Sperm are very different from all other cells
bbc.com
bbc.com
"To prevent this from happening, once a sperm cell has made contact with it, the egg quickly employs two mechanisms. First, its plasma membrane rapidly depolarises – meaning it creates an electrical barrier that further sperm cannot cross."
We are some pretty special and die-hard persons.
It also depends on how much of the sperm cell comes from the fathers genes, and how much is generated from the new DNA. I didn't find a clear answer to this but the following indicates that the new sperm cell is at least somewhat generated by the sperm DNA:
Because nucleotide recombinations can occur during meiosis I, the genetic code of chromosomes of gametes can differ from that of somatic parent cells (ie., progeny cells might express cell-surface antigens that are recognized by the host [the father] as foreign and thus be eliminated by humoral or cellular immune mechanisms). Occluding junctions that interconnect adjacent Sertoli cells shield secondary spermatocytes, spermatids, and spermatozoa from autoimmune recognition.
And I'm completely ignoring the mitochondria (Dad's copy are not passed on so should be mostly irrelevant to sperm selection pressures). I'm pretty ignorant of this whole topic - high school biology only.
The best sperm will likely result in the next generation of sperm also being good.
We look at the human as the organism, the sperm as the gamete - but perhaps our logic is anthropocentric - perhaps the sperm is the organism, and we are just the ridiculously elaborate reproductive mechanism.
I remember something like they may have sexual reproduction and even form blastulas, but the Wiki article is not very clear, so I may be misremembering.
That is the point behind Dawkins' The Selfish Gene.
The book well argues that genes are what reproduce, and that everything else is just complications to reproduce genes.
Humbling book - not light reading but I love it and I read it again and again because of the unobvious insights into systems.
but each chromosome could be "grandfathers" or "grandmothers", and usually those chromosomes have one or two crossover events, so, the chromosome goes FFFFFFFFMM for example. (where F = grandfather and M = grandmother)
So If you think about it girls get ~5% more genetic material from their father than boys.
this is probably why distribution of traits in men has fatter tails than in women.
Fascinating to think about the variability introduced by having only a single copy of the X chromosome. Lot's of interesting genes in there, MAOA/MAOB (primary neurotransmitter breakdown pathways), AR (androgen receptor), OPN1LW/OPN1MW (red green color blindness), G6PD, etc.
There are rare exceptions for genetic disorders relating to sex development, but generally speaking the above is true.
A lot of what's behind that "selection" there is still unknown; in principle all sperm are, more or less, the same.
There's also so many external effects in play that no single sperm cell may actually have a significant advantage over others; e.g. the behavior of the seminal fluid (ph, viscosity), the physical location of the egg, etc.
The cartoonish image of sperm swimming towards the egg is pretty much that ... a cartoon. In reality, they're pretty much drifting and their movement is much more like brownian motion than anything else [1].
Reminds me of this sperm race thing that took the spotlight a month ago, after watching the videos [2] ... come on, man.
Only someone who is extremely ignorant and/or is lacking severely on their mental abilities (bordering on idiocy), would believe that thing was true.
1: When the sperm is really close to the egg, however, there seems to be a hormone gradient that guides the sperm, preferentially, towards it.
Length of survival is dependent on factors largely put side of sperms control, but sperm lifespan does test for massive genetic abnormality.
So yes 99% never get a chance to compete, but meaningful competition still occurs.
It's the peacock's tail effect: what relevance does a brightly colored tail have to a male peacock's actual fitness?
IVF is also associated with congenital malformations etc. Though it’s hard to separate issues preventing normal conception from issues associated with IVF, it’s likely less viable sperm result in a less healthy fetus.
Otherwise clinically infertile men have children who are both healthy, capable and also not infertile.
Which is solid evidence that "sperm quality" is an extremely poor proxy for useful phenotypes in the resulting human being.
Extremely poor is a qualifier that isn’t backed up by your previous statements or actual scientific studies. Further births are very late in the process, rates of success is a critical metric here.
