Since most mutations are detrimental, this gene complex is an advantage to its holders. Now you might wonder about beneficial mutations—they do happen occasionally, so wouldn’t the unmutable be at a disadvantage? But in a sexual species, a beneficial mutation that began in a mutable can spread to the descendants of unmutables as well. The mutables suffer from degenerate mutations in each generation; and the unmutables can sexually acquire, and thereby benefit from, any beneficial mutations that occur in the mutables. Thus the mutables have a pure disadvantage.
