Sometimes The DNA Is Damaged by Chemicals or Radiation, or The DNA Code Isn't Copied Exactly in The New Cell. Either Way, The New Cell Is Slightly Different From The Old Cell. Such a Change Is Called a Mutation. Mutations Cause DNA Code to Be Changed in Passing From Parent to Offspring. Well Adapted Than Parents.
Mutation Tends to Die out.
If Mutation Is Minor, Has Little Immediate Effect on Offspring. Mutation Doesn't Die Out, But Is Passed On. Most Organisms are already Well-Adapted, so any change is likely to be harmful. Mutations rarely have any immediate effect. If conditions change, then the mutation may be better for the organism, and those organisms without the mutation may die out.