Evolution by natural selection is the key mechanism by which species retain their mutations in their gene pool. Mutations as such do not confer fitness advantage for the species. If species is imagined as a product of a factory, mutations make the proteins that are the raw materials for the product. The selling factor of the product is the fitness advantage (reproductive or survival). Since the selling factor is determined by demand/supply rather than the raw material the product is composed of, demand/supply is analogous to natural selection. Just as demand/supply determines the selling factor, natural selection determines the fitness advantage through which the species propagates. If the product sells well, the factory will make sure there is a good amount of raw materials in store, similar to the proteins that are conserved through the mutations in the gene pool. I rest my case.
