  RNA editing can be crucial for the expression of genetic information via
inserting, deleting, or substituting a few nucleotides at specific positions in
an RNA sequence. Within coding regions in an RNA sequence, editing usually
occurs with a certain bias in choosing the positions of the editing sites. In
the mitochondrial genes of {\it Physarum polycephalum}, many more editing
events have been observed at the third codon position than at the first and
second, while in some plant mitochondria the second codon position dominates.
Here we propose an evolutionary model that explains this bias as the basis of
selection at the protein level. The model predicts a distribution of the three
positions rather close to the experimental observation in {\it Physarum}. This
suggests that the codon position bias in {\it Physarum} is mainly a consequence
of selection at the protein level.
