Yes it has been shown and repeatedly. Pre-flood and even pre-Adam critters show about the same genetic diversity as today.
You, of course are free to stick your fingers in your ears and shout "Nah nah nah, can't hear you" or shut your eyes and shout "I can't see what you wrote" but honest people have and can.
You even took part in the Oetzi thread where genetics of humans, plants and animals that would have been contemporary with Adam had Adam existed were discussed and there are also studies from humans, plants and animals that lived 10,000, 20,000 even 50,000 years ago and nowhere is your nonsense supported. In fact, humans today show greater genetic diversity than those from over 50,000 years ago.
Anyone so limited that they can only spell a word one way is severely handicapped!