Mutations are important in evolution because they cause variation within the gene pool .
A mutation can occur in any cell, but those mutations which occur during the production of gametes are of importance in terms of the population and of evolution , because they have the potential to be inherited. Most mutations are gene mutations and involve the addition, deletion or, most commonly, the substitution of one or more nucleotides. Since the nucleotide sequence of DNA codes for the amino acid sequence of protein product, the effect of even a single nucleotide substitution can have far-reaching effects upon the development and life of the individual, by altering the structure and function of the protein. Diseases such as haemophilia and sickle-cell anaemia are the result of relatively simple gene mutations. Mutations can occur as the result of errors in copying during DNA replication, but this process normally has a high level of failure.