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Biodiversity is a result of semi-randomly combining the genes of two parents, which allows for mutations that can provide benefits. In order to have a sufficiently diverse population to persist, you need a certain minimum number of individuals to avoid being wiped out by lack of diversity. For example, the modern banana is basically all a single set of chromosomes copied over and over, so it is now at risk for a single fungus wiping out the whole population.

So my question is this: Would having three sexes increase or decrease biodiversity in a population? Stated another way, would a 3-sex species be able to thrive with a lower minimum threshold of genetic diversity, or would it actually need a higher population to guarantee stability?

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    $\begingroup$ How do your three genders function? Do all three need to contribute genetic material to every offspring? How do their chromosomes work? If you have three genders, but gender differentiation is based on environmental factors and they all have the same chromosomes, for example, you'll have a very different answer than if each has a unique sex chromosome. Gender is also commonly used to refer to a socially constructed identity, so 'sex' might be a less ambiguous term for this question. $\endgroup$ – ckersch Nov 2 '16 at 21:20
  • $\begingroup$ Three distinct sets of chromosomes contributing genetic material to the zygote (each giving 1/3 of the complete information). Thanks for the tip too. $\endgroup$ – thanby Nov 4 '16 at 15:53
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Note I am assuming all other factors that can impact biodiversity are equal and I will refer to the third gender as zemale.

Would having three genders increase or decrease biodiversity in a population?

Depends, if they get three copies of each gene, then yes it would increase biodiversity. If only two of the three parents pass along their genes, then it is a break even.

Would a 3-gender species be able to thrive with a lower minimum threshold of genetic diversity, or would it actually need a higher population to guarantee stability?

Depending on how you implement the third parent's role you may need more individuals to cover the genes that third gender provides in addition to the first two. In other words if you have men to pass along the Y-Chromosome you will need zemales to guarantee the diversity of the Z-Chromosome.

A way that it can be implemented to need less individuals is to allow hermaphrodites in your species, which also helps simplify managing three sets of genes. One way it could work is that X, Y and Z chromosomes contain only gender specific elements and so members of the species can have three Y chromosomes and still be viable. Which ever of the three you get are the genders you get. Some examples:

X X X = Female

X X Y = Male & Female

Z Z Z = Zemale

Z X Z = Male & Zemale

Y Y Z = Female & Zemale

X Y Z = Female, Male, and Zemale

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  • $\begingroup$ Interesting ideas. I think a simpler way to handle it would be to have the combinations be XX, XY and YY (as the "zemale") for a set of three, so that would cut down on the number of possible sexes. But I'm guessing there's a reason nature hasn't done that...? $\endgroup$ – thanby Nov 4 '16 at 16:07
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    $\begingroup$ If the Y-Chromosome suppresses female gender, then why not the X Chromosome suppresses zemale gender. That would only leave having the zemale gender suppressing male gender somehow. $\endgroup$ – Anketam Nov 5 '16 at 3:13
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No. Adding additional sex chromosomes won't increase genetic diversity.

The benefit of sexual reproduction lies in the fact that a population can create offspring with any combination of chromosomes that are present in the population. An asexually reproducing population, on the other hand, can only create offspring with a set of chromosomes that is already present in the population.

The important thing to keep in mind is that the sex chromosome is only one of a large number of chromosomes. Just like offspring receive one of two sex chromosomes from each parent, the offspring will receive one of each other chromosome from each parent, as well. Which chromosome the offspring receives from each parent for non-sex chromosomes is independent of which sex chromosome they receive from that parent. Two parents with the same AB heterozygous chromosomes for all chromosomes but their sex chromosomes, for example, can produce offspring that are any combination of homozygous (either AA or AB) or heterozygous for all of their non-sex chromosomes. Adding a third sex chromosome won't increase the size of your potential gene pool, since it's already 'all of the genes'.

On the other hand, if three individuals with three different sex chromosomes must be present in order to reproduce, your species is more likely to go extinct at small populations, since if all of any of your sexes die off, your species will perish. This is compounded by the fact that, should a genetic defect exist on one of the sex chromosomes, the population of individuals with that sex chromosome will be smaller compared the portion of the population with each sex chromosome in a two-sexed species. (i.e. 20 individuals rather than 30 in a total population of 60.) This will make it more difficult to breed out that defect, since the population of chromosomes for that specific sex chromosome will be smaller. A defect will also be more likely to wipe out your species because it makes up a larger portion of a smaller pool of individuals.

