Nonsense. Adam couldn't possibly contain all the genetic diversity of humanity within him. He was only one man, and only had the DNA of one man. It's not as if Adam had more chromosomes than us, or as if his genome was bigger or longer than ours. Adam did not simultaneously possess all the genes coding for blood types A, B, O, AB. Adam's blood was not simultaneously Rh positive and negative. There are 35 different blood group systems identified so far (Rh and ABO being the 2 basic systems everybody knows about), with over 600 identified antigens so far, and there's no way a single individual or pair of individuals could contain that much diversity. Nor could Adam have the genes coding for every inch of possible human height, from the shortest to tallest, nor the genes for every type of fast-twitch and slow-twitch muscle fiber, every hair colour and hair type, etc. That's simply not how DNA works. Adam had the genes for his blood type, not all the other thousands of blood type combinations. Therefore---in the proposed YEC scenario---the different traits and diversity we see today must've evolved afterwards, based almost entirely on mutations to Adam's genomes. We're not just talking about selection of existing genetic material; we're talking about new traits arising, which we observe still arising in human populations today.
Therefore, no matter whether you believe in a young earth or an old earth, you've got to admit that the human genetic diversity we see around us is (mostly) due to mutations, because Adam and Eve can't have contained it all. No two people could contain it all. The only question is how fast those mutations happened and how long the process took. If the world and the human race are old, then it's simple: the mutations happened at the same speed we observe them happening today. But if the world is young, then human diversity must have evolved orders of magnitude faster. The same goes for all the supposed animal "baramins" on the ark. If there were only two cats on the ark, and they gave rise to all the vast feline diversity we see on planet earth during just 200-400 years after Noah's flood, then that presupposes mutations and evolution on an incredible scale, far beyond anything that evolutionists themselves believe happens.
That's one of the big problems with YEC: in order to avoid evolution over millions of years, they're forced to propose a theory of hyper-evolution over just a few centuries!
*throughout this post the term 'Adam' refers to the the YEC understanding of an historical Adam created as the sole biological male progenitor of the human race.