When do mutations not cause a change in amino acid sequence?

The genetic code is degenerate- this means that amino acids can be coded for in more than one way. In an mRNA sequence coding for a protein, each amino acid in the protein is encoded by one codon (set of three nucleotides). Some amino acids can only be coded for by one particular codon (eg tryptophan, UGG) but most can be coded for by at least two different codons which differ in their third letter. This means that if a mutation occurs in the third letter of a codon, there is a significant chance that it will still code for the same amino acid.

It has been suggested that this flexibility is necessary for life to evolve, as it makes natural selection more lenient towards mutations that would otherwise be harmful.