Without genetic variation, some key mechanisms of evolutionary change like natural selection and genetic drift cannot operate.

There are three primary sources of new genetic variation:

1. Mutations are changes in the information contained in genetic material. (For most of life, this means a change in the sequence of DNA.)  A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations with small effects.
2. Gene flow is any movement of genetic material from one population to another (e.g., through migration) and is an important source of genetic variation.
3. Sex can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation.