Descendants, common ancestors, and intermarriage

Almost exactly four years ago, I posted my analysis of the descendants of Queen Victoria and Prince Albert. At that time, there were 836 of them living; now, my source, Allan Raymond, lists 875, but hasn't updated since May last year. It's not that I find the Royals terribly interesting; but I think they provide useful evidence in favour of the fascinating findings of Rohde et al that the most recent common ancestor of all living humans lived only a few thousand years ago, probably in south-east Asia. (One can debate whether this also applies to apparently isolated populations like the North Sentinelese, but they may not be quite as isolated as reporters like to claim, and also are actually in south-east Asia.) A related statement, which I think is strongly supported by all the available evidence, is that all of us with European or part-European ancestry are descended from Charlemagne. (A paper published last year finds it probably that all Europeans share common ancestors from within the last 1000 years; Charlemagne died in 814.)

One point that was made in discussion of my previous post was that the number of marriages between relatives must surely slow down the overall rate at which one's ancestors increase going back in time, or descendants increase going forward. Examples were given such as the Grand Dauphin, who had only four great-grandparents rather than the usual eight, his parents being first cousins and his grand-parents being two pairs of siblings. But these cases are very unusual. I revisited the Victoria and Albert data, and found that while indeed about a quarter of their living descendants can trace more than one line of descent back to them, this ratio has not increased much over the last forty years. This graph shows the overall trend in multiply-descended individuals as a percentage of all individuals descended from Victoria and Albert since the birth of Prince Waldemar of Prussia in 1889:


As you can see, it starts rather low, then zooms from about 5% in 1933 to about 23% in 1970, rising rather more gradually in the decades since – actually falling at the end of the last century, before picking up to the current 27%. I note that in my previous post, I had the average rate of annual increase of descendants at 2.6%, but the rate since 1970 at around 1.9% only, as if a quarter of the overall rate of increase had disappeared; it's interesting that this decrease is close to the overall ratio of multiply-descended individuals. The 900-odd current descendants of Victoria and Albert may represent up to 1200 possible lines of descent.

I suspect that the ratio will never again rise as quickly as it did from the 1930s to 1970. The era of such dynastic intermarriages is basically over. There have been 21 marriages between descendants of Victoria and Albert, including Liz and Phil, and the king and queen of Spain, but the most recent was in 1981 (Prince Andreas of Leiningen and Princess Alexandra of Hanover). Only two others have taken place in the last fifty years (King Constantine II of Greece and Princess Anne-Marie of Denmark in 1964, Prince Franz Wilhelm of Prussia and Grand Duchess Marie of Russia in 1976). This compares to seven such marriages in the 1930s, when the overall pool of descendants was only one-seventh its current size.

Just as the Grand Dauphin is a bit of an outlier among dynastic practices, the inter-war royals were much more likely to intermarry than their post-war children and grandchildren, and I suspect much more likely to intermarry than the general population at any time. Given that the intermarriage factor has failed to slow down the growth in the total number of V&A descendants by much, despite the flurry of marriages between cousins in the 1930s, I think it's pretty clear that this does very little to change the likely date of a most recent common ancestor for Europeans, or indeed for humanity as a whole.