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I have in mind this alien species that can reproduce with humans. The result will always be of the same species as the alien; the human is used only to vary the genetics so the newborn won't be a clone of its mother.

Only females of that species can do this. Males can only impregnate its own species' females. Yes, there are males and females.

The females can do this with certain other species too, from a list of compatible ones (all fictional except for humans). This species was genetically engineered... how its designers achieved this is unknown and an explanation is not a requirement. The topic will just be avoided in my story as the designers are considered a civilization with a level that would take us thousands/millions years to reach.

The above-described features are all story requirements. I may negotiate some but let's try to avoid it. In particular, the "hardcoded" list of compatible species is a story requirement, but let's leave the details of how it works out of this, except if you have an idea that may work. If it helps, pretend this feature is not there.

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    $\begingroup$ Not an answer, but possible help: the movie Alien (and its sequels) has the Xenomorph species, which can breed with most other species. Given that the movie aimed to be somewhat rigorous (for Hollywood), if you search the Internet for the backstory of that movie, you may find more information on this topic. $\endgroup$ – SRM Feb 7 '17 at 6:52
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    $\begingroup$ Genetically based reproduction between humans and aliens would be virtually impossible. However, there are other, non-genetic options, described, for example, in Philip Jose Farmer's book "The Lovers". $\endgroup$ – Alexander Feb 7 '17 at 7:49
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    $\begingroup$ @SRM, I think I have consumed the entirety of the Alien franchise, except maybe for the written material such as comics. The idea of a larva implanted in other species is amazing, and making it to take genetic influence from the host makes it even better, but they never tried to explain how it works (of course). Anyway, thank you for make me remember Alien. I'm reading the fandom theories from Wikia in case one inspires me. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 8:57
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    $\begingroup$ No, the definition of species is that they can only reproduce with each other. If your alien could breed with humans, biologists would consider it the same species as humans. $\endgroup$ – Superbest Feb 7 '17 at 17:33
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    $\begingroup$ @Superbest has the real issue with this plan, although I might word it differently. Our concept of a "species" is an artificial construction that we find convenient for making sense of our world. Such a creature would simply upset the entire concept of "species," rendering it meaningless in this case. We have similar issues with bacteria from different "species" that manage to trade little fragments of DNA in ways that don't quite count as reproduction, but yet have similar end results. $\endgroup$ – Cort Ammon Feb 7 '17 at 19:04

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Not using anything that remotely resembles genetics as we know it.

What you are talking about is fairly similar to the Asari from Mass Effect, who likewise use the genes of other species to "modify" their own. The problem with this is that even if there is a mechanism for combining DNA arbitrarily, you can't plug bits of DNA into another species with a different evolutionary path and expect to come up with something meaningful. Even supposing they happen to look like humans, they will be completely different on a chemical level. They will probably not be able to produce proteins from our DNA, and if they can, said proteins will most likely be either useless or toxic.

The reason why mating makes sense biologically is because you're combining your DNA with that of another organism whose genes are close enough to your own that the child will be able to benefit from those good (or at least viable) genes. Here, the child will not benefit from the fact that the other species has good (or at least viable) genes, so what's the point? You could say it's just randomly mutating to prevent the child from being a clone, but in that case you might as well just randomly mix up DNA and hope something comes of it, saving the tedious difficulty of mating.

Some bacteria do swap genes randomly with bacteria of other "species", but this only makes sense because #1 bacteria are much more structurally simple than multicellular organisms and therefore producing a new protein has a much lower chance of simply killing them and a fairly decent chance of providing some benefit, and #2 bacteria reproduce so fast that they can afford to take these risks.

All this is without even getting into the issues of the species being alien. They might not even have DNA.

Now of course since these organisms are both alien and artificial you could say that they have a completely different chemistry. Maybe they even have "smart" DNA that is somehow able to analyze the DNA of its partner and pick out bits that might be useful. But this is getting into realms of bio-engineering so far beyond feasible speculation that it might as well be magic. Not that there's anything wrong with that.

One interesting solution that is more well thought-out than most is presented in the webcomic Schlock Mercenary. In it there is an intelligent, amorphous species that reproduces through binary fission, and passes a part of its actual brain to the child. They are capable of "breeding" with other species, not biologically or chemically, but by using their own idea of the other individual's personality as a template to build the brain passed to the child, so the child winds up with some character traits of the other "parent". In other words, they mentally construct the traits of their child and use another organism as "inspiration".

