A reassessment of genetic diversity in Icelanders: Strong evidence from multiple loci for relative homogeneity caused by genetic drift


Helgason, A., G. J. Nicholson, et al. (2003). “A reassessment of genetic diversity in Icelanders: Strong evidence from multiple loci for relative homogeneity caused by genetic drift.” Annals of Human Genetics 67: 281-297.

In the article published in 2000, Helgason and his colleagues compared mtDNA variation of the Icelaners to that of other European populations and argued that Icelanders experienced bottleneck recently.  They observed large θπ but small θk and θS among Icelanders and argue that genetically heterogeneous Icelanders reduced their genetic diversity after the historic population declines.  Arnason (2003) responds to Helgason and his colleagues and says that mtDNA variation of Icelanders is product of admixture during the founding or history of Iceland. 

In this 2003 article, Helgason and colleagues respond back to Arnason, and they show that admixture and genetic drift affect mtDNA sequence diversity differently.  If a new population was founded by two genetically distinct populations, the new population is expected be genetically more diverse than source populations.  However, they show that when a population was founded by two genetically similar populations (for example two European populations), the new population has Tajima’s D and cumulative frequency of polymorphic haplotypes which are intermediate between two source populations or are very similar to those of source populations.

On the other hand, the populations that experienced reduction in effective population size, so genetic drift, have reduced number of rare haplotypes and higher number of haplotypes in intermediate frequencies.  In the end, they concluded that number of distinct haplotypes (k), number of polymorphic sites (S), θk, θS, and Tajima’s D are sensitive to recent demographic events, so genetic drift reduce number of distinct haplotypes (k), number of polymorphic sites (S), θk, and θS.  The haplotype diversity (heterozygosity) and mean pairwise nucleotide differences are less sensitive to recent demographic events and are less likely to be affected by genetic drift.

I believe this is a very interesting case study showing how a historically known event may have affected mtDNA variation of modern Icelanders, and the results of this study can be applied to other genetic studies investigating mtDNA variation of anthropologically interesting populations that may have experienced recent bottleneck.  Reconstruction of past demographic events among these populations tends to be challenging, because they lack written documents.  Unfortunately, probably because their focus is Icelanders, they did not address how you can tell the genetic signature of admixture from demographic expansion.

References:

Árnason, E. (2003). “Genetic heterogeneity of Icelanders.” Annals of Human Genetics 67: 5-16.

Helgason, A., S. Siguroardottir, et al. (2000). “mtDNA and the origin of the Icelanders: Deciphering signals of recent population history.” American Journal of Human Genetics 66: 999-1016.

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