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I’ve seen a lot of questions on here about the possibility of more than two sexes but I’ve found nothing on the possibility of only a single sex. The closest thing I’ve found is isogamy, is there any way to scale that up into complex sentient beings? Or is there another way?

Question: Is it possible for a species to have a single sex? - As in no male or females only one reproductive type.

(As much more neatly put in the comments by Slarty: There is just one sex but two organisms of the same type are required to reproduce.)

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    $\begingroup$ I'm a little confused. Are you asking whether something like this is feasible for a specific species? Otherwise, hermaphroditism seems to answer your question. $\endgroup$ – HDE 226868 Oct 23 '17 at 19:06
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    $\begingroup$ Let me introduce you to the all-female lizard, the New Mexico whiptail. $\endgroup$ – Azuaron Oct 23 '17 at 19:07
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    $\begingroup$ I think this question needs to be clarified. As I see it following the edit, the two key points are: there is just one sex but two organisms of the same type are required to reproduce. $\endgroup$ – Slarty Oct 23 '17 at 19:31
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    $\begingroup$ have you considered snails? They are mostly undifferentiated physically, though some do have behavior (safe for work) that leads to only one of two carrying offspring at a time. $\endgroup$ – user25818 Oct 23 '17 at 19:47
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    $\begingroup$ I would like to refer to HDE 225858 , hermaphroditism seems to fit your description see. answers.com/Q/Are_earthworms_hermaphrodites $\endgroup$ – P Chapman Oct 23 '17 at 19:54

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Remove the Y chromosome

I'm going to use mammalian humanoids since that's easier for my brain to work with.

Regardless of whether there's a single sex or multiple sexes, the embryo always has to start from an egg. The egg must have plentiful resources for growing and must contain all the DNA required to mature into an adult organism. Lacking either of these characteristics, the egg won't fair as well as an egg that does have these characteristics.

If the author requires two parents, then it's not possible to choose "males" since male gametes lack the cellular infrastructure to grow into a fully-formed embryo. Thus, we have two egg donors that mate (somehow). Which or both of the parents gets to be pregnant is left as an exercise for the author.

Fertilization

Somehow (handwaving) the two eggs find each other and fuse together. Some mechanism transfers DNA between the eggs and that DNA is recombined with the host egg's DNA. After DNA transfer is complete, the two eggs separate to find their own spot on the placenta. The process of becoming an embryo begins in earnest. As a result of this process, pregnancies are almost always twins. Although, depending on how the rest of the organism is designed, resorption (WARNING: icky pictures) to produce only a single offspring may be possible.

Conclusion

With two egg donors, it's entirely possible to have a species that only has a single sex.

There are some very interesting implications on nurturing juveniles with this system. Females with non-precocious young usually spend a lot of time caring for and protecting their young while males go off and do things (namely, get other females pregnant.) Since both parents are the same sex and are both liable to get pregnant, I suspect a more communal child rearing environment would work better.

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  • $\begingroup$ It would be simpler and closer to existing reproductive systems for two haploid eggs to fuse to produce a single diploid embryo than for two diploid eggs to exchange DNA and subsequently form two embryos. $\endgroup$ – Mike Nichols Oct 23 '17 at 19:42
  • $\begingroup$ @MikeNichols see? You're using terms I haven't look at in a decade. :) Either way, two isomorphic gametes turn into one or two viable offspring. $\endgroup$ – Green Oct 23 '17 at 19:45
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    $\begingroup$ @Green Pregnancies almost always becoming twins is actually very good for my species. They live in an ant-like colony sort of way were most inside one colony are identical to one another. $\endgroup$ – Axolotl Oct 23 '17 at 20:00
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I think it would be very difficult, though you could create a world where it could occur.

If we may work backwards, the tremendous ubiquity of sexual reproduction should be a strong indicator suggesting that there's an advantage to it over other reproductive approaches. Indeed, we see sexual reproduction later in the evolutionary tree than other approaches such as isogamy, which suggests that it has a higher fitness for reproducing "advanced" organisms than other approaches.

Now I won't be able to bring up a full scientific argument for why this is so. Sexual reproduction is buried so deeply into the genetics of our ecosystem that it would be folly to try to claim to have "the one reason" why sexual reproduction works better. However, with a little hand waving, we can try to build enough of an argument to build a world which stymies these advantages, leaving readers to ponder whether it is indeed possible.

If I wanted to grab the biggest part of the advantages of sexual reproduction in one topic, to make it easy for a reader, I'd say the big advantage is differentiation. Sexual reproduction creates a very strong opportunity for differentiation into two different classes of individuals with common DNA between them. If there is value in differentiation, this provides a powerful place to do it.