This is fairly plausible even for non-amorphous creatures; after all the mind affects the chemistry of the body and the chemistry affects the development of a child, so it is possible for the mind (at least the subconscious, emotional parts) to affect the development of a child. Humans have a relatively small number of chemicals that regulate emotion, but if this species has a much more complex mind-body chemical connection, their thought processes might be able to have significant impact on the traits the (technically cloned) child develops.

This kind of "cross-breeding" by it's very nature doesn't need to involve any kind of traditional mating or indeed any physical contact at all, although you could make a version of it where a physical act is necessary for "initiating" the process. Perhaps they naturally do this to some extent when mating with their own species, and are also biologically capable of cloning, but if they clone themselves while in the presence of another sapient being they can mentally pass traits of the other organism to their child.

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  • $\begingroup$ All good points. If I want to avoid this to be taken as something of magic nature I have to justify somehow why things are done as they are done, at least show that I gave it some thought. Now, in speculative interspecies breeding the need for sexual intercourse is hard to justify, if it can happen at all. Taking inspiration from the other individual (of the other species) is good idea. Maybe I can say that somehow the females of my alien species need a sample, with genetic information, of the chosen male, but then comes the problem of "that sample can be obtained from nearly anything". $\endgroup$ – Hatoru Hansou Feb 7 '17 at 10:10
  • $\begingroup$ More than inspiration or smart DNA, while reading your answer, I was thinking of something along the line of a smart reproduction system, that somehow look into "alien" DNA for clues about what is needed to be successful in the current planet environment (if the male reached reproduction it can be considered a valid model). About DNA compatibility, I may have the solution for that but maybe in another question I will ask for help in checking its believability so I won't touch the topic now. Again, all good points. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 10:18
  • $\begingroup$ One more thing, I know the Asari, I played to Mass Effect 2, someday I will play Mass Effect 3. And, please, believe me, I have this idea before ME 1 got released, they just released first. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 10:21
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    $\begingroup$ @HatoruHansou The thing is, evolution is just a pruning mechanism on top of a random generator. It doesn't plan. It doesn't change multiple things at the same time. Most traits will not stick even if they are beneficial. To really get your aliens going, you need genetic engineering - something that can analyse the host, find out which traits make it better suited for the environment, and try to reproduce it with its own biology. This very likely requires intelligence - a way for the individual to consciously affect the process. $\endgroup$ – Luaan Feb 7 '17 at 14:13
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    $\begingroup$ @HatoruHansou Keep in mind that we are only barely able to "analyze" DNA of species we have studied. DNA isn't a blueprint, it's assembly code - it's basically impossible to tell what will emerge from a particular code without watching it develop. Development generally involves interaction with the parent's body or eggs as well; DNA alone isn't enough. For a species' reproductive system to analyze an unknown organism from its genetic code alone it wouldn't just need intelligence, it would need to have capabilities far, far beyond anything humans can comprehend - practically magic. $\endgroup$ – IndigoFenix Feb 8 '17 at 6:19
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Check out the Asari from the Mass Effect universe. They're a lot like what you describe. There is a long wiki article on the Mass Effect Wiki, but to summarise the important part:

Asari don't have sex to reproduce. Instead, they perform a process called "Melding". The base template has both genes provided by the mother, and Asari can Meld with both male and female members of any sapient species*.

When they want to create offspring, they seek out a mate of a different species who they think has appropriate genetic traits. Then they join their nervous system with that of their mate, seeking out the genetic traits they want in their offspring and using the rest of the genes to randomise the remaining genetic code of their offspring.

* There are hints that it's also possible with merely sentient species, but besides WAY too much porn, the fan base has also created the hypothesis that melding with a non-sapient creature might overload the creature's nervous system, leading to the death of the creature.

