Hooby are you being obtuse on purpose?
Let's say one individual in a population of 10,000 acquires a favourable mutation. That has no effect on how many offspring the other 99,999 have. But over many generations the percentage of individuals with the mutation will gradually increase. Each generation of individuals with the mutation will do slightly better at leaving descendants. No change in the overall population will occur but the percentage of the whole population who have the favourable mutation will increase from 0.001% to closer to 100% while those without it gradually move in the opposite direction.
It really is a very simple concept.