To survive in the wild, Buck has to let his instincts take over. This means he learns to hunt and to forget the comforts he had known when he was a domesticated dog. He can no longer rely on an owner to feed him and take his time eating for example. He learns that to eat enough means defending his food against the other dogs for example. He becomes a ruthless hunter.
The second part of the question is more difficult. I think you are referring to the other sled dogs but you may be referring to the domesticated dogs who were sold in Seattle along with Buck. The second set of dogs either adapt to life as a sled dog or they die. The dogs who are already living as sled dogs and somewhat wild - already live by their instincts.