for me it’s not about metaphysics, it’s that genomics doesn’t work like that
the reason we can observe microspeciation in single cellular organisms is not because of the short lifespan/reproduction, it’s because of the size of the genome
when you’re working with say, idk I’ll pull from experience and go with a bacteriophage with a genome of about 40,000-70,000 base pairs
across a 47k bp genome, you might have 70 or so genes. each gene is coded with a start and stop and the lengths are variable.
a single nucleotide variation might do nothing, or change the protein structure, or be fatal. mutations are most often negligible or fatal.
on the next level, that single variation can change the codon. a codon is a group of 3 nucleotides which forms an amino acid. this can be negligible, or change the function of the codon. a mutation causing a stop codon to form mid gene is another fatal mutation.
a different nucleotide often will not change the amino acid, because there are multiple ways to form the same one, at which point the coding error is corrected and everything forms as usual.
in a small gene, a different amino acid can cause somewhat significant structural changes. these often do not lead to any phenotypic variation however, or they are typically suboptimal and die out.
the rate at which these mutations are retained is significantly higher in single cellular organisms because they reproduce asexually. in the absence of a new, untarnished set of genetic data, the cell will more likely repeat the same error
but we’re already at some pretty unimpressive odds for the occurance of mutation, that mutation then not being negligible or corrected, and then subsequently being inherited
when you raise the degree of complexity, this already meager number gets blown to astronomical improbability
the genome of a dog is 2.5 BILLION base pairs, the length of a single gene is larger than the entire genome in the bacteriophage example. there are far more moving parts, which means need for more significant change to affect anything and a higher potential for fatality if there is a structural change.
things don’t form part way slowly, the idea that hands to feet was a gradual change makes no sense when you consider that any in between stage is unusable. there’s no proto-foot, and there’s no reasonable way for that to form except a sudden massive change. and then that has to be passed down, which is anyone’s guess as to the probability of that because it depends entirely on the structure of the gene itself and the fact that most large scale change is affected by more than one gene
and when you go from asexual to sexual reproduction, you are now battling the fact that you have two sets of DNA and that in the presence of mutation or transcription error DNA will often be pulled from the other X chromosome.
no matter how generous you are with your probability estimate, and even if the earth is as old as the most generous estimate, there’s no way we could possibly have had enough time
because no amount of time will ever be enough to accidentally go against all laws of nature on a macro scale
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