It's my understanding during slavery, assuming we're thinking American Black slavery here and not something else like Egyptian slavery etc, slaves were allowed to have their own families which they picked and were not selected for some sort of "superior" trait in children. The death rates for slaves was 1.8% and for whites was 1.2%, so I don't think that would have been a factor either.

I wonder if any previous human civilization did actually select slaves that were stronger to try birth stronger slaves... horrifying to think about really, but that doesn't mean it hasn't been done.