In Guns, Germs and Steel what is the biggest factor that caused the Maori and the Moriori to evolve differently?

One of the refreshing aspects about Diamond's ambitious
study of world history is the way that he constantly affirms the way that differences in
the level of technology and sophistication in societies are the result not of genetic or
racial differences but a result of differing environments. In Chapter 2, Diamond uses
the case of the Maori and the Moriori as an excellent example of how our natural
environment impacts our level of technological sophistication. Even though the two
societies had only been separated for 500 years, the radically different environment of
the Chatham islands clearly displays the role that environment plays in placing one
nation above another.

Note how Diamond comments on the
context of the Chatham Islands:

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It is easy to trace how the differing
environments of the Chatham Islands and of New Zealand moulded the Moriori and the Maori
differently. While those ancestral Maori who frist colonised the Chathams may have been
farmers, Maori tropical crops could not grow in the Chathams' cold climate, and the
colonists had no alternative except to revert to being hunter-gatherers. Since as
hunter-gatherers they did not produce crop surpluses available for redistribution or
storage, they could not support and feed nonhunting craft specialists, armies,
bureaucrats, and chiefs. Their prey were seals, shellfish, nesting seabirds, and fish
that could be captured by hand or with clubs and required no more elaborate
technology.

Thus is is that
we can see the biggest, overwhelming factor that predicated the difference in the
evolution between these two societies, that formerly were part of the same society, was
environment.