GC: Evolution comes with change, biology is not infallible and immutable and can change, therefore sex is not immutable and can change?

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[–]Britishbulldog 5 insightful - 1 fun5 insightful - 0 fun6 insightful - 1 fun -  (4 children)

Evolution: the advantageous (or neutral) alterations that occur in an organism due to DNA mutations over millions of years. An organism doesn’t wake up one day with a radical change in its genome. With regards to sex, humans (and most higher organisms) reproduce sexually. This is to mix the DNA of two people, which at the population level gives higher genetic variation than an organism that reproduces asexually. Higher genetic variation means an organism is more capable of adapting to environmental changes. Sex is crucial to this- sex evolved to do this very function. Humans won’t evolve to not have sexes anymore. Further to that, genes unrelated to sex are stored on the X chromosome, such as clotting factor VIII (this is why haemophilia almost always affects males). The X chromosome contains 5% of the whole genome, so it would take a mass translocation to get rid of the chromosome. This would involve genes and non-protein coding genetic elements (promotors, enhancers, suppressor regions) to also translocate. This fusing of chromosomes only happens in cancer cells (specifically where cellular mechanisms preventing this have failed).

Your point on cancer is interesting (mainly because I’ve done oncology at uni and it is depressing as heck but also incredibly interesting). Almost all types of cancer occur after reproductive age. Evolution is basically the adaptation of the genome to selective pressures over millions of years. Organisms adapt to reproduce, any changes affecting life past are reproductive age have no evolutionary benefit. A theoretical mutation X that prevents all cancers (this is actually impossible as cancers are highly diverse) would not affect who reproduces and who dies before this point enough to increase the frequency of mutation X. The obvious exception to this is childhood cancer but these are almost always due to an existing mutation, either inherited or a mutation in the gametes, and fortunately it is not common enough to have an evolutionary consequence at the population level with respect to mutation X.

[–]Not_a_celebrity[S] 2 insightful - 2 fun2 insightful - 1 fun3 insightful - 2 fun -  (3 children)

Thank you! I got another question. Since evolution comes with change, how come over all these millions of years there hasn't been a single organism that produced a third gamete, therefore no third sex ever existed and will ever exist? Why only 2 sexes? And not 3 or more sexes in all organisms, humans included? I learned from you why there are 2 sexes instead of no sexes at all, but I can't understand why only 2 sexes and not more than 2 sexes?

[–]Britishbulldog 6 insightful - 1 fun6 insightful - 0 fun7 insightful - 1 fun -  (2 children)

I’m not completely sure tbh but I’ve got a few thoughts. Humans are diploid. That means we have 23 pairs of chromosomes. Each pair contains different alleles of the same genes (except the sex chromosomes). Meiosis the process by which a gamete is made. Gametes are haploid (1 set of 23 chromosomes), so when 2 fuse, you’re back to 2 x 23 chromosomes. So the number of sets halves to then double back to the ‘correct’ amount. This couldn’t work with 3x23 (you can’t split a chromosome).

The maths would work if we were tetraploid (4 x 23), but there is a practical aspect. Reproduction in mammals is almost always penis (supplies male gamete) in vagina (supplies female gamete)- the gametes actually come from other parts of the sex organs but for simplicity it can be assumed as above. How the heck four gametes from four individuals would come together in such a way I’ve no idea.

Interestingly there are organisms that don’t have 2 sets of chromosomes. The only organisms with 3 sets are sterile crop cultivars as they are incapable of meiosis. Asexually reproducing organisms often have one set. There are organisms with 4 or 6 sets but these are always plants, often specific cultivars. Some sugar cane plants have twelve sets.

[–]Not_a_celebrity[S] 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (1 child)

This is very interesting to me. Do organisms that are tetraploid, and more, have a third sex, a third gamete, or are they always sterile?

[–]Britishbulldog 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

Triploid are always sterile or asexual. They lack different sex-determining genes. The one example of a tetraploid animal I found (Australian burrowing toad) still only has two sexes, so I’m not 100% sure how that works.