In my long post below, Celts to Anglo-Saxons, in light of updated assumptions, I had a “cartoon” demographic model in mind which I attempted to sketch out in words. But sometimes prose isn’t the best in terms of precision, and almost always lacks in economy.
In particular I wanted to emphasize how genes and memes may transmit differently, and, the importance of the steps of going between A to Z in determining the shape of things in the end state. To illustrate more clearly what I have in mind I thought it might be useful to put up a post with my cartoon model in charts and figures.
First, you start out with a large “source” population and a smaller “target” population. Genetically only the migration from the source to the target really has an effect, because the source is so huge that migration from the target is irrelevant. So we’ll be focusing on the impact upon the target of migration both genetically and culturally.
To simplify the model we’ll imagine a character, whether genetic or memetic, where the source and target are absolutely different at t = 0, or generation 1. Also, these are discrete generations, and the population is fixed, so you can presume that it’s at carrying capacity. Migration of the outsiders into the target population from the source means less of the original native population in absolute terms (to be realistic this is bidirectional, so people are leaving the target too, but that’s not our concern here).
There are two time series which illustrate the divergent dynamics on both the genetic and memetic dimensions. In one series you see gradual and continuous migration from the source to the target population over 13 generations. In another there are two generations of massive migration, before and after which there is no migration. For the genetic character, imagine disjoint allele frequencies at generation 1. So at generation 1 the target population is at 100% for allele A, while the source is at 100% for allele B. Therefore migration of from the source to the target results in a decrease in the proportion of allele A, which is what is being measured on the y-axis. For the memetic character, imagine that it’s language. So at generation 1 100% in the target zone speak language A, while everyone in the source zone speak language B. Again, the frequency on the y-axis is of the proportion who speak language A in the target zone.
As a follow up to the previous post I’ve spent some of this weekend looking for the results which might shed some more light on the genetic impact of Germans on the British landscape between ~500-600 A.D. There are some problems here even assuming all other conditions are met: Northwest Europeans are already genetically rather close. This does not mean you can not distinguish an Irishmen from a German, but, once you take into account the clinal variation in gene frequencies due to isolation-by-distance models it becomes somewhat difficult to ascertain admixture in the spaces in between. After all, populations spatially between northern Germany and Ireland, the Brythonic Celts, are presumably going to be genetically between these two populations as well. That being said, space is not the only variable. Culture is another. The Celtic dialects spoken in Ireland and Britain are extremely distinctive, being of two branches of the broader language family, but there was still a clear resemblance between the two cultures (e.g., druids and other aspects of religion). In other words, the Celtiberians of Spain, the Galatians of Anatolia, the Gauls, and the various peoples of Britain in Ireland, all traded in a common cultural currency. Not only are they likely to have had some common ancestors, but there is going to be a dialect continuum across much of this cultural zone, facilitating continuous gene flow. And speaking of continuity, the fact that Britain is separated by water from the European mainland and Ireland is at least something that works in favor of genetic inferences, as that is a structural constant which mitigates in favor of distinctions and between group variation.
As I stated below, the ideal case would be a data set where you compare genetic variation as a function of geography within the British Isles (with linguistic information intact, so that Welsh speakers are labeled) against a north German reference. The problem with a pooled Northern European reference is that there’s also a “Finnic dimension,” which is pretty much irrelevant when you’re comparing Germans to British. And usually there isn’t the fine-grain that you’d want. Irish are not the ideal proxies for Brythonic Celts. Welsh, or perhaps Cornish or individuals from Devon, might be.
One of the peculiarities of the synthesis of 19th and early 20th historical linguistics and biological anthropology was the perception by many British thinkers that the English, as the scions of the Anglo-Saxons, were fundamentally a different race from the Celtic nations to their west, the Welsh and Irish, and the Scots to the north (yes, I know the Scottish nation emerged is a mix of Celtic and Anglo-Saxon elements which were preponderant at different times and periods). In other words English nationalists would characterize their own race as a branch of the German peoples. English was a Germanic language, and the linguistic chasm emphasized more starkly a distinction from the Celts who inhabited Britain prior to the arrival of the Germans, and gave the island its name before they were marginalized and pushed to the “Celtic fringe.”
The historical context of this does not need to be elaborated in detail. The Emerald Isle’s integration into the United Kingdom of England, Wales, Scotland and Ireland was always a difficult affair. This was due in large part to religion (the lack of an effective Irish Reformation may have had other structural causes); the Irish were a Roman Catholic populace at a time when Roman Catholicism and loyalty to the monarchy were presumed to be contradictory. In 1800, before the potato famine and the English demographic explosion, Ireland accounted for one third of the population of the United Kingdom (I do not put much stock in the linguistic difference, as the Welsh speaking regions were firmly Protestant and so not perceived to be sources of equivalent dissension despite their cultural marginality). With the rise of taxonomic science what was a crisp social chasm was reconceptualized as a biological and evolutionary gap along the Great Chain of Being.
In the twentieth century the tide turned, today most scholars would assert that the shift from Celtic to Anglo-Saxon speech and culture in what became England was a matter of emulation, not genetic replacement. Personally I suspect that the pendulum has swung too far, but it does show how strongly influenced by fashion these sorts of preconceptions are.
Modern genetics can clear up the confusion to some extent. A new paper in The European Journal of Human Genetics surveys samples from Dublin, the south & southeast of England (the heart of Saxon Britain), Aberdeen, Portugal, Bulgaria and Sweden. Population structure and genome-wide patterns of variation in Ireland and Britain. I’ll just focus on the figures of interest in relation to the questions I aired above.