How can Buck's change from a civilized dog to a complete product of the wilderness be described?

Buck, whose father was a St. Bernard and his mother a Scotch shepherd dog, begins his life as a pampered pet, living in the Santa Clara Valley of California on Judge Miller's place. In the fall of 1897 when the Klondike strike brings men from everywhere in search of gold, this influx of prospectors creates a great demand for sled dogs. So, when one of the workers named Manuel finds himself in debt from gambling, he decides to secretly sell Buck to a couple of men as a way of paying his debt. One of these men wears a red sweater, and he beats Buck severely in an attempt to "break" him. 
After he is sold to two French Canadians—"a new kind of man" to Buck—he quickly learns that they are not mean. When he is put with other dogs, Buck is amazed at their behavior. Thirty or forty huskies have surrounded a dog named Curly, and they strike then leap away. By the time the men come with clubs, Curly is dead. Another shock is dealt Buck when he finds himself harnessed, and set to work, hauling firewood. François and Perrault later set out on the trail with the mail that they are to deliver. Buck begins to adapt to the wilderness and snow, even learning how to cover himself with it at night. Because he is large, Buck becomes the rival of Spitz, who is the lead dog of the team. Eventually, they fight and Buck kills Spitz in a brutal fight.

Mercy was a thing reserved for gentler climes. . . . Buck stood . . . the successful champion, the dominant primordial best who had made his kill and found it good.

Buck begins to enjoy his new life as the lead dog, and with him at the front of the pack, the carriers make record time. Unfortunately, François and Perrault later sell the team to another mail carrier, who overloads the sled and overworks the dogs. By the time the dogs reach their destination, they are physically spent and one dies. The carrier sells the dogs to three Americans, who know very little about dogsledding. They have not brought enough provisions, the woman insists upon riding in the sled, and they have too much baggage. By the time the trio stagger into John Thorton's camp, only five out of fifteen dogs are still alive. Thorton tries to warn the two men and woman that the ice over which they have been traveling is dangerous because it is melting. But, one of the men named Hal refuses to believe him. When the dogs begin to walk on, Buck refuses to move. Hal beats him without mercy. Thorton rushes Hal, knocking a knife from his hand. He cuts Buck's straps and saves him. Hal curses and moves forward with the sled, but he and the others only travel about a fourth of a mile when the ice breaks and humans and dogs are drowned in the icy water.
As Thorton's dog, Buck is privileged again. He proves his mettle by saving Thorton from drowning one day. Buck also attacks a belligerent man who seeks a fight with Thorton in a bar. One day Buck manages to pull a sled holding a thousand-pound load, and Thorton wins his bet of $1,600. With his winnings, Thorton, along with two other men, sets out for a hidden fortune of gold. For Buck it is "boundless delight, this hunting, fishing, and indefinite wandering through strange places." Because he has time to lie by the fire, Buck dreams of the "other world" that his instincts remember. In this dream, there is a hairy man who sleeps restlessly. Sometimes he peers with fear into the darkness of the night. Then, too, Buck hears "the call of the wild" in the forest primeval. Impulses begin to seize Buck and he ventures forth into the thicket, filled with "wild yearnings and stirrings" that he does not yet understand. One night he sees a wolf. After a while, Buck makes friends with this wolf, but he heads back to camp at night. Returning to his primordial state, Buck hunts and kills a bear and a moose after he begins to sleep away from the camp for days. When he finally kills the moose which he has tracked for days, Buck returns to camp, but as he is a couple of miles out, he smells something that makes the fur on his neck stand. When he nears the camp, Buck finds dogs with arrows in them. Then, he sees the Yeehat Indians dancing around the wreckage of the lodge. "Passion usurped cunning and reason" as Buck leaps upon them, ripping and tearing flesh. The Yeehats try to fight him off, but suddenly they decide to flee as they believe he is "the Evil Spirit." Buck finds his friend Thorton dead.
"From far away drifted a faint, sharp yelp, followed by a chorus of similar sharp yelps." It was the call, and without John Thorton to whom he could return, Buck realizes that "the claims of man no longer bound him." The pack runs at Buck, but he manages to kill several and fight off the others. Then, the wolf that Buck knows comes forward and they touch noses. Finally, an old wolf approaches and they sniff noses until the old wolf sits and howls at the moon. Now accepted, Buck joins the wolf pack; eventually, he becomes their leader. In fact, he is the topic of Indian legends, a Ghost Dog, who "has cunning better than they" as he steals from their camps and kills their dogs. Buck "defies their bravest hunters."