Well, thankfully, that theory cannot be tested, can it?
Why "thankfully"? Because it would be a problem for your dogmatic beliefs?
Well,
thankfully, it isn't so much a theory as it is a fact.
Consider this examples: remove 99% of all dog breeds and only keep great danes and st bernards.
Do you think mating them will produce a chiwawa?
See? Killing 99% of the population means losing 99% of the genetic variation. I used dogs as an example because of the many many breeds. It makes it easier to understand.
But even within the same breed, this remains factual. Within single species, there exists a LOT of variation.
Consider another example....
The human races. Black, caucasian, asian, aboriginals,...
Now kill everyone except a handfull of asians.
Do you think they will have black or white descendants?
See?
And the same goes if we go a level deeper still. As deep as the actual genetics.
We even see this in human populations. When humans migrated out of africa, the largest population of human stayed behind. The group that migrated, had far less genetic variation then those that stayed behind.
This loss of variation is so extreme that we take dna samples of a random white french person and a random chinese person, there is actually LESS genetic variation between them then between 2 random (unrelated) black people in
the same city in Africa.
So, to conclude...
Yes, this is very testable.
And the results are not in favor of biblical stories.