Changes in the DNA code are the basis of evolutionary change. However, European research is looking at small DNA alterations that do not cause protein alterations but are still somehow important in the selection process.
The codon, as the basic unit coding for a single amino acid, comprises three nucleotides. A so-called synonymous or silent substitution in this unit has no effect on the amino acid it codes for. Nonsynonymous substitution results in a change in the amino acid and therefore should be more important in evolution.
Further details: A molecular view of evolution