Selective Breeding aka, artificial selection is a way of humans selecting mates for an organism based on certain traits to produce offspring with those traits.
When African Americans were held in the USA, did their masters breed them with the intent of producing "better" slaves? Or is this even possible biologically? I would not think it is impossible, since a lot of phenotypical (physically viewable) traits are influenced by genetics and the environment.
IE: Male slave A ~ phenotypically Large , Female slave B ~phenotypically Large
Their intention: A + B ~ phenotypically large child.