I think we can generally say that any sufficiently hostile environment to make an interesting story (say, Australia) is going to select for differentiation. It's just too useful. However, perhaps your planet can cause differentiation to happen later.

For expository purposes, let's call the two differentiated sides of your species masculine and feminine. We H. sapiens are familiar with the meaning of those gendered terms, so it's convenient. Now what if your planet had a long chaotic cycle, on the order of 30-50 years. Perhaps its weather, or maybe it's some alignment of some god-awful trinary star system (the stability of which is a separate question). Regardless, it may be very effective to have a large number of non-sexed children, waiting to see what the planet does before adjusting the mixture of masculine and feminine to fit. Maybe 10 years into their life, we find that the planet has created an ecosystem where boisterous strong stubbornness is needed. Then an entire generation can take the path towards the masculine side right then and there.

If your species took this path, it would have to take care not to rely on sexual reproduction. If the phases of the planet demand a vastly masculine approach, there may not be enough females to sustain the population. Likewise, if the population needs to be mostly feminine to get through a subtle crisis, there may not be enough genetic diversity left in the few masculine individuals. This would create a strong incentive for the species to be able to mate regardless of such a gendered arrangement.

The price, of course, would be that the differentiation happens later. If there were value in differentiating early (such as teaching one half of the species child rearing from day 1, literally), then that would encourage sexual differentiation. Since you don't want that, you need to make sure that there's value in waiting until you have more information about the perils that your species faces. That'll ensure there's selective forces to make the differentiation happen late.

If the planet isn't enough of a force to compel your species to evolve for late differentiation, you can always borrow some spiders from Australia. I don't know if a swell of spiders will encourage masculinity or femininity, but if you'll excuse me, I'm going to go jump in my wife's arms now while shrieking like a little girl.

Spiders!
(Credit: National Geographic)

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Sexual reproduction between diploid beings (have double chromosomal endowment, have it and mix) is one way our specific evolutionary lineage found to "mix & match" genetic characteristics from one generation to the other.

This fact (mix useful mutation and spread them, not necessarily the way we do it) gives a huge boost to whatever being "invents" it.

It is arguable there are other mechanisms to achieve something similar (in truth we already know this is possible, at least for unicellular beings), but it's clear that, once the method has been discovered, all beings lacking it are at a serious disadvantage.

This for two reasons:

  • diploid beings are much more resilient to "single damage" (this is more serious than it seems, because the "damage, if not immediately fatal, can be the stem of "further development).
  • new developments can be "tested in the wild" with less negative impact.

Currently haploid beings are limited to:

  • unicellular
  • specific subsexes (male bees)
  • some very strange occurrences

From there evolution of "sex" as we know it is a "natural consequence", but may well not be the only possible consequence.

So the answer is: Yes, it's conceivable to have beings exchanging genetic material in a "aequalitarian" basis, but what it could be is really pure speculation and I won't venture in it.

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  • $\begingroup$ Yes Bees have interesting genetics creating some unusual effects. Bee species as a whole are Haplodiploid which always strikes me as odd $\endgroup$ – Slarty Oct 23 '17 at 21:48
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A neonatologist (doctor who cares for newborns) I talked to recently pointed out that a larger percentage of babies need serious medical care after being born. This change came about about a generation after some serious improvement in the way we care for premature and sick babies in the 70s and 80s. He concludes that the genes that make it impossible to give birth without the help of advanced medical infrastructure are already spreading in our society.

If you take this doctor's assertions to the (possibly ridiculous) extreme, you might imagine a race of highly advanced being with no reproductive organs at all who are only capable of reproducing by combining genetic material in a lab. You and your partner both swab your cheeks, pay the fee, and in nine months the lab calls you in to make you parents.

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Sequential hermaphroditism is not exactly what you're asking for but may be of interest.

Some fish (and amphibians and invertebrates) spontaneously change sex either as a normal part of their life-cycle or in response to their social environment.

In a fictional world, it could potentially occur in intelligent species.

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Yes.

While many reproduction strategies exist, they boil down to two basic methods. Asexual reproduction, where a single organism reproduces using its genetic material in the offspring, and sexual reproduction which involves two organisms who exchange genetic material after it was shuffled by meiosis.

There are over 65,000 species of Hermaphroditic animals (not including plants), so there is no reason that I see why an organism could have a reproduction technique such that both partners give and receive gametes during mating.

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Male and female crocodiles have all the same chromosomes. Their sexual differentiation is determined by the ambient temperature when they develop in the egg.

There are flowering plants which produce flowers with both male and female structures, combining both sexes in one structure. So that means individuals are all the same sex.

