Theoretical ancestors vs. the local breeding population
We are all the product of a successful connection — successful in a simple biological sense. The pool of possible partners — the breeding population, in the technical register — is determined by the coupling horizon: how far a person could realistically travel to find one. Before roads, railways and air travel, that horizon was probably 5–10 miles, with longer journeys rare. 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 will go on to have children. Change any one and the ceiling shifts; change the underlying technology of movement and all three shift together.