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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act. Answer: mutation
Output:
What phenomenon, crucial for evolution, creates new genetic variation in a gene pool and is how all new alleles first arise?