it doesn't need a source
you should understand these concepts if you're making this argument
heterosis is where the offspring of two biologically distinct parents are superior in size,growth,fertility,etc
(of which non is observed in humans
if you have population A average height 180cm
population B average height 170cm
mixed population would be 175cm and shorter than A)
and the dramatic effect from heterosis comes when the the populations are highly inbred (not inbred in human terms)
no human population is highly inbred
even if there's some statistically negligible effect on risk reduction of x disease
one could argue mixing is not beneficial because mixed individuals have lesser chance of finding bone marrow matches
mixed people are not superior to non mixed ones