Why are global population differences not sufficient? Is there an a priori reason to expect a different distribution of genetic factors in cities versus rural areas that feed city migrations?
Yes, but are they all ethnically skewed in the same way? If the concern is genetic factors, it doesn't really matter if cities have different ethnic demographics than rural areas in all two hundred countries as long as the rural/urban racial skew is different in the different countries.
For example, in the US rural areas may tend to be whiter than urban areas, but in countries that aren't predominantly caucasian, the opposite may be true.
The only places I feel have been homogeneous with their rural areas surrounding them, have been some of nordic places like Copenhagen, Oslo, Stockholm and Tallinn.
1 - https://ggwash.org/view/6497/maps-show-racial-divides-in-gre...
But Tulsa, OK vs the surrounding non-metro counties? I'm less certain without diving into the data.
The median US city size is Naperville, IL at 149,104 people.[0]
[0] https://en.wikipedia.org/wiki/List_of_United_States_cities_b...
Naperville is a suburb of Chicago, and without looking I can confidently tell you that it's more white than Chicago, but less white than a nearby rural area. I will look though: https://www.census.gov/quickfacts/fact/table/chicagocityilli...
Cities in mostly ethnically homogeneous states tend to still have things of value that might draw people of diverse ethnic backgrounds. I'm in Nashville Tennessee and there are far more people of Asian or middle Eastern descent here in the city than there are in the surrounding rural areas, due to the universities and the job opportunities which draw people from out of state.
I've done splicing work for several farming co-ops out west that ran fiber to the homestead. Small town of like 200 has GPON to most houses.
But if by “a priori” you mean something innate in humans or mathematical, I think you’ll be hard pressed to find that. Or what that really means in this context.
A priori = Beforehand, or before analysis performed. Standard phrasing in research, defines reasonable hypotheses to test.
For example, most americans have a-priori knowledge that rural americans are demographically whiter than urban americans, and could hypothesize global trends.
If that counts as an a priori reason - I’ll take it. In summary: “A priori” is ambiguous here / not a very useful modifier afaict.