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Creatures on Earth has 2 modes of reproduction, sexual and asexual. Most species employ only one of the two strategies, but a handful use both. Those that engage in sexual reproduction are either monoecious or dioecious. Most on Earth are dioecious (each individual has one type of genitals), but some, like certain worms and snails and plants, are monoecious (each individual has all types of genitals).

Notice how on Earth, "monoecious" is completely synonymous with "hermaphrodite"? On the planet I'm worldbuilding, most of the species engaging in sexual reproduction are monoecious, but not hermaphrodite either. They're all just one gender, producing only one kind of gamete capable of merging with the gametes of all other individuals of their species.

This strategy seems to yield better reproductive success as you can mate with all individuals of your species just like hermaphrodites, and at the same time only produce one type of gamete thus cutting energy costs unlike hermaphrodites who have to produce two types of gametes at the same time.

Is this idea plausible? A whole planet where the vast majority of multicellular organisms living there are monoecious. Why didn't any organisms on Earth ever come up with this configuration for reproduction?

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    $\begingroup$ "Producing only one kind of gamete" is called isogamous, not monoecious. A monoecious (= Greek for "one house") species has individuals producing both microgametes and macrogametes. And we do have multicellular isogamous organisms on Earth, for example the ubiquitous water silk Spirogyra. $\endgroup$
    – AlexP
    Jun 7 at 18:17
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    $\begingroup$ Given that multicellular life almost completely abandoned that plan, it apparently does not yield better reproductive success. $\endgroup$ Jun 7 at 20:39

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Figuring out the bill:

What I suspect this comes down to is figuring out where the costs associated with your reproductive plan come in. Different reproductive plans might work better or worse for any given species.

Do your species incubate young internally or produce large nutritive structures (eggs)? Do they tend their young, or watch the eggs?

For higher life forms, these are specialized functions. Child rearing and reproductive organs are both expensive propositions biologically. So is moving about seeking mates to spread your genes. If all members must perform both tasks, this is complex and expensive for your species. If all members can reproduce and don't migrate around to spread their genes, inbreeding becomes a significant risk. This is okay for very simple organisms, but as you get more complex, it starts being a problem.

Offspring care is demanding. If you have two individuals who have sex and both get pregnant at the same time, they are both being loaded with the biology cost of caring for young. This either means one cannot support the other, OR that both must work to raise the offspring and one of them is not free to wander and spread their genes to more individuals.

You can say that all members are a single sex, but what that really means is that all members must be able to perform all tasks of both sexes and be able to perform both jobs biologically. So your one sex still performs both functions, regardless of how complex the job is. Compared to having both sexes, it's a distinction without a difference.

Gendering is also a product of selfish genetics. An individual with a gene allowing them to mate with many partners and themselves not take on offspring-raising will spread their altered gene to many offspring as they continue to mate without having to raise their own offspring. The half of offspring getting the altered gene spread it around, while the half that don't continue to child-rear. Oops, we just invented male and female.

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Reproduction by the fusion of identical gametes is not optimal, at least for multicellular lifeforms. For the gametes to find each other, at least one of them must be motile. Motility is improved if the gamete is small. On the other hand, the resulting zygote must contain enough energy and macromolecules for several rounds of cell division before it can embed into the uterine wall and begin to receive nutrients from the mother. If fetal development takes place entirely inside an egg, the egg must contain all the necessary materials for the new individual and must be quite big. Evolution has solved the contradicting requirements by making the sperm small and agile swimmers, while the larger egg stores a lot of material and does not move on its own. Apparently the benefits of this system outweigh the cost of having to find an individual of the opposite sex to reproduce with, otherwise we would see more examples of isogamous reproduction.

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