There are many examples from the book that are meant to prove Diamond's point. Let me briefly discuss two of them.
First, there is the example of the Polynesia societies that is discussed in Chapter 2. In that chapter, Diamond establishes that some Polynesian cultures were more advanced than others. He also shows that the more advanced societies were those that were on islands that could support more agriculture. This proves that environmental conditions lead to agriculture (or lack thereof) which determines which societies become advanced.
Second, there is the example of the Aboriginal Australians, discussed in Chapter 15. There, Diamond notes that the Aborigines were one of the most primitive societies in the world. He goes on to prove that this was due to their isolation and the lack of resources that were native to their homeland. This, too, works to defend Diamond's overall thesis.