In my previous post on Finnish population clustering I should have emphasised that the map was constructed only from individuals who had both parents from the same geographic/linguistic region; this obviously provides a lot more power to detect a correlation.
The close match between genetic and geographic ancestry in these selected individuals indicates that there hasn't been a huge amount of long-range admixture between Finnish populations over quite a long period of time - if there had been, there would be no reason to expect much correlation between the two maps. However, data in the supplementary section of the paper show that there has been substantial mixing over more recent history:
This graph shows the number of individuals from the cohort that had both parents from a single region. You can see immediately that the single largest category is the "mixed" category indicating parentage that crosses region boundaries. It seems likely that many of these individuals come from border areas between the regions (in which case excluding them from the map will have resulted in a rather artificial increase in the separation between clusters), but no doubt there are also many examples of long-range admixture resulting from increases in population mobility during the 20th century.
As mobility continues to increase the correlation between geographical location and genetic ancestry will decline rapidly. We really are in a brief and privileged moment in time: a moment when we have the tools to perform genome-scale analyses of ancestry, but before globalisation [and urbanisation] begins to erase information about historical population structure.
Chiara Sabatti, Susan K Service, Anna-Liisa Hartikainen, Anneli Pouta, Samuli Ripatti, Jae Brodsky, Chris G Jones, Noah A Zaitlen, Teppo Varilo, Marika Kaakinen, Ulla Sovio, Aimo Ruokonen, Jaana Laitinen, Eveliina Jakkula, Lachlan Coin, Clive Hoggart, Andrew Collins, Hannu Turunen, Stacey Gabriel, Paul Elliot, Mark I McCarthy, Mark J Daly, Marjo-Riitta JÃ¤rvelin, Nelson B Freimer, Leena Peltonen (2008). Genome-wide association analysis of metabolic traits in a birth cohort from a founder population Nature Genetics DOI: 10.1038/ng.271
It won't persist, because everyone is moving to the cities now. Never mind globalisation, urbanisation is destroying the structure.
Fair points. I've added urbanisation to the post, and deleted the question mark from the end of the title.
there hasn't been a huge amount of long-range admixture between Finnish populations over quite a long period of time
These particular populations are not ancient though, this area was not colonized by Finns until from the 1500s onwards. In many cases much later.
"...this area was not colonized by Finns until from the 1500s onwards..."
If you define colonization as "big populations staying over the winter", that is correct. However, the Finns from the south stayed in the area for the summer (between tilling and harvest) already millennia previously, hunting and gathering food for the winter. These clans had their "own" hunting grounds where they returned year after year. It was a kind of semi-colonization. The population substructure is older than colonization.
Anyway, after WW2 the refugees from Carelia were resettled all over Finland, many of them in the North, where there still was uninhabited land.
It's worth noting for other readers (in case it isn't clear) that children of post-WW2 refugees would have been mostly but not entirely excluded from the map - the cohort consisted of individuals born in these provinces in 1966, and the map includes only those who also had both parents born in the same region.
...clans had their "own" hunting grounds where they returned year after year. It was a kind of semi-colonization. The population substructure is older than colonization.
The population substructure is older than colonization mainly because the colonists came from different populations. There may have been an element of pre-colonization but the far more obvious influence of the population history of Savo, for example, should be addressed. The Savo people had not hunted in Kainuu for millennia (they themselves hadn't existed as a people that long!) yet most of the colonists came from there.