Yes, I'm aware of what a hypothesis is. I am claiming that your hypothesis already has existing evidence against it that needs to be accounted for (and has not been). Moreover, what you are framing as a null hypothesis is most certainly not one. In order to shift the burden of proof, you need substantial grounds to falsify the old theory - the old theory being that any person from a literate society could perform the same feats of verbal memory as a person from a non-literate society because there are no intrinsic differences in memory. If you have invalidated this claim, then it has escaped me. If not, then you are very much working with an alternative/experimental hypothesis.
I don't think you quite realize the scope of what you're claiming here - for a gene that selects for memory to be wiped out in every literate civilization in no more than a few thousand years implies a massive evolutionary pressure to the contrary. This would be a massive, groundbreaking claim. And to imply that there's no evolutionary trade-off would actually shake the foundations of evolutionary theory, since there must be a huge difference in reproductive success. Genetic drift (the common cause of loss of a previous adaptation) simply doesn't work that quickly. And, moreover, a significant fraction of the world's population is only a few centuries removed from living in pre-literate civilizations, which most certainly hasn't had time to exert substantial evolutionary change, let alone through drift. This is part of the evidence that works against the evolutionary change claim. It doesn't mean it didn't happen, but it makes it very, very improbable.
What you just made is a positive claim, your positive claim being that "any person from a literate society could perform the same feats of verbal memory as a person from a non-literate society because there are no intrinsic differences in memory."
I provided an example of the incredible feats of memory that non-literate societies are capable of. You need to provide a counter-example of the same from someone in a literate society.
Also, again, the evolutionary difference proposition was a case of me innumerating the potential for difference between literate and non-literate societies. You are entirely glancing over the part where I followed that up by saying an evolutionary difference may not even be necessary to explain these differences, and it could just be a simple matter of childhood development taking advantage of neural plasticity.
This argument is also dragging things significantly off from the original point, that original point being that there is a qualitative difference between the memory of a non-literate person in a non-literate society, and a literate person in a literate society. This difference can be supposed to be compounded if that literate society was in turn born from another literate society (say, the US from England,) and that in turn from another literate society (England from France and Germany) and that in turn from another literate society (France or Germany from the Western Roman Empire).
This is a long enough line of literate societies that it would be more strange to suppose we are the same than that there is likely some difference that has developed. For instance, in Kenia, they practice a coming of age ritual in which at age 13 they rub mud on a young boy's face and then let the mud harden. Then, they subject the child to some manner of physical pain. (I don't remember this clearly, but I think it was actually a circumcision ritual.) If the mud on their face cracks as a result of them cringing from the pain, their tribe judges them as cowardly and the women of the tribe judge him as an unfit mate.
This, over the generations, has sexually selected for pain tolerance. This pain tolerance explains why Kenyans do so well at Olympic level running. That's a rather ground breaking qualitative difference in evolution right there, and it doesn't take many generations to develop.
This doesn't prove there is an evolutionary difference. It just opens the door for the possibility, and the number of generations separate between a western culture and a non-literate culture are so vast that if you take my example of Kenya into consideration then you would have to be pretty wrong-headed to claim that we of western civilization are genetically identical to people in non-literate cultures.
My only actual claim though is that the tests for westerners prove nothing about how the memories of a non-literate culture would perform, and you have to re-do all the tests on those non-literate cultures in order for the models to be accurate.
Now, do I have to pull up other examples such as how people who are born blind have larger temporal lobes than people born with eyesight? That would be the other non-genetic angle from which to approach this. Because your claim is that we are identical to people from non-literate cultures and my negative claim is that we are not identical, I can attack this from literally any angle. If you don't like the genetic example, then I'll give it to you and start on the neural plasticity route since you are getting so hung up on that.
EDIT: (Also, it is not hard to imagine what the negative pressure for memory would be. Memory is a pretty resource intensive process. Therefore, the negative pressure is constantly applied and requires a positive pressure to equalize it. If there is no longer a positive pressure selecting for memory, the simple removal of that intense positive pressure by offloading the need into written word would be enough to allow it to degrade so those resources can be re-allocated to something else... such as creativity.
This edited-in argument BTW can apply equally to both the evolutionary as well as the neural plasticity explanations. For the neural plasticity angle, how many people over the age of 30 can remember their friend's phone number today? Now, same question, when you were 10, could you have remembered your friend's phone number back then? The simple introduction of the smart phone seems to have altered our memory capabilities for phone numbers quite a bit within the same lifetime. This isn't even childhood neural plasticity anymore. This is adult level neural plasticity, which is far lower than a child's.)