In Chapter 2, Diamond is arguing that it is geography, and not culture, that determines what kind of a society springs up in any given place. To prove this, he looks at evidence from Polynesia.
Diamond tells us that all the people who inhabited Polynesia came from the same ethnic and cultural background. Therefore, if culture was what really mattered, they should all have ended up with similar societies. Instead, what happened is that various Polynesian societies ended up with very different levels of economic and political development. Diamond says that those that sprang up on large islands or island groups became more advanced. Those that arose on small islands with fewer resources remained small and relatively primitive.
This, to Diamond, proves that it is geography rather than culture that determines what a society ends up being like.