The novel seems to argue that a person's situation is largely influenced by his or her environment but that there is the possibility for one to escape the clutches of fate given determination, struggle, and the right opportunity.  On the reservation, Junior sees many people who have gotten sucked into a horrible fate because the environment is harsh.  For example, his own sister Mary really wanted to be a writer when she was in school, but she ends up living in his parents' basement for a while before getting married and moving to another reservation only to die in her trailer started from a careless fire (she was drunk when the fire happened).  One could argue that Mary's fate was "sealed" by the harsh reality of alcohol abuse on the reservation.  However, Junior decides for himself that he does not want to fall victim to the fate of others around him, so he opts to go off the reservation to Reardan for his education.  The decision was not easy, and Junior has the support of his parents to make the choice a reality (his parents drive him to school when they can).  Junior has fought for this opportunity and arguably has thwarted the fate others suffered based on their environment.