Monday, November 8, 2010

Inferred Background Populations for Tuscany, Northern Italy and Spain

Working from the possibility of three different and possibly combined migrations from the Fertile Crescent (Cypriot, Syrian and Southern Anatolian) to Tuscany, Northern Italy and Spain, it is possible to infer background populations for these groups.

The proportions for the Fertile Crescent populations and their inferred Southern European background proportions for Tuscans, Northern Italians and Spaniards are shown in the following table:
Table of Southern European Admixture Components

It is clear that Tuscans have the highest contribution from the Fertile Crescent (24 to 42%).  The contribution from the Fertile Crescent to Northern Italians and Spaniards appears to be less (between 10 and 20%).

It is interesting to plot the inferred background populations in terms of their Western Asian vs. Northern European components and in terms of their Southern European vs. Northern European components.

In the graphs, the Tuscan background population appears in blue, the Northern Italian in yellow and Spanish in red.

The three points of each triangle are inferred by Syrian, Cypriot and Anatolian migrations, in the proportions defined in the table above.  These points are labeled respectively S, C and A.

In the Southern European vs. Northern European plot, Basques, plotted as a star, are added as a point of reference.

What can these graphs tell us?

First, the Northern Italian inferred background population appears to be very similar to the Tuscan background population.  The difference is in the proportion.  We can also see that while it is possible that the sole West Asian contributor to both Tuscans and Italians came only from a direct Fertile Crescent migration, it seems likely that a small portion of the West Asian component in both Northern Italians and Tuscans came by a Danubian route.  The Myres et. al. and Curdy papers would indicate that this occured with Bronze and Iron Age migrations.  If that's true, then it implies that Danubian Farmers absorbed some West Asian populations either in the Bronze or Iron Age.   That seems likely, as even Lithuanians have a small proportion (7.4%) of the West Asian Component.  We can't know for certain when this West Asian component was absorbed into the Danube and Dnieper-Don migratory paths, but whatever the timing, some of this West Asian component acts to pull the "possibility space" triangles for the background population of Northern Italians and Tuscans off the zero West Asian axis.  By comparison, the Spaniard background population hugs the zero West Asian axis, indicating that they are less influenced by the Danubian Bronze and Iron Age Migrations.

The plot of the Southern European vs Northern European possibility space also hints at the difference in the population of Spain in comparison with Italy.  The Spaniard background population matches the Northern European position of Basques, but has less of the Southern European component.

The Italian background populations, by way of reference, trend toward having both less Southern European and less Northern European than Basques (and correspondingly more West Asian).  At the tip of the Tuscan possibility space, assuming an Anatolian Fertile Crescent contribution, the background population for Tuscans matches closely with that of French Basques.

It should be noted that the "possibility space" as plotted does not include uncertainly.  The populations described in Dienekes' ADMIXTURE results have a variance, and were that variance known, it would be possible to add uncertainty into the graphical description of these background population possibility spaces.

1 comment:

  1. Excellent blog!

    Thank you for the technical depth in your articles!

    The scientific analysis is greatly appreciated!


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