I believe that some that believe in the Great Flood may also believe that mutations and therefore 'new information' can be added. Call it microevolution. So I guess new alleles are fine, its just the big changes as a result of multiple genes having evolved that many have trouble believing. These are not my beliefs, I'm jsut paraphrasing from what I've heard.
Nonetheless, you are correct that the genetic diversity that exists in terms of the number of alleles (and recently I heard the number of copies of genes that humans have) doesn't make much sense after only 4500 years of evolution.
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