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In this world, most things that could be considered animals or animal-like reproduce in a genderless, sexless, or asexual manner, primarily through processes like apomixis or other means excluding birth(either from womb or egg) which excludes parthenogenesis. However, in my personal opinion (which may not be biologically, philosophically, or morally and politically correct), if a creature gives birth, it is considered female for the purposes of this question, irrespective of how alien its appearance is or whether the entire species has only one gender.

The only entities that possess:

-Sex

-Gender

-Anything related to sex,

are fantasy/alien humanoids/semi-humanoids, including humans, and possibly some bacteria or other smaller creatures in this world.

The question arises: Can this world truly exist, and if so, how would these creatures survive when sex offers so many advantages?

Advantages:

As far as I know, genetic diversity is a great advantage, and sexual animals tend to be selective with their partners, which creates competition, which itself is good, people and species that decide to not compete, are outcompeted.

Details:

Regardless of my lack of understanding of sexual biology, reproduction where the individual can just split itself into two to clone itself or release roots/pieces of the body that can then grow into full adult animals, or spores and similar things are preferred over things like lying eggs or giving live birth.

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    $\begingroup$ Lots of plants have male and female parts, but the organism itself is neither male nor female. So that your typical plant with perfect flowers (sometimes called synoecious, "common house" in Greek) is an example of such an alien species with no males and no females. As do monoecious plants. Isogamic organisms would also qualify. (When asking about alien reproduction, it is always helpful to look at plants. Plants are aliens.) $\endgroup$
    – AlexP
    Jul 27, 2023 at 22:52
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    $\begingroup$ More likely a world without males. Females are the default. Males are modified humans. $\endgroup$
    – Mike Wise
    Jul 28, 2023 at 12:47
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    $\begingroup$ @MikeWise that is only true for animals using the XX-XY or X0-XY system. other animals are males by default and become females later in life or just don't have any gender at all $\endgroup$
    – user104995
    Jul 28, 2023 at 12:56
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    $\begingroup$ You need to learn a whole lot more biology. "genetic diversity is a great advantage"... it's a heaping huge advantage, which is why it is used by all life greater than a few cells. $\endgroup$
    – RonJohn
    Jul 28, 2023 at 17:40
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    $\begingroup$ I'll just mention Ethan of Athos by Lois McMaster Bujold as a story featuring a planet with no human females (with the male population using artificial reproduction). $\endgroup$
    – cjm
    Jul 28, 2023 at 20:04

8 Answers 8

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We have only one data point to work with: Earth. Worse, as much as humanity has learned about genetics and the evolution of life, we're really just scratching the surface of the subject. We're getting better at "why did it happen this way?" but we're still a long way away from casually answering the question, "OK, so why didn't it happen that way?"

But, while our mandate is to help you build an imaginary world (by definition, you control all of the rules of your world), one of the more satisfying goals of the Stack is to help you rationalize the world using what we know about Real Life.

A quick divergence

Just to make a point, when it comes to reproduction things are a bit more complicated than just sharing DNA and moving forward. For example, an observation from our sister Stack, Biology.SE:

Take bees for instance. There are basically three types of individuals in a hive: one female queen, which lays eggs, male drones which fertilize a queen (or try to), and then die, and neuter workers, which feed the queen, raise the eggs & larvae, and determine whether a particular egg will become a new queen or drones.

So for bees, only two types of individuals actually participate in sex, but all three types are required for the hive to survive and reproduce. (Source)

While you're asking if you can have a world that's fundamentally sexless in pretty much all cases, it's worth noting that we should avoid the temptation of creating all life (or manipulating the representation of life) in the image of humanity. When you get into bacteria and fungi, there can be many more sexes involved in reproduction than just two. Or three. Or four....

Why this quick diversion? We're going to need it a bit later.

Why did nature evolve multiple-sex reproduction?

Those species that exhibit sexual reproduction have an evolutionary advantage over "cloners" in that there is more diversity in their offspring. This diversity allows the species to adapt more quickly to a changing environment, or to increase its chance of survival in the existing one. — Tim Waterfield, Cambridge England

I'm no biologist, but if I remember the basics, life started as sexless cloners (that's really what you're asking about. Please correct me if I'm wrong). As time (and meteors, and ice ages, etc.) moved on, "life" discovered that hedging its bets was a good idea.

From that perspective, a world that evolved with fewer life-altering and life-ending catastrophes might favor single-sex life over multi-sex life. Thus, we have the answer of "you can have it" with a reasonable rationalization.

But it might be more fun to hedge your bets a bit like the bees

What if life on your world is, indeed, a bunch of cloners... that evolved "helpers" like the neuter sex in the bee world that acted to ensure the continuation of the species, but never assist in the actual act of procreation.

