Daniel Voshart replicated the Roman emperors based on historical clues and statuary, coming up with this chart showing the gradual decline from a Nordic-Germanic population into something more closely approximating the Asiatic and North African admixture we see in Italy today.
The Roman Emperor Project is worth perusing, as is the list of Roman emeprors.
It shows how empire leads to diversity, diversity creates instability, and trace admixture obliterates any chance of getting out of the trap. Once you mix it up, you go third world (or nearly so, as with Italy today) forever.
Tags: admixture, italy, miscegenation, trace admixture