dogs can interbreed....and sometimes there are characteristics which people work hard to 'select' for...or to get rid of. There are 'breeds' of dogs, not 'races' of dogs. Same general thing goes for humans.

We all know that Cocker Spaniels are different than Dobermans, because of small differences in genetic makeup...but we seem to lose that insight when thinking about people.

If we think about it carefully, and define terms precisely, there can be little confusion....but it is not adventageous to some folks prejudices to adhere to careful definitions.