The idea meets your basic desire of not wanting "females" (e.g., not wanting a second or more participants in the act of procreation), but it feels more "real" in that "life" (the mindless process of evolution) came up with a way of better preserving the future — even though it did it without using the generally binary sex model developed here on Earth.

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    $\begingroup$ A correction to the bee thing - a worker bee is actually genetically female; she carries eggs. The organs lacking are those required to mate and fertilize the eggs, but when a worker lays, it will always produce a male bee. When the queen fails, workers will lay male eggs in the hopes of continuing on their genetics by mating with a queen from another hive. The original point of the question is difficult to circumvent if you follow the societal norm that an asexual reproducer is still arguably female $\endgroup$ Jul 28, 2023 at 12:03
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    $\begingroup$ Too bad you couldn't have worked birds into your answer. A question about sex produces an answer about birds and bees? Priceless. $\endgroup$ Jul 28, 2023 at 12:37
  • $\begingroup$ Good answer. The Waterman quote that "diversity allows the species to adapt more quickly to a changing environment" is, however, not a reference "life-altering and life-ending catastrophes", however. While such catastrophes and mass-extinctions have been important in shaping the evolution of life on Earth, I believe the evolution of sex is thought to have been encouraged by smaller and more continual changes in the environment, both abiotic and especially biotic like parasites and pathogens. $\endgroup$
    – Kirt
    Jul 28, 2023 at 17:02
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Have most life be starfish like

Most life on your planet can be based around organisms like starfish with somewhat decentralized brains spread across multiple limbs. They can reproduce by splitting and making copies. This will make evolution of intelligent life notably harder since you're cutting off a lot of routes to intelligence, but stories tend to involve a lot of luck anyway.

Have a plague block sex

Have some sort of common bacteria actively interfere with sexual reproduction. Anything that evolves sexual reproduction will tend to die on this planet. That will remove the advantage of sexual reproduction.

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I believe there is a life form on this planet that exhibits what you are looking for.

The zygotic adult gives off gametic spores. If a spore lands in a viable location, it grows a small organism. When a second spore lands on this, a zygote is produced and an adult grows.

They are called Ferns.

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If you're talking alien biologies, then you have options. The purpose of sex is to allow recombination of genetic structures, both for improvement and for resilience. I'm sure you're familiar with the "improvement" end of evolution, but it sounds like you aren't considering the resilience end of it.

Without genetic recombination, you get monocultures. This results in cases like the banana, where an entire species is susceptible to a virus, bacteria, or fungus. We've already had cases where entire species of banana have been wiped off of the planet by a single infection.

Recombination doesn't require distinct sex/gender assignments. Most plants are hermaphroditic. They all produce pollen, and they all have flowers. The pollen is the transmission mechanism, and the flowers are the incubation mechanism.

There are actual cases of "spermcasters" and "broadcast spawners" in the animal kingdom. Coral comes to mind. When mobility is available, this allows selectivity of breeding partners, which is a far more effectual genetic strategy. You may find this paper interesting:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5031625/

Overall, you don't have to go very far to find examples of genetically recombining creatures without gender, but they are always hermaphroditic. You need a sender and a receiver.

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The "advantages" of sex are less obvious than it seems. What is the benefit to me and my genes of my species being genetically diverse - why should my genes care about them? They only care about their own reproduction and increase. Being selective about partners is a consequence of sex, not an advantage. Indeed the more you dig, the less obvious advantage sex seems to have.

So have your species divide by "budding" (although I don't really see the difference between "budding a child off the exterior" and "budding a child in a womb". Or splitting, or setting spores that develop into a plant-like autotroph which in the alternate generation produces babies...

All of these will produce clones - and this isn't a problem per see. I'd say in such a world, there are no males (female is the default, males are a modification of female, at least in mammals)

Or you can still have sexual reproduction, just without the sex: Every spring we go down to the sea to release our gametes into the ocean. Our "children" swim with the plankton for a few years and the lucky few that don't get eaten eventually crawl on land, metamorphose and start their adult life. You get the genetic diversity of sex, without giving birth. (for added ickiness, have the adults die after spawning - Futurama had something like this.)

You'll need to work through the consequences of all this. But there is no fundamental law of nature that you're breaking.

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    $\begingroup$ "What is the benefit to me and my genes of my species being genetically diverse?" Richard Dawkins wrote a lot about this. One of his key points is that to a gene controlling eye colour, the genes coding for skin, circulatory system, etc are all "other entities" which the gene is "cooperating" with; there's not really a difference between that, and "my genes" vs "your genes". The benefits to a gene of belonging to a thriving species are much the same as the benefits of belonging to a thriving genome - if humanity dies out, there won't be any hosts for copies of the "blue eye colour" gene. $\endgroup$
    – IMSoP
    Jul 30, 2023 at 10:16
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They do not breathe oxygen

It is certainely not completely proven yet, but a great deal of research are pointing that way:

One major advantage of sex may be its promotion of recombinational repair of DNA damage during meiosis

If your creature do not breathe oxygen, they may never have evolved to sexual reproduction, and continue to use some form of cloning for reproduction.