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  • $\begingroup$ I'm aware of the similarities. I played ME1 and ME2. I didn't remember the word "Melding" though, so time to read. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 10:24
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    $\begingroup$ This is like having a computer program that borrows bytes at random from another compiled computer program to gain its features. . . Mass Effect is science fantasy, not science fiction. It isn't a science-based suitable answer. $\endgroup$ – Yakk Feb 7 '17 at 15:05
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    $\begingroup$ @Yakk Evolution has come up with quite a few weird things that are similar to Asari genetics. We got single cell organisms like viruses that hijack the genetic code from living beings to reproduce. Extremophiles (life that lives in extreme conditions like hot water springs or either arctic desert) have genes that have been borrowed from other bacteria. While we have no knowledge of multicellular organisms borrowing genetic code from other multicellular organisms, genes from viruses have been found in our genetic code. it's not necessarily impossible. $\endgroup$ – Nzall Feb 7 '17 at 15:47
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    $\begingroup$ @nzall Coevolution is one way this works. Another is increadibly high failure rate. Single cell organisms and viruses use R-selected strategies; many "offspring", very high failure rate. An offspring failing is a relatively low cost. No macroscopic lifeform can be anywhere near as R-selected as bacteria or viruses; there isn't enough room. $\endgroup$ – Yakk Feb 7 '17 at 15:56
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    $\begingroup$ @Yakk Depends on when they fail. I don't know enough about genetics to know if this is plausible, but maybe the vast majority of non-viable combinations fail before they even start cell replication? $\endgroup$ – Tin Wizard Feb 7 '17 at 17:43
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If you allow for parthenogenesis to occur and bend the required reason for interspecies mating a little (there will be no DNA sharing), something at least somewhat similar occurs in ants: a queen ant can lay viable unfertilized eggs, which will all become male, or she can lay fertilized eggs which will become female.

Specifically, Pogonomyrmex rugosus and Pogonomyrmex barbatus, the rough and the red harvester ants must mate with each other:

For their mating system to operate, females of each species must mate within a brief couple of hours with males of both their own species and males of the other species. [...] If she mates only with her own species, she can produce only reproductives, and, with no worker force, the incipient colony withers and dies.

--The Sting of the Wild, Justin O. Schmidt

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Reproduction as we know it works only within the same specie, because it's all about matching a huge number of genes in the DNA.

The only way I see to make what you want possible is that the females have a built-in genetic lab that, after collecting the xeno-sperm, sequentiates its DNA, cut the interesting parts (i.e. knowing the war of the world they may want to get the sequence giving immunitary response to flu) and integrate them in their eggs, which are then parthenogenic bred.

Bacteria do it already by exchanging plasmides.

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  • $\begingroup$ An organ or system that works like a genetic lab is one of the possibilities I imagined. Of course, I have no ideas about its internals so the topic needs to be avoided in the story. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 6:40
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    $\begingroup$ A system that works like a genetic lab is called a virus. As long as the alien species also uses DNA as the genetic code, it should be possible to design a virus that alters the human DNA to be compatible with the alien one. $\endgroup$ – fishinear Feb 7 '17 at 11:02
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    $\begingroup$ "Reproduction as we know it works only within the same specie". Not quite. Mules and Hinnies are offsprings from 2 different species: donkeys and horses. Note that both are sterile (i.e. cannot breed at all) $\endgroup$ – Nolonar Feb 7 '17 at 12:26
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    $\begingroup$ "Reproduction as we know it works only within the same species" - See also Ligers and Tigons. Note also that all of these are only usually sterile. There are recorded instances of all four having produced offspring (though it is much commoner in the cats than the equines). $\endgroup$ – Martin Bonner supports Monica Feb 7 '17 at 12:56
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    $\begingroup$ @Nolonar "Species" isn't as sharp as a boundary as it may seem. Even mules and hinnies can be fertile, though it's rather rare - it depends on the individuals and chance. The definition of "species" is mostly "Individuals of the same species can easily reproduce with each other" - it's nothing exact. Some human pairs may find reproduction harder than a horse and a donkey :D $\endgroup$ – Luaan Feb 7 '17 at 14:06
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As other answers have mentioned, reproduction in humans produces one full set of DNA - half from the mother, half from the father. The half-DNA from each parent combines, making a full set.

However, your aliens may not reproduce the same way; the mother may provide the full DNA set to her offspring, and have no need for a mate. Of course, this has a problem: any mutations in the mother's DNA will be passed on, even weak mutations, eventually resulting in a 100% infant death or miscarriage. With no way to breed out mutations, the species as a whole will die.

To fix this, pregnancy begins with a seed - not like a plant seed, but like a random number generator seed. The seed-DNA is not combined with the original DNA, but rather is used to set one half of the DNA to matching values. The two strands of DNA may not even be the same length, but that doesn't really matter. Only as many values as available in the donor DNA will be used; the rest will be either random, or matching the female. The offspring will always be the same species as the female.