There’s significant research in trying to artificially create similar selection criteria because it increases the odds of a successful pregnancy and live birth. https://pmc.ncbi.nlm.nih.gov/articles/PMC7365522/
So if you still disagree, how about presenting some actual evidence.
Both reasons for the high count of sperm cells are true.
There must be many sperm cells to survive the adverse conditions, but there is also intense competition with the sibling sperm cells.
The DNA of the sperm cells is generated by a random generator, which is the meiosis mechanism, which randomly shuffles then randomly discards half of the father DNA.
The sperm competition then discards the random choices that happened to be bad, implementing an optimum search algorithm.
The sperm competition is only a first filter for rejecting bad random choices. Many embryos will die very soon, without ever developing, rejecting other bad random choices.
Only a very small percentage of sperm (less than 5%) are chromosomally abnormal. Meanwhile, the vast majority of sperm are morphologically abnormal in some way. So there's not really a tight relationship between genetic problems and sperm fitness. Men with infertility due to low motility, for example, are capable of having perfectly healthy children with those low motility sperm through IVF.
>The DNA of the sperm cells is generated by a random generator, which is the meiosis mechanism, which randomly shuffles then randomly discards half of the father DNA.
Meiosis also occurs in women (technically in the female fetus), but women generally produce only a single egg each ovulation.
>Many embryos will die very soon, without ever developing, rejecting other bad random choices.
A very large number of zygotes/blastocysts survive until implantation, upwards of 50%. And of those that do, maybe 20-40% are miscarried before 12 weeks. All things considered, about 1 in every 4 fertilized eggs results in a successful pregnancy.
So yes, it's absolutely true that the body filters out chromosomally abnormal germ cells and zygotes. But an egg is orders of magnitude more likely to survive than a sperm (even if you take into account the eggs that die in the uterus without being released). And the overwhelming reason is that the egg is simply in a much less hostile immune environment.
> A very large number of zygotes/blastocysts survive until implantation, upwards of 50%. And of those that do, maybe 20-40% are miscarried before 12 weeks.
That’s with sperm going to a filter process for genetic abnormalities before fertilization. If hypothetically 5% fewer zygotes survived that would have real consequences not just for humans but in terms of increasing variability in litter sizes for mammals with multiple births etc.
There is a competition between sperm from different males.
- You can reject 99.99% in thousands, not millions
- How is swimming fastest relevant to the genetic information quality inside?
It’s useless for all that multicellular goodness that separates humans from fish. But making viable single cells is a prerequisite for everything that comes after. DNA that can’t make cell walls etc can’t make a person as such there’s a host of genetic anomalies that don’t result in a fetus let alone a live birth.
Add luck doesn't explain millions either, it would sound the same if thousands dropped to hundreds
Also, it’s “fittest” as category not most fit as a ranking. Perhaps a better conceptual model is people who finish a marathon are a fitter group than those who start a marathon even if someone who failed was potentially in better shape than the winner. Many species have thousands of offspring because the odds any one of them reproduces is very low, a valid strategy not some sign of incompetence.
> Add luck doesn't explain millions either, it would sound the same if thousands dropped to hundreds
The actual cause is the full evolutionary history going back to the first life form and the environment each generation lived in, but we can still examine individual elements of what’s going on.
“Luck” is a multiplier on the number of sperm needed. The ability of an individual sperm to optimize its odds of success is a sign of cellular function. There’s zero contradiction between those statements. ~1:10,000 * ~1:100
> Many species have thousands of offspring because the odds any one of them reproduces is very low, a valid strategy not some sign of incompetence.
But having millions instead of thousands that would decrease the chances of survival (for example, by the offspring exhausting food resources and starving) would be a sign of incompetence.
Anyway, this is a different, simpler, argument, it explains "many with low chances", but doesn't explain 50 millions wether thousands would work just fine.
> There’s zero contradiction between those statements
My issue is there is zero connection. You can't justify millions this way.
