One suggestion is the personality of Jack. His physical unattractiveness and harsh ways prevent him from being elected over Ralph. His treatment of the choir (one boy faints from marching), his contempt of Piggy, and his temper (displayed as he repeatedly slams his knife into tree trunks) strongly indicate his tendency toward savagery. His initial hesitation at killing the pig would make him seem at least a little vulnerable, were it not for his chilling vow to show no mercy the next time. Thus he is introduced as a foreshadow of what will come.
Another suggestion is what has happened outside the island. While Ralph and Piggy are first getting to know each other, Ralph asserts with child-like confidence that his father and the Royal Navy will come rescue them. Piggy however, is not so sure:
"How does he know we're here?"
Because, thought Ralph, because, because. The roar from the reef became very distant.
"They'd tell him at the airport."
Piggy shook his head, put on his flashing glasses and looked down at Ralph.
"Not them. Didn't you hear what the pilot said? About the atom bomb? They're all dead."
So the boys are cut off completely from the outside world, and they may not be able to sustain themselves on the island.
Finally, the boys pull further away from civilization, even in the first chapter. All of them bring childhood perceptions of society to the island, in particular, the need for organization. Piggy tries to organize the boys by finding out their names in order to keep track of everyone. His failure to do this is due both to his ineffectiveness as a leader, and the boys’ general immaturity and unwillingness to cooperate. Ralph shallowly sees the need for the names but does not really understand why. Jack does not see its necessity at all. In lessening degrees, with Piggy being the most civilized and Jack being the least, the boys distance from order brings them closer to savagery.