The comment got me thinking. In a nutshell, pedigree collapse is where ancestors marry distant relatives (and sometimes not to so distant relatives), resulting in a narrowing of the family tree the higher up you go. Instead of having an exponential amount of direct ancestors, some ancestors will be found in more than one spot of your family tree.
This brings to mind a comment made at one of the sessions I attended at last year's Atlanta Family History Expo. In the session Breaking Down Brick Walls with Location Based Genealogy, presenter Bernie Gracy said that prior to the advent of faster methods of travel, ancestors often stayed in the same area their entire lives, marrying other people from that area. Since area residents were often family members of some degree, this resulted in intermarrying with family members.
If you trace your family tree far enough back, you will see that pedigree collapse is bound to happen at some point. As the number of possible direct line ancestors grows exponentially, the number will soon reach an amount that is higher than the world's population at that particular point in time.
In the end, you will find that certain ancestors have more than one relationship to their descendants. This is especially true of royal families where spouses were selected based on noble blood, which inevitably meant that the couples were related in some way.
Examples of pedigree collapse in my tree include
- My grandfather's third wife (I descend from his first) married into the family several times. My grandfather was one marriage, another was to his first cousin and a third marriage was to a man I'm fairly sure was his first cousin once removed.
- My great grandfather's niece married my maternal great grandmother's brother. The children from this marriage are my double first cousins twice removed.
- I have several instances of siblings of one family marrying siblings of another family.
- My 3rd great grandfather second marriage (I descend from his first) was to the niece of his brother's wife. Two of his wife's sisters married brothers of his daughter-in-law.
- My 2nd great grandparents were second cousins.
- My 7th great grandparents were first cousins.
When I was in elementary school, I remember we did a segment on local culture and history. We learned how to milk a cow, churn butter, weave on a loom and make candles. At the end of the week, the mother of one of my classmates played her banjo and sang folk songs. Some were sad; some were serious. The one that cracked all of us up was "I'm My Own Grandpa," a ballad documenting the series of marriages that led to the song's author becoming his own grandfather.
Further Reading on Pedigree Collapse
Family Tree Magazine: What is Pedigree Collapse?
International Society of Genetic Genealogy Wiki: Pedigree Collapse
Ontario Genealogy Society: Pedigree Collapse