With that system, the female would be able to use almost any cellular matter, as long as it had a vaguely similar DNA structure; even a virus would work. There is likely a genetic compatibility test that the female's body performs, to make sure that the DNA-seed is at least somewhat comparable before use: roughly the same length, not damaged, generally the correct structure, etc. Certain stimuli on the female, along with the donor DNA strand, begins the process; from there, it operates like a normal pregnancy.

However, this system has significant downsides. For one thing, one male may provide DNA that results in strong, healthy children, while another of similar genetic stock results in stillborn offspring. It's possible that an outside material could transfer DNA - a hair, a blood cell, even a virus. To overcome the possibility of unregulated mutations, it's likely that developing offspring newborn infants have a very rigorous development process, resulting in miscarriage unless the offspring is quite healthy; thus, females will likely need to try multiple times to conceive. With miscarriages being frequent, the reaction to a miscarriage will likely be quite pragmatic.

As an added note: there exists a species of fish, the Amazon Molly, that reproduces via gynogenesis - the fish must mate for procreation to begin, but the male does not actually contribute any DNA, except possibly under rare circumstances.

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  • $\begingroup$ That solves nearly everything. Maybe the downsides may be avoided by a mysterious feature that our current knowledge cannot explain. But with this I can have a believable scientist character formulating a theory that have sense of how this species reproduce. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 21:38
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    $\begingroup$ It may be that non-viable offspring miscarriage within days, so there is never a trace; there may also exist a "filter" that only accepts half-strands of DNA, to weed out other cells with full DNA from interfering. $\endgroup$ – ArmanX Feb 7 '17 at 22:04
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In order to have a realistic organism capable of creating halfbreeds, all species involved must have a VERY close common ancestor.

As an example, consider real dogs. A Chihuahua and a Great Dane are incapable of directly breeding without artificial insemination due to geometric isolation. If there were no other breeds of dogs between them to bridge the gap, they would be considered different species. However, both of these dogs could breed with some third dog of medium size.

Assuming you are looking at a science fiction setting, this probably means that all your alien species are human derivatives. This can occur if inter stellar travel is slow/rare, which would cause distant colonies to act as isolated populations. There are two ways that these isolated groups could have diverged.

They could simply have evolved differences over a very long time. This would probably work best over a period of ten thousand to a hundred thousand years. That is probably long enough to create significant phenotypic differences without necessarily developing any insurmountable genetic incompatibilities. To keep the populations separate long enough, you would probably need for the secret of spacetravel to have become lost for some reason. As an additional advantage, it is far enough back that it would not have interfered with recorded history. Moreover, you could jump directly from modern humanity to your desired tech level by finding the ruins of an advanced ancient civilization someplace on earth.

The other option is that the difference has arisen due to self applied genetic engineering. This cuts down the required isolation period to a few hundred years for the first few generations at each colony to have died off. Isolation on a timescale like this can be simply explained with a slow interstellar drive. If you want a much quicker drive for your plot, that can have been a far more recent invention. The other advantage of this is that your species can be FAR more anatomically different without the differences being insurmountable to genetic engineering.

If you are operating in a fantasy setting, things become easier. You can set up all of your races as sharing human genetics. Racial differences can be explained in this setup as being caused by heritable enchantments.

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  • $\begingroup$ Welcome to WorldBuilding.SE! Nice answer. $\endgroup$ – Secespitus Feb 16 '17 at 13:34
  • $\begingroup$ I would say more sci-fi than fantasy specially in what refers to this species but a degree of fantasy exists (no hard science in the design of the fictional species but I want to show that I gave it some thought). What you propose is very compatible with my story, some of the characters will come with a theory that all the fictional humanoid looking species share a common ancestor (humans aren't natives from Earth in the theory). In the end, that theory will be proven wrong. It's hard to explain without spoilers so I will stop here. $\endgroup$ – Hatoru Hansou Feb 16 '17 at 17:33
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There have been a lot of answers as to why it's impossible but I'm not sure it's completely impossible.

1) For this to be at all possible we have to assume the aliens are based on the same genetic code we are. This assumes that either we actually share an origin with them (panspermia hypothesis) or that there are only a few possible answers. (Which gives a list of some species yes, some species no.)

