A new paper by Jukka Palo and colleagues investigates the population history of Finland:
The Finnish population in Northern Europe has been a target of extensive genetic studies during the last decades. The population is considered as a homogeneous isolate, well suited for gene mapping studies because of its reduced diversity and homogeneity. However, several studies have shown substantial differences between the eastern and western parts of the country, especially in the male-mediated Y chromosome. This divergence is evident in non-neutral genetic variation also and it is usually explained to stem from founder effects occurring in the settlement of eastern Finland as late as in the 16th century. Here, we have reassessed this population historical scenario using Y-chromosomal, mitochondrial and autosomal markers and geographical sampling covering entire Finland. The obtained results suggest substantial Scandinavian gene flow into south-western, but not into the eastern, Finland. Male-biased Scandinavian gene flow into the south-western parts of the country would plausibly explain the large inter-regional differences observed in the Y-chromosome, and the relative homogeneity in the mitochondrial and autosomal data. On the basis of these results, we suggest that the expression of 'Finnish Disease Heritage' illnesses, more common in the eastern/north-eastern Finland, stems from long-term drift, rather than from relatively recent founder effects.
So you’ve got a cline of genetic variation. How do you explain it? This paper reminds us that for a single locus there are always multiple explanations: asymmetric migration, natural selection, founder effect and population growth are the simple unicausal scenarios. Considering a cline by itself, there’s no reason to prefer any of these except for assumptions that come from outside that gene – maybe you know something about the history, maybe the gene’s function gives you a clue.
If you’re going to test these hypotheses with genes alone, then you need to sample multiple loci, and you need to make an adequate spatial sampling of the population. And when you do, sometimes the evidence points in a different way than you had expected.
Palo JU, Ulmanen I, Lukka M, Ellonen P, Sajantila A. 2009. Genetic markers and population history: Finland revisited. Eur J Hum Genet 17:1336-1346. doi:10.1038/ejhg.2009.53