As the last ice-age began, it became necessary to move down to below the tree-line to hunt game. At its peak, the ice shelf within Europe extended down as far as southern Ireland, the middle of England and across northern Germany. Scandinavia was entirely covered. The sea-ice pack extended as far as northern Spain and tundra covered much of continental Europe. The tree-line at the height of the ice-age was as far south as Southern France, Northern Italy, north of the Balkans and across the Black Sea. Haplogroup I populations predominantly took refuge in the Balkans although there is good evidence that the Franco-Cantabrian region was a refuge for this haplogroup also.
As the ice-age ended and the fauna and flora were able to move northward again, people in Haplogroup I also migrated north.
Defined by the SNP markers P30 and M253, Haplogroup I1a has the most frequent and widespread distribution area among Haplogroup I sub-clades, most common in Scandinavia; Swedish, Norwegian and Saami populations. Evidence to support the Franco-Cantabrian theory includes a higher STR (short tandem repeat) diversity in western Europe and the I1a haplogroup being the most common of the I haplogroups in France. A possible coastal migration route exists and there appears to be a population expansion from the Scandinavian region at the start of the Holocene epoch (from 10,000 years ago). Haplogroup I1a shows a good correlation with Haplogroup I1c and also with the mtDNA Haplogroups of V and U5b.
Haplogroup I1b was derived within Viking/Scandinavian populations in northwest Europe and has since spread down into southern Europe where it is present at low frequencies. Defined by the SNP marker P37, Haplogroup I1b is the most frequent clade of haplogroup I in Eastern Europe with maximum frequencies in Herzegovinians (64%) and Bosnians (up to 52%). The gradient is steep westward as frequency drops dramatically to almost 1% in northern Italy. I1b extends eastward in moderately high frequency into the north Balkans, the Slavic populations and eastern Ukraine.
A glacial refuge for I1b is likely within the Balkan/Dinaric Alp region given the high frequency and diversity, combined of which are good identifiers for origins of expansion.
An interesting clade is I1b2 which has very high frequency in the Sardinians (about 40%) and is found almost exclusively on that island. There is a possibility that the I1b2 clade separated from I1b before the LGM (last glacial maximum) given its present distribution.
Defined by the SNP marker P223, Haplogroup I1c is possibly older than both I1a and I1b. It can be seen in highest frequency in the German and Dutch populations (12 and 10% respectively) but with a generally broad distribution from the Volgas across to the British Isles. A likely refuge for I1c was the Franco-Cantabrian region and it has a good correlation with Haplogroup I1a but also with the mtDNA Haplogroups of V and U5b. A possible coastal migration route exists and there appears to be a population expansion from Germany and Holland at the start of the Holocene epoch (from 10,000 years ago). A younger marker, M284, appears to show a distribution centered on the British Isles.
Credit: Definitions for haplogroups is borrowed from both
Family Tree DNA