For monocellular creatures, cloning is trivial and the "offspring" quickly mature to the same size as the "parent".

For more complex creatures, you cannot have a direct cloning of a full size adult, you'll have to have the parent release a mini-clone (I know you don't want to call it an egg but at this stage it becomes just semantics). A single stem cell which will multiply and diversify into all the organs needed for the creature.

You can then choose how your "parent" creatures handle the rearing process. It can go from "just drop it somwhere and let it be" up to "drop it in a sheltered area, watch over it and protect it until the clone reaches maturity", with everything in between allowed.

Note that some ratio still have to be respected. For example in the case of just "dropping the clone" somewhere and letting it take its chance. If the clone chance of survival until adulthood is 1%, then it means the parent will have to drop at least 100 mini-clones just to ensure the specie survival.


The long explanation:

The first organisms were all sexless cloners. They powered their life processes by extracting chemical energy from the minerals in the soup around them. Then some organisms (cyanobacteria) discovered (or rather evolved) photosynthesis. Yay, they thought, now we can get our power from the sun instead of having to lick the side of underwater mounds (hydrothermal vents). That also came with an advantage, sunlight is way more abundant than localised hydrothermal vents. So yipee, these organisms thrived and spread across the whole water near surface instead of being limited to specific locations. This worked so good for them that quickly their sheer number started to upset the balance. The byproduct of their process is dioxygen (O2), and they were all releasing more and more of it. Problem is at the time oxygen is a poison for anything alive. The explosion of the cyanobacteria population is thought to have caused the Great Oxidation Event which raised the percentage of oxygen in the water and in the atmosphere. This unfortunately killed between 80 to 99% of species at the time (by direct poisoning but also because the now free oxygen started to react with the iron and other minerals, depriving some species of their food supply).

But life finds a way. Some survivors of this great extinction decided to tackle the problem head on. Instead of fearing the oxygen, and since there was so much of it, let's make use of it. And presto, some organisms evolved to use oxygen to power their life process (one of them is your gran-gran-gran mother/father, still no sex at the time). On it went, the population of aerobic organisms grew, but they were soon faced with another hard problem: Breathing oxygen and having it power the biological process in your cells means the oxygen must be present in the cell at some point. And oxygen is extremely corrosive, it wants to react with nearly everything, including the amino acids which form your DNA. Give it enough time, and one day some oxygen will damage your DNA. Now if you are a cloner, when you reproduce, your clone inherits a copy of your DNA. If this DNA was damaged, your new clone's DNA is already damaged from the start. Then it gets damaged further, and you clone a new you with even more damage. You see where this is going, the damages are cumulative and over enough generations you could loose enough DNA to affect primary functions and become unable to live on (or to reproduce).

But once again, life found a way. Back to the first quoted sentence, sex is a great way to mitigate DNA damages caused by oxygen. When you combine the genetic materials of 2 individuals , the recombination process will only keep one final version. If one of the DNA sequence was damaged in a parent and not in the other, the recombined DNA of the child has 50% chance of getting the undamaged sequence ... compared to a cloning process where you have 100% chance of passing on the damage.

Ok that's obviously oversimplified near the end, and the real number is not 50%, but it's to illustrate that it was for nature an excellent way to hedge the bet.

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  • $\begingroup$ "They do not breathe oxygen". The problem is that oxygen is just about the perfect oxidizer (pun not intended) for life: reactive enough to do things quick enough for active life, but not so quick that things instantly corrode (like with fluorine). Carbon is another one of those "sweet spot" elements. $\endgroup$
    – RonJohn
    Jul 28, 2023 at 17:44
  • $\begingroup$ @RonJohn, I know. It seems specially useful for complex life. While it's easy to find example of anaerobic life, nearly all the existing examples are simple archea or bacterium, all unicellular... $\endgroup$
    – Hoki
    Jul 28, 2023 at 18:02
  • $\begingroup$ Although it appears that a few previously oxygen breathing eukaryotes evolved to not need oxygen after all, and continued on to evolve in anaerobic "animals", from 1mm in size, to the largest one at ~8mm. May be they just haven't got their chance yet, after the next asteroid wipes us out, they might grow to dinosaur size :D $\endgroup$
    – Hoki
    Jul 28, 2023 at 18:02
  • $\begingroup$ "May be they just haven't got their chance yet" or maybe chemistry limits the maximum amount of activity they can have. $\endgroup$
    – RonJohn
    Jul 28, 2023 at 21:01
  • $\begingroup$ Anyway, than you for those links. Extremely interesting reading. $\endgroup$
    – RonJohn
    Jul 28, 2023 at 21:02
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It seems to me you are asking about the plausibility of two different things.

