I think it’s funny that Brian Mansfield thinks I’m afraid to discuss evolution with anyone. Fine, Brian, you are herewith challenged to either a) a written debate on population genetics and neutral theory, or alternatively b) a discussion on Rebekah Davis’s channel. We’ll see who lacks what you describe as “the balls”.
“Vox Day tries to refute this on his web site but doesn’t have the balls to do it here.“
My account has been blocked on YouTube for seven years. For me to create a new account and post there would be a violation of their terms. I’m also personally banned from setting foot on Google’s Mountain View campus as it happens. So I will refute his woefully outdated critique and attempt to bring him up to date on the state of the art of his own field here. Anyone reading this is welcome to post this in its entirety in response to his comment.
“The only Kimura ‘equation’ I have mentioned is the probability of fixation of a new neutral allele — 1/2N — so he seems very confused.”
Yes, and that’s the problem. 1/2N is the fixation probability. It tells you the chance that a given neutral mutation will eventually fix, given unlimited time. It contains no time variable. It says nothing about when. The Kimura equation Brian should have used is Kimura’s equation for fixation time: 4Nₑ generations for a neutral allele, or t ≈ (2/s) × ln(2Nₑ) for a beneficial one. Without the time equation, his calculation has no temporal constraint. The fixation time equation is the Kimura equation that Brian omitted, and that’s the equation that produces the result he calls “ludicrous” because it’s the one that forces time back into a calculation from which it was omitted.
Everything Brian says depends upon k = μ applying, and he has obviously never examined the domain conditions under which that identity actually applies. It doesn’t apply over changes in either time or population. It also doesn’t apply to any sexually-reproducing species above approximately 10,000 census. Even setting aside the mathematical domain problem, comparing Bergeron’s (2023) pedigree-measured mammalian mutation rate against the required substitution rate from Yoo et al. (2025) gives k = 32.3μ, not k = μ. Brian doesn’t know the related literature and he hasn’t done the relevant math.
“For Vox Day to say that ‘genetic drift doesn’t happen in any population over 10,000’ is incomprehensibly stupid. Genetic drift happens in every population and his claim otherwise is mystifying. Population size only affects which alleles are effectively neutral.”
It’s only mystifying to those who don’t understand the relevant math and haven’t taken the limits of reproduction into account. What Brian is revealing here is that he doesn’t understand population genetics at all beyond the idealized textbook version. Population size absolutely imposes a hard limit on genetic drift. What he’s doing here is appealing to k = μ again, failing to realize it has been mathematically, empirically, and logically proven to be inapplicable to any real-world population, across more than 100 species.
“Empirically, each human zygote has about 100 new mutations. The question is how many of these are expected to drift neutrally to fixation — that is, they are neutral and they segregate independently of a selected variant.”
The question is not how many mutations are expected to appear or to drift to fixation under irrelevant steady-state assumptions. The question is how many can actually complete the journey to population-wide ubiquity within the available generations, given the actual population size and its history. Expected value under k = μ and realized completions under finite time are totally different quantities. Brian is treating the first as though it answers the second.
“If even just 2 of these 100 are neutral — which is certainly way under the actual proportion — then in a population of size N there are about 2×N new neutral alleles introduced each generation. The probability of fixation of each one is 1/2N. So, the expectation is that there will be on average 1 neutral fixation every generation if just 2% of new mutations are neutral.”
This is k = μ. Input rate × fixation probability = expected output. The algebra is correct. It is also an asymptotic steady-state identity that holds only when the fixation pipeline has been running at constant population size for at least 4Nₑ generations. For humans at variance Nₑ, that’s billions of generations. The pipeline is not full. The identity does not apply. He has derived the delivery rate of a full pipe without checking whether the pipe is full. I have actually checked the pipeline using the ancient DNA data. The empirical evidence supports my math, not his error-filled textbook assumptions. And he’s obviously not aware of Franco Chalub’s 2022 solution of the Neutral Kimura equation with two integral constraints or its implications.
“Day’s calculation of just six fixations over 9 million years is ludicrous. To get this he erroneously takes the time to fixation of a neutral allele, which is very slow. I assume this is the ‘time-related’ equation he refers to.”
