Mutations
A substitution may be synonymous, change an amino acid, introduce a stop, or affect a regulatory site. An insertion or deletion within a coding region shifts the reading frame unless its length is a multiple of three. Not every mutation changes a protein, and not every protein change alters function. Germline changes can pass through sexual reproduction; somatic changes generally remain in a cell lineage. Mutations provide variation without anticipating what an organism needs.
Changing an mRNA codon from GAA to GAG preserves glutamate in the standard code. Deleting one nucleotide near the start of the same coding region changes the downstream grouping.
Mutations are not always harmful, and a silent coding substitution is not proof that every possible cellular effect is absent.