Mutations occur all the time, but only certain mutations are important for evolution: those that are heritable. For single-celled organisms like bacteria, all mutations are heritable. A bacterium reproduces by dividing, and any changes that occur in the organism's DNA before division are passed on to the daughter cells. That's not the case with animals. Most of the cells in the human body, for example, don't contribute directly to the next generation. Only the germ line (eggs and sperm) do, so only mutations in those tissues can be passed to offspring.
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