I always envisioned each planet / great house / ethnic group to be ethnically homogeneous, although different from each other.
I don't think it's very likely there would be some black Fremen and some mediterranean looking ones in the same sietch. Even if the planet was colonized with a diverse population, it would have been blended out by the time the events Dune take place.
But also it's soft sci fi so tons of things already aren't realistic. Ethnicities are a weird one to get hung up on.
But they trace their line to the ancient greek Adreides which fought in the siege of Troy. So their origin would be Greek, whatever mixed in over the millenia.
Genetically it doesn't make sense to privilege one ancestor or ancestral line as 'their origin', over others that are merely 'mixed in'. They're all just ancestors. Take Paul for example, is his ancestry Atreides, or whatever Jessica's background is*? He's just as much her son as he is Leto's.
That is of course correct. Isn't it even so that every 4th man in Asia can name Genghis Khan as his ancestor?
I think in general, making large assumptions how humanity looks even very few generations in the future is pure speculation with the current speed of globalization and mobility.
I thought Sharon did a fantastic job as Kynes, one of the more naturalistic performances in the whole movie.