Sperm are very different from all other cells (opens in new tab)

Is this really the case? Do the fittest sperm become the best people? Feels like a streach

Not really, the reason so many sperm are needed is because a woman's reproductive tract requires an aggressive immunological response to foreign bodies (which sperm are). The vagina provides a direct route for pathogens through the cervix and uterus to the fallopian tubes (which can be scarred by inflammation resulting in infertility) and they themselves open up directly into the peritoneal cavity (potentially exposing a woman to septic shock or death if an infection reaches it). To protect against that, the vaginal environment is highly acidic, has layers of mucus that shields the cervix, and a high concentration of immune cells proliferate throughout. Men need to produce so many sperm because they need to be able to temporarily overwhelm these defenses.

This doesn't help much:

- You can reject 99.99% in thousands, not millions

- How is swimming fastest relevant to the genetic information quality inside?

I’ve actually had this question myself, “is there evolutionary pressure for imperfect genetic copying and repair mechanisms, since perfect ones would halt evolution and leave a species unable to adapt?”

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.

But there are also Microsatellites https://en.wikipedia.org/wiki/Microsatellite#Mutation_mechan... that are repeated sequences in the DNA. It's difficult to copy the correct number of repetitions, so they have a higher mutation rate. It's like like a controlled localized increased rate of mutation.

Sperm are actually quite good at finding the egg if they make it to the fallopian tube, they have their own chemoreceptors that can detect very small changes in chemical gradients, but only a few hundred make it through the preceding gauntlet.

Let the games begin.

There's this fascinating phenomenon called "micro chimerism"where somehow they've found that some of the cells in women's bodies are actually descendents of cells from past sexual partners, and they can be found in places far removed from the reproductive tract. the relation to your comment is that the tonsils and throat are known to be susceptible to this.

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.