> The actual cause is the full evolutionary history
That's not a cause, but a description of what happened. There's a lot of irrelevant info in that history. Also a lots of random things with no benefit
Again fittest is a classification. In Darwinian terms, the phrase is best understood as "survival of the form that in successive generations will leave most copies of itself."
After several generations it could be shown that a tree did have the form of the fittest without itself surviving.
> But having millions instead of thousands that would decrease the chances of survival (for example, by the offspring exhausting food resources and starving) would be a sign of incompetence.
Offspring of many species consume their siblings, millions is generally inefficient for other reasons. Some trees could have millions of viable offspring in their lifetime but random dispersal is really inefficient.
> My issue is there is zero connection. You can't justify millions this way.
I already did.
Odds multiply here, you want competition and also have to contend with random dispersal. 1 in 10,000 * 1 in 100 is 1 in 1 million. It’s very easy to look at the combination of multiple factors and see why millions is an efficient use of resources for animals as large as humans. Trying to argue for a single justification doesn’t work because multiple factors such as our size is involved.
> That's not a cause, but a description of what happened. There's a lot of irrelevant info in that history. Also a lots of random things with no benefit
Randomness is part of evolution. There’s ultimately no particular reason we got the particular version of various mirrored molecules that we did, but once that selection happened it was unlikely to change.
The actual process isn’t just the high level overview we talk about but the actual interplay down to individual subatomic particles. Multiple paths could have resulted in an organism with your specific DNA sequence but only one path actually did result in you existing and having your specific layout of carbon 14 nuclei etc. So argue all you want that an equivalent organism could exist, just realize you’re arbitrarily lowering the threshold between the actual process and a simplified abstraction.
> "This preservation of favourable variations, and the destruction of injurious variations, I call Natural Selection, or the Survival of the Fittest." – Darwin, Charles gqiyoh
> This preservation, during the battle for life, of varieties which possess any advantage in structure, constitution, or instinct, I have called Natural Selection
Dumb meteor luck doesn't care about preserving favorable, doesn't care about any advantage in structure, so there is no fit going on even if you constrain it to a binary classification
So coming back to your first comment, your understanding of "fit" doesn't help *at all* in "drastically cut[ting] down on the issues". You don't cut anything bad if you don't filter out bad/fit for good, but instead have dumb luck making dumb choices.
“preservation of favourable variations, and the destruction of injurious variations, I call Natural Selection, or the Survival of the Fittest”
“varieties which possess”
Fit in those quotes means having favorable variations and not having injurious variations, at no point does it refer to a specific organism’s survival.
> You don't cut anything bad if you don't filter out bad/fit for good, but instead have dumb luck making dumb choices.
Sperm which successfully fertilize an egg are more likely to have specific characteristics, that’s all that matters here as that alone results in survival of the fittest.
Sperm with identical characteristics die clinging to a sock on your bedroom floor, but they quite literally don’t matter because of the Trillions of sperm being produced. Humans are large animals we can take the minor metabolic hit to produce a for us a trivial number of cells each of which have extraordinarily low odds of success. Producing even more so fertilized eggs have a marginally higher probability of live birth is a winning strategy.
This is way too generic. Which specific characteristics? How are those "favorable"? How are "injurious" filtered out?
> Producing even more so fertilized eggs have a marginally higher probability of live birth is a winning strategy.
So why not billions? Now try it with the eggs. Why have a few instead of millions? Again, nothing you say here helps you get to 50mil because it's all too generic "the higher the better; not that expensive" argument quality. And you won't have a higher probability of live birth if your meteor discarding filter don't filter out the defects that decrease this probability
> but they quite literally don’t matter because of the Trillions of sperm being produced
No, they don't matter for this conversation because it's about the meaning in differences in the ability to reach the egg. If every cell has a 0% chance, there is nothing to discuss.
Mobility for one, within close distance to an egg the sperm which can orient on the chemical gradient beat out those unable to move.
> So why not billions?
