This seems like a sort of non-story; what it takes for there to be a new family tree is someone entering the field without an advisor in this dataset (i.e. prob

This seems like a sort of non-story; what it takes for there to be a new family tree is someone entering the field without an advisor in this dataset (i.e. probably an advisor from a different field) and then taking on PhD students, etc. And that seems unlikely at different points in time for a variety of different reasons.