# Population Genetics > mtDNA Haplogroups >  Why have some mtDNA haplogroups declined so much since ancient times ?

## Maciamo

Dienekes posted a new article about the genetic discontinuities between Etruscans and modern Tuscans. He also notes the great variance in ancient and modern mtDNA haplogroups in Britain and Central Europe, among others.

I was wondering what could be the reason for this radical change in haplogroup distribution over time. I have come up with the following hypothesis. As is often the case hypothesis are not necessarily exclusive but could very well be complementary. 

1) some haplogroups confer a physical advantage, such as increased fitness or resistance to diseases, or increased fertility for women. If this is the case, then the same disadvantageous haplogroups should be declining in every region, regardless of the local geography, culture, wars or migrations.

2) some haplogroups are better adapted to some specific climates than others. There is already some evidence of this, for example with hg J, which is associated with a higher production of body heat (hence beneficial in colder climates like Norway).

3) The Indo-European migrations (and other migrations) have partly (and possibly massively) replaced indigenous haplogroups, both for Y-DNA and mtDNA. I see increasing evidence for this, especially since the evolution between ancient and medieval populations is much more dramatic than between medieval and modern. 

Of particular interest is the sharp decline in haplogroup N1a in Central Europe since the Neolithic period.

In Scandinavia, it is especially haplogroups I and X that have experienced the biggest drop. In Britain, U5a1, T, W and X have all fallen considerably in frequency, while H and K have augmented.

Overall, it seems from the data I have observed that H and K are generally on the rise, and also happen to have the most favourable medical associations. The widespread distribution of N1a, I, W and X seem to indicate that they have been once been common all over Europe, but are now disappearing.

I have noted a similitude in the distribution between mtDNA hg X and Y-DNA hg G, as well as between mtdna I and Y-DNA hg I1. N1a might be related to Y-DNA I2a. If that is so, the Indo-Europeans could have been responsible for the demise of these haplogroups, to the profit K and some branches of H and U (U3, U4, but not U5a1). Combined with a physiological advantage, the new haplogroups would have continued to progressively replaced the older ones well after the presumed period of violent invasion. Of course this is nothing more than a wild personal guess at the moment.

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## Maciamo

Mishmar et al. argue in favour of a climatic selection that would lead to regional-specific haplogroups.

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## Maciamo

Dienekes posted a new argument in favour of climate-driven mtDNA selection.




> The implications of this are manifold, and they relate:
> 
> (i) to the use of mtDNA to assess population relationships. For example, South Asians have a strong affinity with West Eurasians in their Y-chromosomes and with East Asians in their mtDNA. Nonetheless they do not occupy an intermediate position between the two in autosomal DNA, but are predominantly aligned to West Eurasians.
> 
> (ii) to the overall mitochondrial time depth of humanity, i.e., the age of mitochondrial Eve, and what that implies about the recentness of our species (it may be older than its mtDNA time depth), and its possible admixture with other human populations such as Neandertals (it may have occurred, but the non-modern mtDNA may have been selected against)
> 
> (iii) to the potential of solving, at least in part, discrepancies between present-day and archaeological mtDNA gene pools, which may not in fact reflect processes of migration, but climate shifts over time.

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## Maciamo

A new study about Scandinavia tested the mtDNA of early Scandinavian farmers (Funnel Beaker culture, from 3,500 to 2,500 BCE) and compared it with that of hunter-gatherers from the same period (Pitted Ware culture). 

The study is somewhat useless to determine the actual origins of the farmers since only three individuals were tested and each belonged to a different haplogroup (H, J and T). Nevertheless, it matches the haplogroups of Neolithic farmers from Central Europe (see above).

19 hunter-gatherers were tested. They belonged mostly to haplogroup U as expected by previous studies. 14 out of 19 samples belonged to haplogroup U4 or U5 (unless some U4 are H1b, which cannot be determined for sure with a HVR test). The rest was J, T, V, and two unidentified haplogroups.

Based on the mtDNA data I collected, 22% of modern Danes and 16% of modern Swedes and Norwegians belong to mtDNA haplogroup U. 

I had not noticed before that haplogroups J and T seems to be inversely proportional to U in most of Europe. J+T amount to only 14.5% of the Danish population, as opposed to 22% and 23% in Sweden and Norway respectively.

Finland has the highest percentage of U in Europe (about 25%) and also the lowest of J+T (9%). Greece is just the opposite, with 24.5% of J+T and 11.5% of U.

The total of J + T + U is fairly stable in all Europe, around 37% (Spain is an exception, with only 21%). Haplogroup H varies from 34% (in Italy) to 48% (in Sweden), with an European average of 41%.

What doesn't make sense is why some northern European countries should have so much more J+T than U, while it is the opposite for their neighbour. I suppose that mtDNA frequencies are not yet that reliable. For example, Belgium, Denmark, Finland and the Baltic are U-dominant, but the Netherlands, Germany, Sweden, Norway and Britain are JT-dominant. Only France and Spain have a roughly equal proportion of JT and U.

It is important to keep in mind that mtDNA could be subjected to strong natural selection over a short period of time. Haplogroup J, although likely to have originated in the Middle East, has been shown to increase the body's efficiency in producing heat, a vital advantage for Europe's northernmost populations. This would explain the very low percentage of J in warmer parts of Western Europe like France (5%) and Spain (4%), that are remote from the Middle East *and* do not need the extra heat production. It also explains why Scotland and Norway have the second highest percentage of J in Europe after Greece. Natural selection made up for their distance from the Middle East. More coastal and temperate Belgium and Denmark would not have experienced such selective pressure. What I can't explain is why the Netherlands would be any different. Maybe it is just the mtDNA sample that is not representative.

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## Shasta

Thanks for the information. Interesting to me as my Y-DNA is Ia2 and mtDNA is N1a.

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## Pallantides

My mtDNA haplogroup is N1a.

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