Genes code for proteins by using a 3-letter alphabet of adenine, thymine, guanine and cytosine (abbreviated to A, T, G and C).  These nucletodes are knwn as 'bases' (are alkaline in solution) and make matched pairs which form the 'rungs of the ladder' of the DNA helix. Substituting one base for another (as happens in many mutations) can change the amino acid sequence of the protein a gene encodes.  Changes may make no impact on survival, allowing the DNA sequence to alter over time.  Changes that affect critical sections of the protein (e.g. an enzyme's active site), or critical proteins like FoxP2, are rare (Image: Wikimedia Commons)