We present a statistical model of bacterial evolution based on the cou
pling between codon usage and tRNA abundance. Such a model interprets
this aspect of the evolutionary process as a balance between the codon
homogenization effect due to mutation process and the improvement of
the translation phase due to natural selection. We develop a thermodyn
amical description of the asymptotic state of the model. The analysis
of naturally occurring sequences shows that the effect of natural sele
ction on codon bias affects genes whose products are largely required
at maximal growth rate conditions or undergo rapid transient increases
.