人種的確是個很有意思的問題，現在還有人不承認人種的明確劃分呢

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，我的印象是現在還不能從DNA完全準確地確定人種。好像曾經有個案件涉及這個問題，一開始從DNA證據確定是黑人，後來發現不對。具體的記不清了。