2) Normal terrestrial reproduction is out of the question for the reasons indicated in other answers. The vast majority of the DNA (and it would have to be DNA) for the child comes from the mother. Lets try a different reproductive strategy:

Alien sperm is obtained (I don't think the notion of sex akin to ours is out of the question. Our sexual apparatus exists to allow conception in an environment similar to the oceans from which we evolved. Like pressures will apply to land-dwelling aliens, I can see convergent evolution producing something akin to sex) and chopped up into individual genes. The mother makes a whole bunch of eggs, each with a few genes from the sperm added in and perhaps a few genes tossed out for good measure. In the first stage of gestation the eggs are soaked in a nutrient bath rather than implanted. Most eggs die, after a period of time the one that does the best moves on to the main gestation phase.

Your alien will have a low fertility rate (most attempts will not produce anything viable) very high miscarriage rate and a large rate of genetic defects. However, it will also have very fast evolution, in a sufficiently hostile and dynamic world the latter effect might make it worthwhile.

I can't see an evolutionary path to this alien, though.

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  • $\begingroup$ Good attempt to satisfy all the requirements. I like it very much. What would you say about the miscarriage rate and genetic defects just going away after some generations? For example, and this is completely random, let's say that in the story after 10 generations mating with humans alone no more genetic defects occur. Is 10 too soon? It sounds OK to you? $\endgroup$ – Hatoru Hansou Feb 7 '17 at 23:00
  • $\begingroup$ @HatoruHansou My system will always have a very high failure rate as the genes aren't being introduced in an organized fashion. $\endgroup$ – Loren Pechtel Feb 7 '17 at 23:14
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The reproductive system should cut both their DNA and the DNA of the species they mate with with the help of several enzymes. After this, another enzyme would have to take care of incorporating all the human DNA segments into their own.

You say this species was genetically engineered, but unless the designers were perfect then this reproduction system might result in the death of the embryo, so as to increase the chances of success the designers might have made the alien let out several eggs to be fertilized at once.

With this method of reproduction humans won't be the only ones these aliens can mate with, unless the enzymes were specifically created to target humans, or the aliens' DNA was specifically made to be able to incorporate their DNA (and the other species you want as well).

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  • $\begingroup$ For aesthetic reasons I don't want eggs. Depending on how this evolve I may change my mind but I will try to not. I don't want to expose the whole thing, but yes, they are engineered with compatibility with those species "in the list" in mind, that's the reason DNA compatibility may not suppose a problem, though the believability of that is a topic for another day. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 10:28
  • $\begingroup$ @HatoruHansou Is the species not made of cells like us? They should have eggs as well if they are. If they aren't then you need some sort of capsule to hold the enzymes and the DNA or else it will be all random DNA cutting all over the place and having offspring will be really hard. $\endgroup$ – user31746 Feb 7 '17 at 10:34
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    $\begingroup$ @HatoruHansou New idea. These aliens could have several small tunnels that each lead to a pair of DNA and enzymes. If the DNA incorporation is successful then the hole will grow with the offspring and kill all other DNA in the other holes. This would fix your problem of not wanting eggs and it would make your species either have no babies or only 1. $\endgroup$ – user31746 Feb 7 '17 at 10:40
  • $\begingroup$ That last one sound very original. As far as I have thought, yes, they are made of cells. I never considered the possibility of using a different building block. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 11:31
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The problem I have with this is simple: your genes don't code for "body parts", or complete systems. They code for proteins. That's all they can code for. DNA does one thing, and one thing only: make proteins.

The reason that a particular protein has an effect on your body is that it's part of a complex system, where having more, or less, of a particular protein has an effect. In some cases it's fairly straightforward - having more melanin production means you have brown skin, or brown eyes. But even in those cases, other proteins also have an effect in that area, and affect how the melanin works - some proteins affect how melanin is deposited, for example, the difference between brown or hazel eyes.

Most, however, have no value outside of the system they're in. There's not a gene for "make two arms" or "make two legs", it's a huge section of your DNA that interoperates to cause you to have two legs and arms instead of four legs, or a tail, or whatever. And it's not necessarily DNA all located together - for many of the larger systems (such as legs, arms, etc.) the DNA is on several chromosomes.

So the problem that I think is really unsolvable is, how does useful DNA get copied/absorbed. Bacteria trading DNA have the advantage both of a simple monocellular organism, and fast reproduction, meaning if something doesn't work there are a million other bacteria with different things that might work. Your aliens are presumably as complicated as we are - meaning that there's no way for this to work without extremely complex genetic engineering ability, and frankly well past the point of 'magic' for now at least. You'd have to have an understanding of how all of these proteins (and genes) interact in order for this to make any sense, well beyond what's meaningfully feasible.

