Mutation

A mutation is a disruption in a gene. The change is permanent and alters the amino acid production that the gene coded for. Some changes are subtle and have no visible effects, while others have drastic and life altering effects. Evolution and adaptation occur through mutations that occur over long periods of time.

The one red berry in this bunch is a disruption in the normal pattern of green berries.
Note: this berry's color is not a result of a mutation, it simply illustrates the idea of one little difference altering the whole.