Mailund on the Internet

I’ve just finished reading this paper in PLoS Genetics:

Sequences From First Settlers Reveal Rapid Evolution in Icelandic mtDNA Pool Helgason et al. PLoS Genetics, 5 (1) DOI: 10.1371/journal.pgen.1000343

Abstract

A major task in human genetics is to understand the nature of the evolutionary processes that have shaped the gene pools of contemporary populations. Ancient DNA studies have great potential to shed light on the evolution of populations because they provide the opportunity to sample from the same population at different points in time. Here, we show that a sample of mitochondrial DNA (mtDNA) control region sequences from 68 early medieval Icelandic skeletal remains is more closely related to sequences from contemporary inhabitants of Scotland, Ireland, and Scandinavia than to those from the modern Icelandic population. Due to a faster rate of genetic drift in the Icelandic mtDNA pool during the last 1,100 years, the sequences carried by the first settlers were better preserved in their ancestral gene pools than among their descendants in Iceland. These results demonstrate the inferential power gained in ancient DNA studies through the application of population genetics analyses to relatively large samples.

The paper has already been discussed by MacArthur at Genetic Future and Razib at Gene Expression so I will only give a very short review here.

Iceland was settled in the late first millennium by Vikings (replacing settlements by Irish monks) bringing with them Irish and Scottish slaves.

Iceland has been pretty isolated since the early settlement. Consequently, it is a very homogeneous population — at least genetically speaking — and is one of the best studied populations, not least by deCODE Genetics.

Analysis of contemporary DNA shows that the mitrocondrial DNA (mtDNA) is primarily of Scottish/Irish decent while Y chromosomes are primarily Scandinavian. You can probably draw your own conclusions from that.

What is new in the PLoS paper is that they have sequenced ancient mtDNA from medieval skeletons and compared them with contemporary samples from Iceland, Scandinavia and Scotland and Ireland.

The data doesn’t change much with respect to the genetic origin of the Icelanders, but an interesting finding is that the medieval Icelanders are genetically closer related to present day samples from the source populations than they are to the present day Icelanders.

In other words, the Icelanders have evolved faster.

No, this doesn’t mean that they are mutating faster or that selection has had a hand in this.  It can quite easily be explained by the isolation of the relatively small population.

Drift, one of the driving forces of evolution, simply works much faster on small (effective) population sizes than it does on larger sizes.

Drift is essentially a question of random sampling, and with smaller effective population sizes the sampling is more random than in larger effective populations.  This is very well explained in Razib’s post, so I will direct you there instead of repeating the arguments here.

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Agnar Helgason, Carles Lalueza-Fox, Shyamali Ghosh, Sigrún Sigurðardóttir, Maria Lourdes Sampietro, Elena Gigli, Adam Baker, Jaume Bertranpetit, Lilja Árnadóttir, Unnur Þorsteinsdottir, Kári Stefánsson (2009). Sequences From First Settlers Reveal Rapid Evolution in Icelandic mtDNA Pool PLoS Genetics, 5 (1) DOI: 10.1371/journal.pgen.1000343

16-27=-11

Tags: ancient DNA, Iceland, population genetics

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