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Comment by mcherm | Hacker News Reader

I think an alien looking at life on earth might find it very peculiar that so many of our life forms split into 2 independent parts that are unable to reproduce unless they match up with a partner of the opposite "sex". This is no stranger than that -- but it's fascinating to us who haven't encountered it before.

The "sex" thing turns out to have big advantages: mostly that it allows for faster evolution by allowing mixing of genes between the two different genders so new advantageous traits can be spread through the population much faster than if each organism simply copied itself. I'll bet this scheme has advantages too -- the smaller size of the components (useful for getting through various membranes and other defenses) is an obvious one, there may be others that are less obvious.

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Might be strange that we have sexes. Might also be strange that we only have 2 sexes. Many plants have two different generational types (haploid and diploid). Meaning each "generation" alternates between two different organism types. Fungi have both sexual and assexual reproduction [2]. But their sexual reproduction occurs when two cells directly fuse and then undergo meiosis into haploid spores instead of a diploid organism. Haplodiploidy occurs in insects where males are haploid by females are diploid [3]. Sex can be much more complicated than what it is in humans. Alien species may have even more complicated systems for genetic recombination.

[1] http://sciencelearn.org.nz/Contexts/Ferns/Sci-Media/Animatio... [2] https://www.boundless.com/biology/textbooks/boundless-biolog... [3] https://en.wikipedia.org/wiki/Haplodiploidy

> Sex can be much more complicated than what it is in humans.

Completely arbitrary anecdotal evidence would suggest otherwise.

GP was referencing reproduction, not politics or recreation.

Fair. I've never actually attempted it in a reproductive manner, so I have no evidence there.

Fungal sexual reproduction has more than two mating types, as well, which is apparently made possible by handling the mitochondria differently from animals. In animals, the two parent cells fuse and the male's mitochondria is destroyed. In fungi, the nuclear material is transferred between cells, and incompatible mitochondria never occupy the same cell.

So a species could have one type or thousands of mating types, and may be homothallic or heterothallic. Meaning that individuals of the same type could be compatible, or not, respectively.

In a dimorphic animal species, if you wander the wild and encounter a random member of your species, it is about 50% likely you could reproduce. For some species of fungus, a random encounter could be as high as 99.999% likely to be compatible.

We don't even need to imagine aliens for this. We have species right here on Earth that are incredibly different from us, and also available to be studied.

> Many plants have two different generational types (haploid and diploid).

For the vast majority of plants, these are not physically separate organisms. The key difference here is that the haploid cells of the plant can themselves undergo mitosis, which animal gametes cannot do. Since we generally consider cells with different genetics to be different individuals, we speak of plants as having alternation of generations.

For the most part however, the haploid cells live in the diploid organism or vice versa, depending on which branch of the kingdom you're considering. It's not like botanists run tests on a particular specimen to determine its ploidy. For any given species, we know what the ploidy of the individual is. For example, flowering plants are all diploid. If you find a flowering plant, the individual has two sets of chromosomes, but somewhere in its tissues there is a haploid cell (descended from the parent) that is undergoing meiosis, and is thus a separate organism.

I doubt it. I suspect that for sufficiently complex creatures, sex is the rule not the exception. The benefits from genetic crossover are so powerful from an evolutionary survival point of view, that it very much outweighs the cost of coordinating said crossover. With asexual reproduction, one small shift in climate, and your entire species risks extinction. There's just not enough genetic variation to deal with all the possible ways that local conditions can change on you.

Crossover is so powerful from an algorithmic sense, that you don't even need spot mutations for a genetic algorithm to converge on a solution. And crossover algorithms will tend to converge on solutions much faster than algorithms using only point mutations.

Basically from the books I've read on the topic of Artificial Life, there tends to be a very strong pull to species developing 2 sexes in simulations. Rarely are there more than 2 sexes though since the cost of coordinating more actors isn't offset by the marginal benefit of having more gene diversity.

There was one ALife experiment I heard of that managed to converge on 5 partnered sex, though I'm not sure if that meant the species had 5 distinct sexes, or if you needed 5 genetic donors to guarantee reproduction.

I'm not near my library, else I'd link some of the books on the topic that I really liked.

Two of my most favorite books tries to shine light on some of the different aspects of your (first) question. Maybe the unexpected findings during the pursuit of this question will be as fascinating to you as it was to me.

The Mating Mind: How Sexual Choice Shaped Human Nature (2000) (Geoffrey Miller)

The Red Queen: Sex and the Evolution of Human Nature (Matt Ridley)

https://en.wikipedia.org/wiki/The_Red_Queen:_Sex_and_the_Evo... https://en.wikipedia.org/wiki/Geoffrey_Miller_(psychologist)...

"The maintenance of sexual reproduction in a highly competitive world has long been one of the major mysteries of biology given that asexual reproduction can reproduce much more quickly."

https://en.wikipedia.org/wiki/Evolution_of_sexual_reproducti... [NSFW: animal porn]

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