Question

how can we say that variations may or may not help an individual but they always help an specie to survive

Answer 1

Variation is beneficial for the survival of species but not necessarily for the individual as variation makes species more adapted to survive with the changing conditions. At the species level, these variations can make difference at each generation but at individual level an organism is not evolving but aging. Variation is not necessary in case of an individual because it does not result into evolutionary change. If an individual naturally accumulates variation in his gene composition through mutation or any other source, it could be harmful. Thus variation at individual level is not so useful as a person does not evolve by these variations.