chemistry1111 Genetic drift since it tends to have a more profound and drastic effect on the gene pool, since the bottleneck effect wipes out much of a population and the founders effect can drastically reduce the gene pool its allele frequency due to migrating populations not genetically reflecting the many alleles found in the initial population (both populations, initial and founded, are usually greatly impacted). The effect of genetic drift can be even more pronounced in small populations, causing some to become extinct. Mutations occur all the time and are often quite small or random. Since they tend to only occur in one individual, they don't suddenly change the entire gene pool. Obviously, natural selection will alter the allele frequencies in response to a mutation and its possible advantages or disadvantages will alter the gene pool. However, you'd need a lot of mutations to have a sudden and drastic impact, and changes tend to occur gradually over time. Genetic drift is usually more sudden in its impact. The level of impact of mutations simply isn't the same as genetic drift tends to be. That said, I might be wrong, but that's sort of what I've been taught.
