Load minimization of the genetic code: history does not explain the pattern

The average e¡ect of errors acting on a genetic code (the change in amino-acid meaning resulting from point mutation and mistranslation) may be quanti¢ed as its `load'. The natural genetic code shows a clear property of minimizing this load when compared against randomly generated variant codes. Two hypo-theses may be considered to explain this property. First, it is possible that the natural code is the result of selection to minimize this load. Second, it is possible that the property is an historical artefact. It has previously been reported that amino acids that have been assigned to codons starting with the same base come from the same biosynthetic pathway. This probably re£ects the manner in which the code evolved from a simpler code, and says more about the physicochemical mechanisms of code assembly than about selection. The apparent load minimization of the code may therefore follow as a consequence of the fact that the code could not have evolved any other way than to allow biochemically related amino acids to have related codons. Here then, we ask whether this `historical ' force alone can explain the e¤ciency of the natural code in minimizing the e¡ects of error.We therefore compare the error-minimizing ability of the natural code with that of alternative codes which, rather than being a random selection, are restricted such that amino acids from the same biochemical pathway all share the same ¢rst base. We ¢nd that although on average the restricted set of codes show a slightly higher e¤ciency than random ones, th