I like the symbol of Mercedes and her possessions. She, and all of her stuff, represent materialism. She believes that having all of her stuff makes her important and powerful. She is adamant that it get loaded onto the sleds even though it is too heavy for the dogs to pull. Mercedes can't recognize that out in the wild material possessions are valued very differently than in cultured society. To Mercedes, her possessions are meant to reflect her value and importance. In the wild, the worth of a possession is based on its immediate value for survival. Mercedes doesn't get it. And it kills her.
I think a second main symbol is the lone wolf. I believe that the lone wolf represents Buck's desire to be free of man and society. Buck wants to be like the wolf and make his own decisions and hunt and kill. If it weren't for John Thornton, Buck probably would have left a lot earlier to follow the lone wolf. But once the Yeehats kill Thornton, any ties Buck felt toward man are severed. Buck goes wild, literally. He kills a bunch of the Indians, then takes on the wolf pack leader, and finally fully embraces his call to the wild.