Why must there be variation in a population in order for natural selection to occur?

In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited. If there is no variation, then all individuals have the same chance to survive and nothing will be particularly selected.

because to irrigate you need purified or rain water to not kill your plants/animals and once you've used your fresh water how would you get more purified water?