The genetic code: What is it good for? An Analysis of the effects of selection pressures on genetic codes

How did the "universal" genetic code arise? Several hypotheses have been pu t forward, and the code has been analyzed extensively by authors looking fo r clues to selection pressures that might have acted during its evolution. But this approach has been ineffective. Although an impressive number of pr operties has been attributed to the universal code, it has been impossible to determine whether selection on any of these properties was important in the code's evolution or whether the observed properties arose as a conseque nce of selection on some other characteristic. Therefore we turned the ques tion around and asked, what would a genetic code look like if it had evolve d in response to various different selection pressures? To address this que stion, we constructed a genetic algorithm. We found first that selecting on a particular measure yields codes that are similar to each other. Second, we found that the universal code is far from minimized with respect to the effects of mutations (or translation errors) on the amino acid compositions of proteins. Finally, we found that the codes that most closely resembled real codes were those generated by selecting on aspects of the code's struc ture, not those generated by selecting to minimize the effects of amino aci d substitutions on proteins. This suggests that the universal genetic code has been selected for a particular structure-a structure that confers an im portant flexibility on the evolution of genes and proteins-and that the par ticular assignments of amino acids to codons are secondary.