There is even more to it than this -- there may be very little apparent 
difference in the total genone of chimpanzees and humans, but  large differences 
in gene activity -- e.g., when turned on and off -- leading to the tremendous 
phenotypic differences between the two species. Also, it has been shown that 
very similar (or even identical) genes can have quite different functions -- 
sometimes even in the same species at different times during development.

As Paul says, looking at one marker is not enough -- in fact, we really don't 
enough yet about the actual *functioning* of genes to say whether two 
populations are the same or not. As another poster said, it is rather arbitrary 
and will probably stay that way for some time. 

All species in the genus Canis (dogs and wolves) can interbreed and produce 
fertile offspring -- but wolves know that coyotes are not the same "species" -- 
the slight genetic differences produce quite significant behavioral and physical 
distinctions.

Gail Mackiernan
Colesville, MD