A mutation is any change in genetic material. A change in the structure of a gene is called a gene mutation. A change in the number or arrangement of genes on a chromosome is a type of mutation called a chromosomal aberration. Gene mutations and chromosomal aberrations occur spontaneously in nature from causes not fully understood. They have also been produced artificially by exposing chromosomes to radiation or to certain chemicals.

Some mutations in the gametes cause harmful abnormalities that lead to the death of the offspring. Others are less harmful, or are beneficial to the new organism, and are passed on to the next generation. In time, physical variations produced by mutations lead to new species and thus carry on the process of evolution.