A major study of the genetic structure of the European population now in press fleshes out and confirms what we already knew from several smaller studies. This is the key graph, with labels for the major population clusters kindly added by Razib Khan.

To summarize very quickly, this study and others like it look at thousands of single-nucleotide polymorphisms (individual spots where DNA differs among humans) scattered throughout the whole human genome. They get the data from members of different populations and use principal component analysis to reduce this mass of data to two or three numbers for each person which can be easily plotted. People from the same population cluster more or less together, and populations that would be expected to be more distant are farther from each other on the graph.

This particular study looked at 500 000 SNPs and several thousand individuals from almost two dozen European cities. You can see that some of the populations overlap, for example the British and Irish, but with this genetic information we can would never confuse an Irishman with an Italian. The populations that would be expected to be more distant are more distant - actually, this isn't that far off from a fuzzy map of Europe as the northern lands are towards the top of the graph and the eastern towards the right.

Also note that there is some genetic difference between regions within countries, though nothing really major. Some countries' data clusters more tightly than others but that does not necessarily mean that there is less genetic variation within those countries. It only means that there is less genetic variation within the particulra city from which that country's sample came from.Then there are a few outliers - makes you wonder if their ancestors included recent immigrants or maybe a handsome Gypsy.

The Finns are pretty distant from everone else, but before concluding that Finns are completely separate from Europeans I'd like to see another study which also includes Russians and at least one Baltic nation. I suspect they'd bridge the gap between the Finns and Poles. The Hungarians, though, in spite of being culturally and linguistically distinct, are genetically very close to their neighbors. It generally looks like language isn't a major barrier to genetic transmission - for example the separation between the Germans and Northern Slavs or between the Greeks and the inhabitants of the former Yugoslavia is not that great.

As new papers come in, we will get a more detailed and accurate picture of the degree of relationships between various nations. I'm personally especially curious how tightly groups such as the Sardinians and Orcadians cluster and how close they are to mainland Europeans.

Several of the earlier, smaller studies are open access, meaning you can read more than just the abstract without paying. Here are some links for those interested in further reading or just looking at more graphs.

Tracing Sub-Structure in the European American Population with PCA-Informative Marker - this one includes a comparison to some Asians and Africans. Figure 2 is a great one to show anyone who says there is no biological basis for race.

Measuring European Population Stratification with Microarray Genotype Data - including some data from Africa, India, Central Asia, and the Middle East

Analysis and Application of European Genetic Substructure Using 300 K SNP Information

Discerning the Ancestry of European Americans in Genetic Association Studies