Who Gets to Survive

Okay so I've been having some massive moral quandaries in my life lately. Without going into detail, they've somehow dovetailed into a post apocalyptic idea for a book.

So I'm not going into the idea too much but I have a question from the hive mind:

At the end of the world, who gets to survive in the society that's left? What do we need to keep a society functioning?

In a real world survivor, who gets voted off the island when you have to make that choice? More importantly, why?