How do variations accumulate in species over time?

I assume that the question here refers to the process of evolution. The "helpful variations" are small mutations that allow the organism to have a small advantage over other - and this organism can then pass this variation to its children better than other organisms, meaning that with time, more organisms will have it. This will accumulate over time and the percentage of organisms with this mutation will increase.

The president's only official legislative duty is to sign or veto bills.