The theory of natural selection (part 2) - How does natural selection deal with recurrent unfavorable mutations?

Recurrent disadvantageous mutation
Natural selection will act to eliminate any allele that decreases the fitness of its bearers, but what about a recurrent disadvantageous mutation that keeps arising at a certain rate?
Selection can never finally eliminate the gene, because it will keep on reappearing by mutation. In this case, we can work out the equilibrial frequency of the mutation: the equilibrium is between the mutant gene's creation, by recurrent mutation, and its elimination by natural selection. This is known as mutation-selection balance.
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