Bad math. A species may have a founding individual who is an ancestor of every member of the species, but not necessarily a "founding pair" who are the ONLY ancestors of the species alive at the time. The founding individual may carry a mutation characteristic to the new species, which every member of the species has, but MANY other individuals alive at the time of the founding individual could still contribute to the gene pool of the species. That is because the SPECIATION (reproductive isolation from the predecessor species) doesn't occur at the time of the founding individual, but later.
There's got to be one turtle on the bottom.
Simple tautology by way of deduction.