Subject: why is there so many differences in the appearance of the modern human race

The differences in the appearance of the modern human population do not 
seems to have been caused as result of an environmental adaptation.  For 
example black skin in Africa, India, Pakistan and not in other parts of 
SouthEast Asia or South America, narrow eyes in China and not in other 
part of the world, hair type in Africa and not in India.  And so many 
other differences. I wonder about this because it is believed that the 
modern human evolved during a long period of time from the same ancestors 
probably in Africa, but I can not find what caused all these differences 
to develop.  Hope you could help me find an answer. 

Re: why is there so many differences in the appearance of the modern human race