How is artificial selection dependent on variation in nature?

Artificial selection is a process by which man chooses the traits it wants to perpetuate.

Artificial selection is not done by nature. It is done by man. Man selects variations according to what he wants. This is to contrast it with natural selection, in which the breeding process is not controlled directly by humans.

Dogs are one of the most well known examples of this process, as humans chose traits like passiveness, obedience, intelligence, and various others to decide which wolves/dogs to breed. This has produced the diversity of dog breeds we see today.

But while dogs are a well known example, almost every domesticated animal and crop plant has been selectively bred for hundreds or thousands of years by humans. One example that fewer people know is that cabbage, brussel sprouts, kohlrabi, kale, broccoli, and cauliflower are all derivations of wild mustard! Humans created these varieties by selectively breeding for different aspects of the plants we found desirable.

Since humans can only pick those individuals to breed based on observable traits, those desired traits must be already present in the species. In other words, we can't breed dogs to fly because that variation is not already present. However, they do have genes that control size, coat color, demeanor, etc., so we can breed based on those and we can get something that expresses certain traits to a much greater extent than any of its ancestors did.