But population replacement follows the 2 n rule, so if the population has a size of billions or trillions and you have 20 generations per year, it still takes a long time. Second over many generations you have three fates
1. The mutation is slightly deleterious and alters tona point in all copies that it can no longer complete its task
2. Its neutral and accumulates mutation and heads toward one a bit slower
3. Its selective and maintains purifying selection moving faster through the population.