Cost vs benefit
> Now try it with the eggs. Why have a few instead of millions?
Spending more resources on a few fetuses is the chosen strategy. Which would run into issues if both sides released millions of cells. The chemical signaling to abort a large number would be complex at implantation, much simpler to release a limited number of eggs.
Both of which you've failed to quantify, so you have no answer as to why stop at millions and not thousands or billions.
> Mobility for one
Your continue to stop at the most important part - relevance to your own criteria. If you care about probability of live birth or cutting out those unnamed "issues", how do tiny variations in mobility help? What's the mechanism connecting the two?
I already quantified it relative to the metabolic cost on an organism the size of a human. There’s a big difference to us between 10 calories (more specifically 5 to 25 calories) and 10,000 there’s not a big difference between 10 calories and 0.01. Thus we spend a relatively but not actually trivial amount of resources for minor benefit.
A fruit fly by necessity operates at a different scale. It would help if you read my posts here.
> How do tiny variations in mobility help.
Mobility is a complex test of a wide range of cellular machinery. Which means that cellular machinery works well.
Lifespan is similarly a great test for the ability to maintain cellular homeostasis.
You haven't, there isn't a single number in that reference. Even now you can't, you completely avoided quantifying the benefit and created a strawman for the costs.
> There’s a big difference to us between 10 calories and 10,000
What about the small difference between 10 and 200? That would move you from 50 mil to 1 bil. What does your "cost/benefit" formula say?
> It would help if you read my posts here.
The opposite - because I did and saw you unable to justify the results even after you tried to expand the original point several times. What would help instead if you tried to focus on a coherent argument instead of making up false claims about the other person.
> Mobility is a complex test of a wide range of cellular machinery. Which means that cellular machinery works well.
So again you have nothing specific to say, what you actually need to prove is that probability of live birth is part of that causal "wide range" of complexity. It could very well be that high mobility comes at the expense of that probability.
Analogy: you can make a car more mobile at the expense of driver safety/comfort while the same generic "mobility complex test wide range engineering machinery" would be true.
So wait now you want a justification for low millions vs high millions? Trying to move the goalpost isn’t an argument. You wanted justification for why not thousands or billions and I provided it.
That said, 200 is ~10% of daily calorie needs directly plus all the cellular machinery to produce sperm, that’s a big deal especially with some reserves for multiple ejaculations. Isn’t it interesting how science actually provides understanding here.
> you actually need to prove
IVF studies have proven this stuff, I’m referring to actual research here. There’s a ton of studies trying to recreate this kind of filtering. https://pmc.ncbi.nlm.nih.gov/articles/PMC7365522/
PS: That’s exactly the kind of thing I would have brought out sooner if I realized you where so emotionally invested.
That could have saved me some typing responding to your other comments.
Anyway, few would contest that being smart and strong is better than being smart or strong. Sperm can at least prove they're stronger.
But they can't prove that their future *human* is stronger!
That statement implies it’s a comparison between potential humans, which is not guaranteed at this stage. Sperm unable to result in live birth even if directly inserted into eggs exist in meaningful quantities.
Sperm are normally required to demonstrate they have traits required to produce a human thus making them categorically better from a biological standpoint than Sperm missing those traits.
Evolution is mostly gambling, and losing a lot.
Ignoring whether or not it would even be possible, a perfect CRC is antithetical to evolution itself, wiggle room for mutation must always be possible, but too much mutation gives you cancer and systemic malfunction. So you end up with these bizarre processes that allow just the right amount of imperfection.
With sperm specifically it ends up closer to a signature check than just a CRC, if the sperm doesn't exhibit behavior that falls under a certain umbrella of expected behaviors, it's rejected by the surrounding environment. The difficulty to comprehend it could even be a feature of the process in many respects, especially when you consider everything in this realm risks getting "hacked" if precautions are not put into place.
So when I see huge numbers like this, I see it as an indirect measure of the precision of the overall process. To put it another way, it's like brute forcing a password you don't know, but happen to have a lot of hints to (since obviously, we are all still the same species at the end of the day).
