How is linguistic evidence used in Guns, Germs, and Steel to draw conclusions about the spread of people in the Pacific?

Chapter Seventeen is the section of this excellent book
that you want to refer to, which discusses the way that Polynesia and the islands in the
Pacific were inhabited. A very interesting point that Diamond establishes is the way
that the population of Java, most other Indonesian islands and the Philippines is
actually rather homogeneous in terms of the languages that they speak and their skin
colour. Note what Diamond observes about the linguistic
similarities:

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Their languages are equally homogeneous: while
374 languages are spoken in the Philippines and western and central Indonesia, all of
them are closely related and fall within the same subfamily... of the Austronesian
language family.

Diamond
concludes that such evidence strongly suggests that Southeast Asians or South Chinese
people spread through these islands, replacing the former inhabitants on the whole. This
evident occurred recently due to the way that skin colour has not changed. The diversity
in languages points to the way that no system of unity was established between these
different islands.