If the old variation is fitter, the new one dies out/is ignored. If the new one is fitter, the old one dies out/is discarded. If they are equally fit, they both spread throughout the population.
(Note, "fitness" is a wide-ranging property. With regards to code changes, if two variants are equally performant and equally simple, then one might be "more fit" simply because it is more widely adopted, and minimising the set of patches you have to others increases the "fitness" of your tree.)
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