Mutation is a change in DNA, the hereditary material of life. An organism's DNA affects how it looks, how it behaves, and its physiology all aspects of its life. So a change in an organism's DNA can cause changes in all aspects of its life.
Mutations are random
Not all mutations matter to evolution
For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. The seeds of this apple do not carry the mutation.
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