Why is the duplication of an entire gene or gene region critical for evolving new functions?

Answer

One copy maintains essential function while the redundant copy can evolve a new function.

Gene duplication creates redundancy; one copy retains the necessary, essential function, freeing the spare copy from strong purifying selection, allowing it to accumulate random changes until a novel function evolves.

Why is the duplication of an entire gene or gene region critical for evolving new functions?
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