A mutation happens when a pair of nucleotides is swapped by another, a pair (or a few of them) is dropped or added to a chromosome. These type of mutations happen during DNA replication, but a large number of them is immediately corrected by proteins involved in the process.

Mutations are caused by different external factors, and can then be very frequent. Changes in DNA leading to mutations can be caused, for example, by X-rays, ultraviolet light (UV), or radiation occurring during decay of radioactive elements. Some chemical compounds like insecticides, preservatives or pharmaceuticals are known to cause mutations.

Frequency of spontaneous mutations ranges from one in 100 thousand to one in a million copies of a single gene. (8) A mutation can have no effect on the organism (a neutral mutation). If, however, it influences a change in the encoded protein or its functions, the mutation is usually harmful. Mutations are the main source of gene alleles. That is why they form a basis of evolutionary changes.