Natural selection acts by taking advantage of natural variations in the traits of organisms within a population what is the ultimate source of this variation

Natural selections can be understand as a check on which hypothetically all too weak to survive must be eliminated for better future of the following generations. With genetic engineering coming more and more popular we will have to replace this check so that it won't kill the too weak but heal them and get them to normal condition. So we could say the ultimate source of the variation is probably the hypothetical check.