Let’s consider some ideas I’ve written about before.
A genetic testing company can in theory define a certain consensus genome as “Puerto Rican,” and it is therefore possible in that case for typical Puerto Ricans to test out as 100% (or 99% or 92 % or 90%.etc.) Puerto Rican. But if the same genome is interpreted in light of the major continental population groups (races), then someone who is “100% Puerto Rican” would then be, e.g., 50% European, 40% sub-Saharan African, and 10% Amerindian. Same genome, different definition.
The gene frequencies that define “100% European” today would not be the same as, say, several thousand years ago. Genetic ancestries which back then would have to be classified as non-European would today be part of the European genome, as intrusive elements have been assimilated. A haplotype today defined by 23andMe as “British/Irish” or “French/German” or “Italian” or “Balkan” may be by the standards of, say, 2000 BC, mostly European (by 2000 BC standards) but with some non-European components (again by 2000 BC standards).
All of this does NOT mean that ethnicity and race are “socially constructed;” those are biologically real and legitimate entities. However, the labels used to describe those entities are defined by people and these labels, and these definitions, can change over time. This also NOT mean that we should blithely accept intrusive elements entering the European genepool today; genetic interests are future-oriented, and what happened in the past affected the genetic interests of the people at that time, not us today. We are what we are, just like they were what they were. One set of changes does not in any way obligate or justify a different set of changes. Everything must be considered in the light of the interests of the people that exist at that time.
The major point, that most “movement” nitwits do not understand, is that people cannot take the labels given to particular gene frequencies or haplotypes at face value in the sense of an absolute and precise entity whose specific definition has the exact same meaning over long periods of time. The actual degrees of genetic kinship based on the raw genetic data are real and concrete and precisely meaningful – to the extent we take into account statistical error and the over-riding importance of parental populations (that is tied into the topic of this post…what is the parental population you use to define “100% X?”) – but the defining labels are not necessarily so.