You can plate a single organism and watch it divide itself into two then four then sixteen, etc, etc. By the time you have a few hundred thousand organisms all from the same originator you will see quite a bit of genetic variation within the population; even though they started out as the exact same organism with the exact same genome.
Even in the case of sexually reproducing organisms you would have to believe that Adam and Eve contained within themselves all the genetic variability of the entire human race in order to deny that this variability came about due to imperfect replication. In other words one would have to assume that Adam and Eve were simultaneously blue brown green and hazel eyed, lactose tolerant and lactose intolerant, had epithelial folds on their eyes and did not have them, had kinky hair and straight hair, had dark skin and light skin, etc etc.
Seeings as how within two individuals there are only four possible allelic differences at any loci (Adam has trait A and B at that loci and Eve has trait C and D), and far more than four alleles at many loci have been described among human populations; it is simply impossible that the entirety of variability in the human genome could be contained within two human individuals.
re: Adam and Eve. I believe we have a topic for civil discussion here.
Yes, I do believe that Adam and Eve had all the genetic information necessary to produce every variation we see today - remember the 1 with the 2017 zeros behind it - the number of possible genetically non-identical possible offspring. The number of hydrogen atoms in the universe is estimated on the order of 10^80th, just for comparison.
I’ve seen pictures of twins of “medium brown” people - one child white and blond, the other brown skinned/brown haired.
I also believe that the harmful variations, the harmful replication errors, mutations, even death itself, are a consequence of the Fall.
If you don’t want to assert Man’s wisdom (”reason”) over God’s literal Word, then there was no death, disease, mutation, or predation (all green plants were for all to eat) before the Fall. This precludes “theistic evolution”.