In Guns, Germs, and Steel, how did the environment affect the diffusion of population in the Pacific Ocean?

The answer to this can be found in Chapter 17.  Basically,
Austronesians diffused into the Pacific Ocean and came to dominate in those areas where
the natives had not already developed agriculture.  Diamond argues that it was
environmental factors that determined whether natives of a given area developed
agriculture.  The environment, then, determined which areas of the Pacific would come to
be dominated by the Austronesians as they diffused into the
Pacific.

Diamond argues that some areas of the Pacific were
less conducive to the independent development of agriculture than others.  Indonesia,
for example, was a poor area for agriculture and so its natives had no agriculture and
did not even have polished stone tools.  Because of this, they were easily dominated by
the Austronesians as they diffused.

So, Austronesians
diffused through the Pacific and settled in every place where agriculture had not
previously been feasible for environmental reasons.