Box 2 | Finnish population history 

The demographic history of Finland is typical of many isolates. A small number of original founders, subsequent isolation, rapid expansion and major bottlenecks have allowed genetic drift to mould the gene pool. The vast majority of Finns descend from two immigration waves occurring about 4,000 and 2,000 years ago⁸. The earlier wave involved eastern Uralic speakers, and the later wave, Indo-European speakers from the south. Both Y-chromosomal haplotypes and mitochondrial sequences show the low genetic diversity among Finns compared with other European populations and confirm the long-standing isolation of Finland. The size of the founding population(s) is unknown, but as late as the twelfth century, the population of Finland was only about 50,000, as shown in the left-hand figure. It reached 400,000 by the mid-seventeenth century, only to experience the great famine of 1696–1698, where one third of the population perished. Since then, the Finnish population has grown relatively rapidly from 250,000 at the beginning of the eighteenth century to its present figure of 5,100,000.

Starting in the sixteenth century, during the reign of the Swedish King Gustavus of Vasa (1523–1560), internal migrations created regional subisolates (as shown in the right-hand figure). The population spread from the early settlement region on the southern and western coastline towards the east and north. The subisolates in the late settlement region were established for the most part by groups of farmers originating from a small area of South Savo in southeastern Finland. They moved to the central, then western, and finally northern parts of the country, clearing the land by fire. Within a century, the inhabited land area of Finland doubled. Until the Second World War, many of these northeastern settlements grew rapidly without further immigration to supplement the descendants of their 40–60 founding families⁶⁶. The reign of Gustavus of Vasa also saw the establishment of a national system of population records, an important resource for later genetic studies of Finns. Using these records, many deceased individuals can be traced to common ancestors, especially in the subisolates of the late settlement region.

Finland's demographic history has led to a unique catalogue of genetic diseases. Around 30, mostly recessive diseases, are highly enriched in Finland. Other diseases, such as phenylketonuria and cystic fibrosis, are almost non-existent. Molecular studies have exposed one major mutation (78–98% alleles) in most Finnish Mendelian diseases and have revealed long genetic intervals of linkage disequilibrium (LD) flanking the disease gene¹⁴, with the length of the LD interval reflecting the age of the mutation.

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