I’ve asked this question to multiple evolutionary biologists, and all of them answered “no” very strongly, strongly enough that I’m inclined to believe it. Apparently the frequency of deleterious mutations is many orders of magnitude greater than the frequency of beneficial ones, meaning there’s little chance perfect copying could be maladaptive. And in any event, evolution always selects for the fitness of the individual, not the species— group selection is a controversial topic in evobio, but the general consensus is that it does not happen, and that the rare things which kinda look like group selection (e.g. eusociality in bees) actually aren’t and can be explained without it.
To me evolutionary pressure isn't an on/off thing, it's like a signal in the noise. It's a vector with a direction and magnitude, facing varying levels of environmental resistance.
To be more specific, if there was enough "magnitude", evolution could potentially arrive at a perfect CRC. But the "resistance" requires a "magnitude" higher than evolution is willing to pay to reach that perfection. Likely in part due to the implicit complexity slope. Considering the systemic malfunction mutation can cause, one might assume this magnitude would be higher than it currently is. However, this is entirely speculation, and not falsifiable.
So when I think of evolutionary pressure I'm considering it as a component of the final vector, where a biologist might more pragmatically consider the total sum of vectors instead. This way of thinking is likely more productive for what they are doing.
As for evolution always selecting for the individual over the group, I'm surprised this is controversial when it's so obviously happening? If that was true how could multicellular organisms even exist? I'm very much not an expert on any of this, but this sounds like perhaps an over focus on DNA itself and not evolution as a whole, but maybe I'm misunderstanding something?
Which is… attempting a rigorous understanding of evolutionary biology, rather than idly ruminating.
I hate to use a dork-ism like “update your priors”, but this is actually maybe a situation where it applies? If you’re serious about a subject it’s more interesting to really incorporate the likelihood that you’re wrong than it is to wave it away as semantics or point of view.
Tell me this, which is more productive in an open discussion? My idle rumination? Or the lazy dismissal that lacks any substantive contribution to the ongoing discussion? I can't help but agree with your distaste for "dorkisms".
Meiosis, which produces both the sperm cells and the egg cells, is not a copying mechanism, unlike mitosis, which generates the other cells of a body.
Meiosis is a random generator, it randomly shuffles the DNA of the grandparents, which is stored in the father's cells, then it randomly selects half of it, producing a unique combination of genes in each sperm cell or egg cell.
The random genome generator together with the following filtering steps that will reject the bad variants, implement an optimum search algorithm for the fittest descendants.
Meiosis has greatly accelerated the evolution of the nucleated living beings (eukaryotes). Because favorable mutations are extremely rare, the probability of a living being accumulating multiple favorable mutations would have been negligible. With meiosis, if in a population 5 individuals have 1 favorable mutation each, there are good chances that soon some individuals will appear who have all 5 favorable mutations, then their descendants may become dominant and replace all others.
To me that seems more to me like a group of academics who just can’t see “how” it’d work. We’ve seen that in field after field where practitioners rarely look outside their own field.
There’s no “magic” in evolution to make it work only on individuals. Evolution is going to follow game theory. In some scenarios the evolution of the group will over time be far more adaptive than that of an individual. It’s a math question, not a biology question.
So is it just misunderstanding? Maybe they don't understand the question, or we don't understand the answer? Or they're hidebound, or incentivized somehow to be blind to the truth? Epistemological questions abound.
It's not that you need millions of sperm or that millions of sperm are competing, its that those sorts of numbers are necessary just to make it probably at least 1 sperm even finds the egg while it's still got energy.
Fun rabbit hole, in since cases they think it's the result of cells from offspring winding up in the wrong side of the umbilical but there are also cases where there was never a pregnancy in which case it has to be wayward sperm but that's absolutely bizarre and far too orthogonal to the sperms primary objective.
And AFAIK they don't have any idea of why this is beneficial to the woman or even to the man who created the invasive cells.