On the other hand, a sexually reproductive species with only one chromosome-based sex will be more robust in the face of extinction than a two-sexed species, since any two individuals in the population will potentially be able to mate and produce offspring, rather than requiring a male and a female.

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  • $\begingroup$ Interesting point about the one-chromosome-based sex, that makes a lot of sense. Are there any examples of that in nature that you're aware of? I know we have some species that can actually change their sex in response to environmental conditions, but I've not heard of one that sexually reproduces with only one sex chromosome. $\endgroup$ – thanby Nov 4 '16 at 15:58
  • $\begingroup$ @thanby Bees and ants determine sex based on the number of chromosomes an individual has, rather than based off of an allosome. Males are haploid and females are diploid. Clownfish and other sequential hermaphrodites don't have allosomes, since their sex isn't controlled by their genetics, and earth worms are hermaphroditic, and as such also don't have allosomes. $\endgroup$ – ckersch Nov 4 '16 at 17:07
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I'm going to assume the genders of the creature at hand works the way the majority of sexual genetics works on earth. I am also going to assume all three genders are required for reproduction and that all three contribute equal genetic material.

First off I feel compelled to say that for genetic diversity reasons this would never evolve. Even further, any kind of need for something akin to a "caste" system is generally handled through non-reproducing offspring. This can commonly be seen in social insects and even in naked mole rats. Without getting too in depth in the varies theories for how genders evolved I'll only say that reading "The Selfish Gene" by Richard Dawkins would be a good place to start in understanding why a tri-gendered species wouldn't evolve.

With that out of the way, if such a species existed, then there would indeed be an increase in genetic diversity. In humans, and indeed in most multicellular species on earth, we have two sets of each chromosome. Each is the result of a long list of processes that occurs during sexual reproduction that I won't go into here. If we added a third individual to the group then it would follow that three of each chromosome would be present. I won't presume to know how that would work out, mostly for the fact that if you were to simply scale up on the two chromosome norm it would be hideously impractical. This, of course, would also break the standard dominate/recessive paradigm. Perhaps three tiers of genetic assertion? Dominate/passive/recessive? The combinations, of course, could lead to interesting results. I'd like to get into some detailed examples but I fear that I could make a mistake and/or everyone reading would get bored!

The conclusion: given the assumptions stated at first, adding a third gender would quite radically increase biodiversity. A slightly inaccurate means of getting an idea of how large an increase this would be is if we said that 10^n where n is the number of genders is the measure of biodiversity.

(I apologize for typos and poor structure, this was written quickly.)

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  • $\begingroup$ Welcome to the site! Great first answer. I'll have to check out that Dawkins book. $\endgroup$ – thanby Nov 4 '16 at 16:04
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Despite what is said in other answer, the answer is yes, provided all 3 are required in creating an offspring and contribute genetic data.

The reason is simple. You have more mixing going on and more/different mutations happening. It's the same reason that 2 parents is better than 1. You aren't adding more chromosomes or a triple strand DNA structure. All you're doing is saying that the chunks of DNA have 3 sources to draw from, rather than 2.

As far as how to create them... Chromosomally you can just have XX, XY, YY, and YX all be different, but there in insects it is done by pheremones and what they eat. There is technically something like 3 to 6 in a number of insects, only 2 mating sexes.

I would also argue that there is some evidence to suggest there are more than 2 sexes in humans, but like with many things we humans have to be weird. I'm not talking about hermaphrodites, but rather that we humans have certain dispositions that lend us towards taking on roles that could be argued are similar to insect castes (sexes) save for that all members of our species are capable of mating. Either way, this is why multiple sexes do not result always in more genetic diversity.

Oh! Also, you might also be interested in chimeras which isn't quite the same thing, but close. The basic information about it is that females of a lot of species are actually chimeras of one sort or another, taking on the DNA of another member of their same species as a normal part of them. This is where Calico cats come from. Their fur is 3 colors because the patches of the 3rd are from parts of them that are actually another cat. Human women have this phenomenon too which you can see in patches of skin being different shades is the commonest, but more rare is that full organs are from other members of the species such as in one case where the eggs/uterus where generated from a different DNA. Women also absorb some male DNA when they have kids that sticks with them. It's an interesting subject ^.^ And you could make the argument that this chimera DNA is actually a "3rd sex" as it can be a 3rd person in the genetic diversity which doesn't fall into what we would normally place in the normal expression of a person. Weird, but meh.

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