A mutation is a change in deoxyribonucleic acid (DNA). An organism’s DNA affects how it looks, how it behaves, and its physiology. So, a change in an organism’s DNA can cause changes in all aspects of its life. Mutations are essential for evolution. This is because it is the only way that new alleles and new regulatory regions are created. Most mutations that are observed either have no effect, little effect or a significant effect on the organism, most mutations in genes affect a single protein product, while much evolutionary change involves myriad structural and functional changes in the phenotype. The most common mutation is when DNA fails to copy accurately. Most of the mutations that we think matter to evolution are “naturally-occurring.” For example, when a cell divides, it makes a copy of its DNA and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation. Mutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural, even in the most isolated and pristine environments, DNA breaks down.