How do helpful variations increase in a species over time?

through natural selection

Explanation:

Natural selection is the driving mechanism of the evolution of species. Organisms can survive more in an environment if they are capable to adapt the changing environmental conditions. After surviving they are also able to pass their modified trait into their future generations (evolution).

Helpful variations accumulate through natural selection, as the organisms that are better adapted to their environment reproduce offspring with the same variations.

0.88m hope i am correct : )