Biology, race, and geography all influence humanity and therefore our cultural expression. Isn't geographical location a consequence of the need to...

I assume that you are positing this possibility as a way
of arguing that Diamond's thesis is invalid. If so, I think that you are missing two
things.  First, Diamond does not argue that all forms of
cultural expression are dictated by geography.  Second, Diamond's major thesis has to do
with the differences between people on different continents, not between, for example,
French people and Scandinavians.

With regard to the first
point, Diamond is talking about economics and politics, not about culture.  He is not
saying that cultural and spiritual differences are determined by geography.  He is
saying that food production and, thereby, technological levels and population densities
are determined by geography.  These things allow some societies to dominate and others
to be dominated.

With regard to the second point, the kinds
of migrations you are talking about have tended to be within continents.  Diamond's
thesis is that geography caused Eurasia to be dominant, not that it caused England to
become stronger than Portugal.  Since Diamond is arguing about things on such a large
geographic scale, the kinds of migrations you are discussing seem much less relevant to
his argument.