The Probability Zero derivation does not use neutral fixation time. It uses Kimura’s fixation time for beneficial mutations: t ≈ (2/s) × ln(2Nₑ), at s = 0.001 (the empirical mean for beneficial mutations in humans from Zeng et al. 2021). This is faster than the neutral time of 4Nₑ, not slower. He assumed which equation was being used without reading the derivation, and he assumed incorrectly.
“This is simply incorrect. It assumes that one allele needs to get fixed before the next one can be considered. But, of course, that is nonsense and thousands of alleles are drifting around independently at the same time.”
The MITTENS calculation does not assume sequential fixation. The LTEE rate of 1,322 gen/fix is a total throughput measurement — the aggregate output of twelve independent populations running every evolutionary mechanism simultaneously, including concurrent sweeps, neutral hitchhiking, and clonal interference. It is the net output after parallel fixation has already occurred. There is no sequential assumption to remove, because the rate was never derived from one.
The Probability Zero derivation at s = 0.001 does compute a per-fixation time, but dividing total generations by per-fixation time to get maximum achievable fixations is not an assumption of sequential processing. It is also a throughput calculation, the same arithmetic you use when you ask “how many jobs can a machine complete in eight hours if each job takes twenty minutes?” The machine can run multiple jobs simultaneously, but total output is still bounded by total time divided by per-unit processing time, adjusted for parallelism. The LTEE rate is the parallelism-adjusted rate.
“His entire premise is a farce and the use of fixation time is an error. The only thing I can guess is that Day seems to think that only 1 allele can be undergoing fixation at any time because if an allele reaches fixation then there are no other alleles in the population.”
He is guessing because he has not read the argument. The Hard Limits paper explicitly addresses the parallel fixation objection under the heading “The Second Objection: Fixations Run in Parallel,” devotes several pages to it, derives the transient fill fraction F(T) = exp(−π²Nₑ/T), and demonstrates that even multiplying by every mutation at every site in every individual across every generation produces an expected number of completed fixations with tens of millions of zeros after the decimal point. The objection he imagines is devastating is the one the paper conclusively refutes.
“So there can be a fixation event every generation at one of these loci, followed by another the next generation at a different locus and so on.”
Only if the pipeline is full. This is the critical point. A pipeline with a 4Nₑ-generation transit delivers one fixation per generation at steady state — that’s k = μ. But reaching steady state requires the pipeline to have been loaded for 4Nₑ generations at constant population size. For humans at variance Nₑ, the transit time is on the order of 10¹⁰ generations. The human lineage has had 252,000 generations. The pipeline isn’t partially empty. It’s functionally nonexistent and empirically confirmed to be empty. The one-per-generation delivery rate he’s invoking is the far-end output of a pipe that hasn’t been fed long enough to deliver anything.
“It would be a remarkably stupid mistake for Day to make, but it is all I can think of that would allow him to get to his impossible conclusion of 6 fixations over 9 million years.”
It is all he can think of because he isn’t aware of the problems with the textbook and he hasn’t read the paper that specifically addresses them. The six-fixation figure (now seven at updated parameters) comes from the beneficial fixation time at s = 0.001, not from neutral drift. The parallel fixation objection is addressed explicitly and closed with a quantitative proof. He has constructed his own version of the argument that would indeed be stupid, attributed it to me, and then refuted his own construction. This is textbook strawmanning.
“The real number is closer to the number I provided, but of course it depends on the real fraction that have been neutral in ancestral populations, which is certainly much higher than the low end estimate I give here.”
His number comes from k = μ. k = μ requires a full pipeline and a steady-state. The pipeline cannot fill above the drift ceiling. His number is zero at any census population above ~10,000. The fraction of neutral mutations is irrelevant to an identity that doesn’t hold.
“And his formula for the probability of 20 million changes is set up to calculate the probability of just one set of 20 million fixations, another colossal blunder on his part.”
There is no blunder at all. Either the specific fixations matter — in which case the Darwillion applies — or they’re interchangeable — in which case they’re neutral noise that can’t explain the observed functional divergence. And furthermore, the drifting of 20 million neutral substitutions is blocked by the Hard Limit and the fact that k != μ in humans.
