Subject: Are different human races simply sub-species?

I could never get a straight answer from my biology teacher. There are appearent differences in the major races of this earth, for instance, skin color, hair, body build and proportians. I realize that all races are classified homo sapien, but how are asians, caucasians, africans and native americans further classified, or are they? And did these changes come about in the supposed 300,000-present time period of the modern human? I'm also curious as to what makes pygmy people of Africa so small? Is it genetics or diet? If it is genetic, then how much do they differ from say, the larger homo sapiens of this planet? I realize these questions are somewhat controversial, and I apologize. Please do your best to answer them.

Re: Are different human races simply sub-species?