I'm willing to admit that I could be incorrect on this one, it's news to me!
However I remain sceptical of how much difference it actually made to the host's genetics.
The old assumptions were that the male populations were multi-ethnic and the female ones remained native to the region, of course within that paper (and the others referenced) it concludes that current evidence doesn't support that assumption and that Roman occupation bought with it increased heterogeneity across both sexes and within the broader communities.
From analysing the data
Trentholme Drive I'd also like to point out that only 43 individual's Isotopic evidence derived from the dentition was included, from the bone samples of Approximately 350 inhumations and 53 cremations, only 35% were of any use, which means we are not getting the whole picture here.
What seems as a high percentage is actually a very small amount of people, furthermore I don't agree with the idea that having foreign soldiers in a country makes it diverse.
An important part of the research is that it was not conducted using only Military grave sites, the overall research was made up of papers referencing grave sites covering Military, Civilian, and mixed Urban areas in York, Catterick, Gloucester, Winchester and Dorchester looking at a mixture of material culture, skeletal and isotope research. (I quoted just one of several papers making up Reading Universities research.)
With regard to Severus, he was born of a North African Pheonician Father, to be more precise his father was what is known as a
Lybico-Punic or in the modern usage
A Berber and an Italian Roman Mother in Libya. He was noted as having dark skin in several accounts of his deeds (although not Black as in a Nubian for example, he was North African rather than Sub-Saharan African). However, his ancestry was not really what I was alluding to, but the nature of the Army he built during his campaigns across North Africa and Nubia, the men, auxiliaries and their families that he bought to Britain with him would have been of multi-ethnic origin, and broadly made up of North and Sub Saharan Africans as well as Europeans and Middle Eastern stock. I would not put too much stock into a bust, as Rome (as Greece before them) did not exhibit colour prejudice and held to a more general xenophobia based on locale, belief and culture rather than a persons Skin Colour.
In any case, I think that Tunney is correct in what he says, and that Roman Britain, like much of the Ancient World was pretty cosmopolitan.