This magazine article was published by Ancestry Magazine in their November/December 2008 issue and is available at their site at: http://www.ancestrymagazine.com/2008/12/features/welcome-to-the-family/ .
Welcome to the Family
If you see the same ancestors popping up more than once in your family history research, don’t automatically assume you’ve made a mistake. It’s entirely possible that your ancestor’s uncle could also be her husband—all because of a phenomenon known as “pedigree collapse.”
Pedigree collapse occurs as marriages between cousins and other relatives made our family trees smaller than we might assume. For instance, if first cousins were to marry, their family trees would collapse by 25 percent because they share one set of grandparents, giving them only three sets between them, rather than the usual four.
The term was first introduced nearly 30 years ago by Robert C. Gunderson, a former member of the Genealogical Society of Utah’s Royal Identification Unit. According to his math, if you started out with two parents, four grandparents, eight great-grandparents, and so forth, you would have had some 281 trillion ancestors alive during Charlemagne’s time. But that was in 800 A.D., when the world’s population was only about 200 million. Somewhere those lines have to cross—there just aren’t enough people otherwise.
University of Tennessee distinguished professor Dr. George K. Schweitzer explains: “It’s a very simple idea. When we go back a certain number of generations, the number of parents you and I had exceeds the population of the earth, which of course is impossible, so the explanation is there [are] cousin-to-cousin and even brother-tosister marriages.”
Numerous circumstances contribute to pedigree collapse, particularly as you go further back in time, says Schweitzer. Two of these are geographical and class selection. With geographical selection, occupants of small villages were limited to a pool of potential spouses who lived within a distance that could be traveled on foot. In class selection, certain situations, including wealthy landowners wanting to keep property in the family and royalty marrying within the limited selection of other royalty, helped lower our ancestral numbers.
Schweitzer uses 1870s backcountry Tennessee as an example. “The people only sparsely settled the land,” he says. “How many young men are of marriageable age nearby? A young man is not going to walk too many miles to court a woman such that he can walk back home and go to bed and then do a full day’s work the next day.”
The family of King Alfonso XIII of Spain (1886–1941) is a more extreme example. In 1999, researcher John Becker noted in the Ontario Genealogical Society’s journal Families that royal intermarriages left Alfonso with a scant eight great-great-grandparents instead of the expected 16—a 50 percent pedigree collapse.
So what happened? Among other examples of intermarrying, Alfonso’s grandparents, Francis of Spain and Isabella II of Spain, were first cousins. Their uncle and aunt by marriage were also their uncle and aunt by kin. In other words, two brothers from one family married two sisters from another family—and then their resulting off spring, who were cousins, married, too.
Schweitzer has seen pedigree collapse happen in his own family, including a great deal in his German lines in the
1500s and in his English lines in the 1550s and 1570s and several other examples in his family line in the late 1700s. It’s normal, he says. “Anybody who has [delved deeply into] their genealogical lines frequently begins to run into it about that time.”
Science writer Steve Olson, whose own research points to each of us sharing a common ancestor as recently as 3,000 years ago, notes that “About 15 to 16 generations ago, everyone in a population that has descendants living today [would be] the ancestor of everyone living in the population [today].”
“Pedigree collapse and common ancestry are the same thing.
… Cousins are people who have a common ancestor, and when people with a common ancestor have a child, that common ancestor shows up in more than one spot on the child’s family tree,” says Olson. Eventually, he indicates, if we’re all related, it has to happen to everyone.
Olson focuses on deeper ancestry, digging into early migrations of large groups. “People think of populations in the past as being very isolated,” he says, “but there was trade, war, slavery, and other things that have happened throughout history.” It all made us much more connected.
But don’t worry if your football-shaped family tree is a bit unnerving. Says Schweitzer, it happens to the best of us—and to all of us. “[Pedigree collapse] is not only possible the further you go back, it’s probable. If you go further back and don’t see any, then I suspect your technique.”
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