Relationship between base composition in non-coding DNA of genes and codon composition

The C + G percentage in third position of codons is linearly dependent on the C + G composition of flanking regions and introns. A similar relationship is shown for the first and second position which significantly influence the nature of amino acid sequence. If mutations would be oriented according to the local base composition, this will imply that genes of the same multigenic family would evolve at different rate.