Human populations today tend to be diverse rather than homogeneous. There is good reason to believe that much of Europe has a history of diversity rather than homogeneity as well. The statistical derivation of a "cluster" or "reference group" that is relatively genetically homogeneous is therefore, to some extent, an artifact of the methodology.
Cluster methodology, for example, automatically places the samples in relatively homogeneous groups, regardless of where they came from, and regardless of how much diversity was originally present in, say, "Scandinavia". It's up to the analyst to discern what degree of clustering is appropriate for the application and what the clusters finally selected actually represent. We can look at the methodology in many different ways, but the "reference groups" will always be proxies or substitutes for the real long-dead ancestral populations that we cannot sample.
Cluster methodology, for example, automatically places the samples in relatively homogeneous groups, regardless of where they came from, and regardless of how much diversity was originally present in, say, "Scandinavia". It's up to the analyst to discern what degree of clustering is appropriate for the application and what the clusters finally selected actually represent. We can look at the methodology in many different ways, but the "reference groups" will always be proxies or substitutes for the real long-dead ancestral populations that we cannot sample.
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