So follow along with me in this thought experiment: Consider yourself as the trunk of your personal ancestor tree that gets bigger as time goes back and you have 2 parents, and 4 grandparents etc, and if you go back n generations, then there are 2^n ancestors. Not all of those ancestors are known to you, but nevertheless you DID HAVE an ancestor in every slot of your pedigree. If one were to go back enough generations(n) to where the amount of ancestors(2^n) at the time was larger than the total number of people even living in the appropriate cultural location of the world at the time, clearly this basic exponential model breaks down. It's a good enough approximation for most small family trees, but insufficient for large ones. That's where pedigree collapse comes in. Pedigree collapse is essentially a nice term and synonym for inbreeding. Pedigree collapse happens in everyone's family tree, and it's a mathematical fact and a certainty that we are all inbred. So get over it. It is a fact that every human being on Earth is a cousin to every other human being on Earth. Your parents were cousins before they were married. My parents are cousins. Everyone's parents are cousins. My wife and I are cousins. YOU are MY cousin, and I can say that with 100% certainty without even knowing who you (the reader) are. Pedigree collapse occurs at every union of man and woman, so the inbreeding issue is not whether we are related, but only how closely are we related. For kicks, here's an article I saw that explains how we're all much more closely related than most realize. So mathematically we understand why pedigree collapse must happen, but what does that actually look like in a family tree? Let's consider the pedigree of one Anna Gevedon - a 4th great-grandmother born in 1822. Her parents were first cousins. And each of her parents' parents were also cousins with each other. And as you go back further, I have trouble even describing it. It's easier just to see with a chart:
Now that's a pretty close family. This is back in Kentucky/Virgina by the way. So how many Ancestors does Anna Gevedon have? Well she has 2 parents and 4 grandparents - so far so good. But she has only 6 g-grandparents instead of the usual 8. She actually has nine g-g-grandparents instead of the usual 16 (Try to count them). And 5 generations back she has 16 ancestors instead of the usual 32! This is what pedigree collapse looks like. Anna Gevedon could have had 32 different unique ancestors 5 generations back, but she doesn't - she has 16. Of course she has 32 slots to be filled in on her pedigree chart, it's just that multiple spaces are filled by the same individuals. In the case of Anna Gevedon's ancestors, I think we can agree inbreeding was a little excessive. But how much is excessive? What does that even mean, and compared to what? To assert that inbreeding is "excessive" is just an opinion coming from the values we've had given to us from the society we are presently in.. and also the marriage laws of whatever State you're from. But society hasn't always been the same way it is now. Consider people that lived 500 years ago or more. People traveled much less back then, and many people actually lived out their entire lives within a few miles of where they were born. Not to mention there were less people in total. Furthermore, literacy and record keeping weren't good, so people may not have even known they were marrying a relative. So for a multitude of reasons, it is certain that inbreeding was much more common in the past than it is today. For the aristocracy too; they especially had the incentives to marry other aristocrats and keep wealth in the family. In a society where everyone was looking to marry "up" in social status, the result can be seen in the genealogies. Within a family tree, inbreeding manifests itself as massive pedigree collapse during the time of aristocratic rule in Europe, and then even more so further back in feudalism where land holdings were tied directly to politics. Remember people didn't marry for love, but Kings married off their daughters to other Kings for the purposes of political alliance. This brings up an important detail about my family tree project. Who does history remember well from 1000 years ago? Aristocrats. Who does history NOT remember so well from 1000 years ago? Everyone else - the peasants and regular folk whom made up the majority of the population. Any further back than a few centuries ago, and my tree is populated almost exclusively by the rich and the affluent. Deep-time family trees like this have what I call Aristocracy Bias or Royalty Bias. The data available to genealogists and historians is heavily biased in favor of kings, queens, lords, dukes, earls, barons etc. That does not mean that we aren't descended from the whole lot of peasants and everyone else across a population. Of course we are. We just have little hope of tracing down their names or relationships because that information is forever lost to the depths of history. So the poor were marrying amongst themselves and having children in small local communities, not travelling very far, while the aristocrats and royalty were marrying amongst themselves separately from the poor, and travelling much further on average. As royalty from different regions all strove to interbreed with each other, they also had more surviving children than did the poor, but only a portion of those children born to wealthy parents remained in the wealthy gene pool. Typically the eldest male child inherited estate, while the other siblings had to make their own way in life, and their descendants tended to trickle down into the ranks of the poor. Seen from the perspective of a genealogist, the aristocracy was like a one-way machine, generating a constant out-flux of progeny with trees of descent that spread out into the lower class and then fade away. It appears that the reverse did not tend to happen. Within my tree, rarely do we ever see it show anyone of common origins rising up into the ranks of the wealthy. But then again, if that did tend to happen how would we know? Since history doesn't record those ancestral lines very far. So it is important to understand that while history remembers the rich and forgets the poor, inbreeding and pedigree collapse occurred in both.
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