You need to think about when the Asians broke off from the rest of the humans, I'd say they broke off relatively early (not as early as the Australians), but early enough to not have large noses like Europeans. They developed from the people they broke off from.
Genetic isolation is another one. If you have a group of people, and divide it in two, the sum total of their genes will not be identical. Thus they could develop completely differently. People flock together with those who have the same heritage as them, and like to remain a homogenous group. This will ensure that their shared gene pool is similar.
Our preference in appearance of the opposite sex is also not uniform. Africans have their preference, Asians have theirs. This has been developed throughout the millennia, and ensures that groups become more homogenous. I don't know if this is environmental, or genetic, but it works.
69 mths ago