Theoretical ancestors vs. the local marriageable population
We are all the product of successful connections — successful in a simple biological sense. The local marriageable pool — what population genetics calls the effective population — is determined by the marriage horizon: how far a person could realistically travel to find a partner. Before turnpike roads, that horizon was 10–15 miles. The pool is a product of three coupled factors: how many settlements fall within that radius, how large they are, and what fraction of their inhabitants are of marriageable age. Change any one and the ceiling shifts; change the underlying technology of movement and all three shift together.