Selection cannot act over a single generation, if by 'act' you mean that an allele fixes in the population (reaches 100% frequency). If a new mutation appears i

Selection cannot act over a single generation, if by 'act' you mean that an allele fixes in the population (reaches 100% frequency). If a new mutation appears in a single individual, it cannot in the next generation be present in every individual, unless the founder's progeny replace everyone else's. In any event this would be a disaster for overall fitness, since the level of genetic diversity would collapse. In general selection propagates a variant exponentially fast based on the reproductive advantage it confers. If you make 1000 progeny and I make 1001, my reproductive advantage is 1.001 - every generation the frequency of my variant relative to yours goes up by this factor. If I start at frequency f and my advantage is s, it will take n=ln(1/f)/ln(s) generations to fix (approximately). For the example above with f=1%, this gives about 4607 generations. There are other factors, notably population size. For a population of size N, the initial f will be 1/N. If N is small, f will be larger and selection will take less time. On the other hand, in a small population, genetic drift is stronger, and weakly selected variants are more likely to be eliminated by drift before they can fix.