The genetic code has been influenced by directional mutation pressure affecting 
the base composition of DNA, sometimes in the direction of increased GC content 
and at other times, in the direction of AT. Such pressure led to changes in 
species-specific usages of codons and tRNA anticodons, and also in amino acid 
assignments of codons in mitochondria and in several intact organisms. These 
code changes are probably recent evolutionary events. The genetic code is not 
'frozen', but instead it is still evolving.