人种的确是个很有意思的问题，现在还有人不承认人种的明确划分呢

http://en.wikipedia.org/wiki/Race_and_genetics

There were often no clear distinctions between the ancestral populations with transitions being gradual. Genetic variations in such groups were larger than genetic variations between groups. How a person is identified racially may be different from geographic ancestry. Especially for groups with mixed ancestry self-identified race is often uncertain and culturally dependent.

另外，我的印象是现在还不能从DNA完全准确地确定人种。好像曾经有个案件涉及这个问题，一开始从DNA证据确定是黑人，后来发现不对。具体的记不清了。