Buck and Spitz fight to the death with huskies surrounding them. This relates to survival of the fittest because you must protect what you own.
Buck emerges victorious, and the huskies finish off the almost dead Spitz. THIS relates to survival of the fittest because only the strongest dogs survive, in this case Buck is the strongest.
Francois places Sol-leks in the lead position. But Buck wants to be lead dog so he shoves Sol-leks aside. This relates to survival of the fittest because Buck feels he is strong enough to be leader, but not Sol-leks.
Finally, Francois and Perrault give up in trying to get Sol-leks in the lead position and let Buck take it. This relates to survival of the fittest because Buck is the strongest dog in the team, making him qualified to be lead dog.
The dogs begin to ride to the town, where Buck and his mates are sold. They are not able to adapt because the new owners were not able to take care of them.
When they are sold, the new owners fail to provide enough food. This relates to survival of the fittest because the new owners only took fit dogs. They left behind the weak dogs to die.