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Selection breeding is waiting for a random mutation with the trait you like - e.g. sweeter or larger plant. Then you reinforce the mutation by crossing it with other plants with similar features. Very slow.
Mutation breeding is artificially inducing random mutations with carcirogens or radiation. Out of 1000 irradiated seeds 900 don't sprout, out of the 100 that sprouted you get one with the trait you like, the rest goes to a bin. Much faster.