In The Call of the Wild, the protagonist gains new knowledge which allows him to adapt to his surrounding. Adaptation in this novel is necessary for survival. When our protagonist is tossed into a new world of harsh conditions and unforgiving men, he needs to learn how to do his job, how to coexist with the other dogs, and how to live while many others die from the conditions. The adaptation we see is physical, mental, and spiritual as Buck is toughened and strengthened by the world around him.
Along his travels in the frozen North, Buck at first resists adaptation, clinging to the life he knows, but later embraces change wholeheartedly.
Buck's "adaptation" to the wild is inevitable, as it is a reversion to his natural state.