All these combinations will produce an offspring that will never produce offspring. Breed an African to an Asian, or a Russian with an Australian, and there will be a child, who in turn can breed and have children. As diverse as humanity is, we are still one species. We are alone, the only family on the branch of Homo to raise our young on the Earth today. Those are the questions of science.
Science holds two main branches, each a mirror of the other.
One branch is the practical science. It gets out into the field and explores the world with the senses, whatever they are. The other branch is theoretical science. It tries to assimilate the results of field experiment and observations. It then tries to put those results into language, first mathematical then ordinary.