First, whether most life could reproduce asexually, i.e. without combining genetic material from different individuals.

Second, whether organisms could reproduce without a dedicated reproductive stage in its life cycle, such as a foetus, egg or seed, or alternatively with a very small one. I assume the distinction you draw between an 'egg' and 'spore' is one of size.

And presumably you still want a range of organisms similar to Earth's, including plants and animals in particular.

No Genetic Exchange

From a genetic perspective, life on our planet has two mechanisms for sex. Bacteria have a subset of their genes in a separate package - a plasmid - that can be donated to another bacterium that accepts those genes.

Eukaryotes - anything with a cell nucleus - have a more organised mechanism where two cells fuse completely and the new cell has two copies of all the genes.

In both cases, a cell is accepting genes from outside. This dilutes its own existing genome, which on the face of it is bad for its existing genes, but nevertheless organisms have evolved to do so because of the great benefits that can come from new genes.

So how to stop sex evolving? One way is to make the costs even higher, and in a sense self-imposed. Suppose the greatest threat to early life on this world was viral infection, i.e. the insertion of foreign genes. Survival would depend on eliminating any genetic material from outside, and at all costs not allowing it into the host genome.

As viruses become more insidious, cells would evolve measures to destroy incoming genes (restriction enzymes, for example). Think of it as similar to rejecting a transplanted orgen. These defences would be more powerful than in our world, and would prevent sexual reproduction ever geting a start. If two cells were to fuse somehow, their defensive molecules would attack each other until one or both genomes was destroyed.

With sex out of the question, it is hard to say whether complex multicellular life could evolve. Most evolutionary biologists think not, but we won't know unless we discover other planets with strictly asexual life. So populate your world with whatever you like - except for sex-related structures such as flowers or peacocks' tails.

Sex, But Not as We Know It

There are various possibilities here, but I'll only describe one. This does involve sex in the genetic sense, but without females.

I mentioned above that eukaryotic sex involves the fusion of two cells with different genes. It is a waste of time, effort and opportunity if two cells with the same genes fuse, yet because cells divide, nearby cells will often be genetically identical. The solution is for cells to have different types. A cell of a given type can divide into two cells of the same type, but can only fuse with a cell of a different type, and therefore different genes.

We are familiar with multicellular plants and animals, which have only two mating types. In this situation, and especially when organisms become multicellular, there is a perhaps-inevitable tendency towards anisogamy. That is to say, the gametes start off all the same, trying to compromise between carrying enough nutrients to develop and being mobile enough to find a partner. But it is much more efficient for one type to grow large and well-stocked - an egg - while the other is cheap, lightweight and optimised for dispersal - sperm or pollen.

But two mating types is not the only game in town. Other organisms, fungi especially, can have more than two, sometimes dozens or hundreds. Gametes of any two different types can fuse. Because they don't know their type of partner in advance, they cannot specialise like sperm and ova.

Without egg cells, species would reproduce sexually through spores in the air or water, like mushrooms. Asexual reproduction is possible too - organisms might divide or bud in the summer, then in the autumn switch to producing spores.

This genetic system doesn't actually preclude more familiar methods of reproduction. You could have three or more types, and females would be disinclined to mate with males of their own type. Perhaps that's what the humanoids do.

Of course, this is just an outline, I have omitted many details, such as whether the haploid or diploid phase dominates the life cycle.

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  • $\begingroup$ Normal viral infections don't insert themselves into the host DNA, they just hijack the host's resources to replicate themselves. Fighting them requires a robust immune system which is cited as one of the advantages of sexual reproduction. What you're describing is "retroviruses", which include some particularly nasty things like HIV. $\endgroup$
    – IMSoP
    Jul 30, 2023 at 10:25
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If you're talking about sex, then the short answer is no. Males are the non-egg producing part(s) of a species with two or more sexes. (Though, I suppose that if all of the females had died off for some reason, then the answer could be yes, but that would mean it's a species that's likely doomed to eventual extinction.)

Of the various sexual systems, the closest you could get is probably androdioecy, where the population consists of only males and hermaphrodites. Note that androdioecy is very rare on Earth.

Another alternative would be that they'd be sexually male for most of their lives, and only be female or hermaphroditic during a portion of their lifetime. This changing of gender throughout a lifetime is known as sequential hermaphroditism.

If you're talking about gender, then yes, since sex and gender aren't the same thing, and gender is really just a social construct. You could have a species which all expressed traits and preferences which our society would typically think of as "male," thus we would gender them all as male (though, they might see finer differences between the behaviors of their sexes than we do). You see this in literature regarding dwarves occasionally, where there is little sexual dimorphism between male and female dwarves, meaning that male and female dwarves look and act essentially same, both having beards, etc..

Hope that helps!

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