In chapter 1, the boys determine that someone needs to be in charge of leading them. As they begin conversations about who that should be, they realize the power of the being who brought them all together:
But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.
“Him with the shell.”
“Let him be chief with the trumpet-thing.”
The conch has power because it has gathered them from their far flung reaches of the island. Since it has power, the boy who blew it also has power. It wasn't even originally his idea, but Ralph is seen as powerful because of his connection with the conch.
In chapter 4, Jack and his hunters have some meat and use that position as a means of power, particularly over Piggy:
“Aren’t I having none?”
Jack had meant to leave him in doubt, as an assertion of power; but Piggy by advertising his omission made more cruelty necessary.
“You didn’t hunt.”
“No more did Ralph,” said Piggy wetly, “nor Simon.”
To eat is to live. Jack realizes this and the power he can exert over the group through his access to meat. Because Jack hates Piggy from the start, he denies Piggy food as a means of asserting his dominance over Piggy's position of wisdom.
In chapter 5, Simon, Piggy, and Ralph are analyzing Jack's character and motivation, trying to determine why Jack is so nasty to them:
"He hates you too, Ralph—”
“Me? Why me?”
“I dunno. You got him over the fire; an’ you’re chief an’ he isn’t.”
“But he’s, he’s, Jack Merridew!”
“I been in bed so much I done some thinking. I know about people. I know about me. And him. He can’t hurt you: but if you stand out of the way he’d hurt the next thing. And that’s me.”
“Piggy’s right, Ralph. There’s you and Jack. Go on being chief.”
This exchange reveals a couple of things. First, there will always be those like Jack who despise people in a position of power. Not everyone desires to live in a society of order or to submit to a form of authority. Jack represents disorder and therefore stands in natural opposition to Ralph, who seeks to exert a form of control over the group. Second, the exchange reveals that those who oppose leadership will sometimes strike out indirectly to harm others close to that leadership. Thus, Piggy realizes that he's in danger simply because of his proximity to Ralph.