So let's combine the two: what if instead of sexual differentiation in the egg, crocodiles just went the way of flowers and each individual just got both types of reproductive organs. They would still need two partners to reproduce but the roles would be interchangeable (or determined by some sort of mating fight, more likely).

I think such a thing would be evolutionarily possible if at the beginning there had been a lot of near-extinctions and communities where the sexual differentiation continued further to form only-male and only-female individuals did not manage to survive them.

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Research at University of Oregon used C. elegans to study this. C. elegans are hermaphrodite and carry both male and female organs. They can be programmed to cross fertilize or self fertilize. So they took two batches, one batch they programmed them to cross fertilize and another to self fertilize.

They found the group that cross fertilize , would show more mutations , and so better able to adapt in the wild.

https://www.youtube.com/watch?v=pEc0DI-n4Pk

Hope that helps.

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  • $\begingroup$ how is this answering the question? you are citing an example of species with still two sexes $\endgroup$ – L.Dutch Oct 24 '17 at 2:53
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    $\begingroup$ @L.Dutch If a species is hermaphroditic then all adults of the species have both sets of organs. In such a species any individual can act as either the male or the female. In such a species there is only one "sex" so to speak. $\endgroup$ – sphennings Oct 24 '17 at 3:17
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    $\begingroup$ Welcome to Worldbuilding, Vikram, can you please edit the sentence in your second paragraph. Presently its meaning is obscure. Enjoy yourself here! $\endgroup$ – a4android Oct 24 '17 at 5:33
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Sure. There are plenty of organisms that mate to reproduce but all have the same sex. However, unless you're going for an organism that can reproduce asexually, this creates a problem of self-fertilization. Gametes are generally produced en masse, and you don't want them to be combining with each other before they leave the body, so there needs to be some way to ensure that the gametes don't recombine. There are two main ways to prevent this.

Isogamy and mating types

For some species, all gametes look basically the same, unlike organisms with two sexes where the egg cell is bigger and the sperm must seek out the egg. To avoid self-fertilization, such species have what is called "mating types" - an individual can only mate with another who has a different mating type. Now this might seem like a cheat (since mating types are basically sexes with a different name) but many species can have up to hundreds or even thousands of different "sexes", all of them physiologically identical, rendering the distinction moot. Any individual can mate with any other individual provided they don't have the same mating type, and the chance of another individual having the same type is very low. Many fungi use this system.

Imagining isogamy in an intelligent species would be interesting, but as long as there were enough mating types, they might not even know that mating types exist until their science develops to that point. Anyone could mate with anyone else, and one out of a few thousand couples would just be unable to have children and nobody would know why. Or maybe they would have pheromones that would simply cause them to not be attracted to another individual with the same mating type as themselves.

Hermaphrodites

Hermaphrodites are organisms with two kinds of gametes, and each individual has both male and female parts. Technically, it would be possible for such an organism to self-fertilize, but they are usually constructed in a way to make this physically impossible (or at least unlikely). Such organisms usually exchange gametes during mating, causing both to produce children as is the case with snails and earthworms. Flatworms will fight for dominance; whoever gets stuck first must bear the children. Many plants are hermaphrodites as well and avoid self-fertilization by having the male and female parts develop at different times.

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there are plenty of Asexual organisms.

I believe the majority of microbes are asexual.

Mitosis is an asexual process and genetic divergence happens with that all the time (cancer).

Even some trees can have an asexual reproduction cycle in the absence of a mate.

Let me clarify Asexual Reproduction

Asexual reproduction is when a single organism reproduces without a partner. Once you add a partner there becomes the ambiguous differentiation of sex. These gender distinctions don't boil down to sperm and egg exclusively but more the roles the different parties play in reproduction. So in effect its impossible to have genderless non asexual reproduction. Even in certain fish who trade off being the male or female are still at one point male, female, neutral.

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    $\begingroup$ The OP stated in the question that they don't want asexual reproduction. $\endgroup$ – HDE 226868 Oct 23 '17 at 19:09
  • $\begingroup$ well missed that, then the OP is self contradictory because those concepts are synonymous. $\endgroup$ – anon Oct 23 '17 at 19:10
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    $\begingroup$ They aren't synonymous. Asexual reproduction involves only one organism; the OP's idea could use two organisms - but both the same sex. $\endgroup$ – HDE 226868 Oct 23 '17 at 19:11
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    $\begingroup$ It is not impossible to have genderless sexual reproduction. The OP's own link to Isogamy is an example refuting this claim. $\endgroup$ – Mike Nichols Oct 23 '17 at 19:32
  • $\begingroup$ It's far from impossible. Check out any of the other answers. $\endgroup$ – user41674 Oct 23 '17 at 20:43

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