That is survival of the fittest because the man in the red sweater is beating Buck to show him who is boss. In chapter 1 paragraph 35 page 9 it says, "You've learned your place, and I know mine."
"Crashed down on mad Dolly's head"
This is survival of the fittest because she did not have what it takes to survive, she fell down and when you're down you're dead. The huskies can survive though, they were the ones who killed her. On chapter , page 15, paragraph 2 it says, made advice to a husky dog the size of a full-grown wolf. There was no warning , only a leap in like a flash, a metallic clip of teeth, a leap out equally swift, and Curly's was ripped open from eye to jaw."
"he cut lose"
This is survival of the fittest because Buck and Spitz are fighting because they both want to be the alpha dog (the leader of the group). That's survival of the fittest because who ever can survive, can survive.
This is survival of the fittest because Francois had to kill Dolly because she went mad. She was not strong enough to survive, survival of the fittest.
This is survival of the fittest, Dave had to be killed because he didn't want to move from his spot.
This is survival of the fittest because Buck is finally was accepted in their pack. Buck has what it takes to survive, survival of the fittest.