They're adaptations to various environments with afforded those
who had them the advantage in survival to pass on their genes...
over time the local related population will then start to take on
these traits because these genes are sucessful in spreading
themselves.
A powerful sun beating constantly on exposed skin will
cause cancer, having increased levels of melamine (hence
darker skin) provides an advantage in survival in such an
environment. And of course skin colour is not the only
adaptation.
To suggest race scientifically is to suggest a different
species, this is not the case given people from different "races'
can obviously breed and have fertile offspring.
As for why Singapore and China Chinese look the same, we're
barely three generations in, lol, hardly enough time for local
change to take place. Additionally modern technology, travel, as
well as general mixing will mean that such selective pressures from
nature would be less pronounced.
However take away technology, reduce us back to a level that
puts us more at the whim of nature, take away easy travel and have
isloated population that have to breed among themselves, you'll
soon see the emergence of a new "race". But this is still very
human.
+1 i'm with this guy...and he's got pretty good points