This was being left behind because they werent fit to survive. They got sick and could not power through it.
This is Polly after she went mad and tried to attack other people. Due to her not controlling her madness she got decapitated.
This is Hal fighting John due to a suggestion and survival. Hal being young and ignorant and listened to nobody. Unfortunately Hal was not fit to survive and was killed by John Thorton.
This is Buck and Spitz fighting to assume dominance and to survive. This is an example of survival of the fittest because Buck killed Spitz showing he was fit enough to survive.
While the dogs were walking on the ice they fell into the ice. Only Buck and John Thorton survived showing that they adapted quickly and it shows survival of the fittest.
This is Buck killing the native americans to avenge John Thorton who wasn't fit enough to survive because he died. Buck was obviously fit to survive because he single handedly killed every native american.