DNA base composition. PNAS 48: 166-69.
From
James Crow's Top Ten List of Significant Events in Molecular
Evolution