When Buck joins a wolf pack after Thornton's death it is part of which plot element?

When Buck joins a wolf pack after Thornton's death, it represents the climax of his transformation and the culmination of the story's themes of survival and the call of the wild. This moment marks Buck's full embrace of his ancestral instincts and his rejection of domestication, highlighting his evolution from a pampered pet to a wild creature. It also signifies his ultimate return to nature, where he finds a sense of belonging and purpose.