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  • $\begingroup$ It seems you agree with IndigoFenix' answer. Human DNA has no use for an alien species, even if they have a similar reproductive system and use the same building block for life, and the problem doesn't seem solvable except we accept the alien DNA doesn't get mixed with human DNA. And the process works by somehow approve or not approve traits and let it influence the DNA of the newborn to avoid the boring result of having an exact clone of the mother. It dodges all the problems and try, in a smart way, to comply with the requirements. $\endgroup$ – Hatoru Hansou Feb 8 '17 at 0:22
  • $\begingroup$ "They code for proteins. That's all they can code for. DNA does one thing, and one thing only: make proteins." - That's not correct at all. "A large part of DNA (more than 98% for humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences." It used to be believed that non-coding DNA was "junk", but slowly over time we've been finding evidence that it is simply used for a different purpose (kind of like how almost every "vestigial" organ in humans has turned out to have an actual function). $\endgroup$ – industry7 Feb 8 '17 at 14:29
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DNA is basically a little, chemical-based computer + assembly mechanism. You can actually use it to run calculations if you have the equipment to set it up and read the result. Fungi on our own world are practically impervious to viruses because their DNA includes a checksum algorithm that identifies and discards code that doesn't fit.

So, theoretically, yes, you could have a species like this, if their DNA computer program is capable of analyzing the incoming half, figuring out how it works, and integrating the proper pieces into the proper places and intelligently filling in the gaps when bits don't fit right.

This would basically have to be genetically engineered by some kind of super(genius|villain) as the natural rate of accrual of code in a genome is probably too slow for such a being to have come about within the lifetime of our universe. Complex organisms moved away from using DNA to provide their actual intelligence a long time ago because it simply doesn't have a good mechanism for adjusting its own programming compared to neurons.

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  • $\begingroup$ More than smart DNA, as I already said in another answer, they are engineered to be somehow compatible with a list of species, including humans. More than smart, the DNA code is like overoptimized program with a lot of hacks and hardcoded stuff. The engineering stage was very smart, the resulting DNA has some "unnatural" flexibility as a result, but isn't so smart by itself. Does that increase believability? $\endgroup$ – Hatoru Hansou Feb 7 '17 at 23:24
  • $\begingroup$ Having a short list of compatible species doesn't affect the "believability" much, but it would, theoretically, reduce the complexity so you might only need a genius genetic engineer instead of a world-class supervillain. $\endgroup$ – Perkins Feb 9 '17 at 22:18
  • $\begingroup$ it has sense. It reduces the complexity of the system. Less things to deal with. Less things to try for stability. $\endgroup$ – Hatoru Hansou Feb 9 '17 at 23:10
  • $\begingroup$ @HatoruHansou But also a higher failure rate with pairings on the extreme ends of the listed species. For whatever that's worth. $\endgroup$ – Perkins Feb 12 '17 at 8:00
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Perhaps you could make them shape-shifters. This could be done through either mental coercion or actual modification of appearance. The shape-shifting is used to fool the males into donating their genetic material through simulated intercourse. In reality, it is just kept in a "pouch". Once in private, the alien female empties the pouch into her bio-lab. Some technological hand-waving can be used to manipulate the genetic sequences as desired, combine it with her own genetic material, and reimplant the fertilised egg.

Depending on the amount of genetic manipulation that you would consider acceptable, you might want to restrict compatibilities to those species with some common biology - compatible with any carbon-based lifeform, for example, or compatible with lifeforms with at least a similar cell biology, or must have DNA, or must have exactly the same base-pair rules, etc.

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  • $\begingroup$ That solves some of the problems I facing, like justifying mating and restrict the process to a fixed list of species. $\endgroup$ – Hatoru Hansou Feb 7 '17 at 21:23
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Actually there is nothing stopping your aliens from reproducing any way they want to and using genetic material from whatever species you choose.

There are organisms (see below) on earth that can absorb and later use genetic from other unrelated organisms, a process called horizontal or lateral gene transfer. Tardigrades are an amazing example that have apparently stolen genes from fungi, plants and other animals, foreign genetic material make up about one sixth of their DNA. Among the super powers that this DNA has conferred on the water bears are the ability to withstand the vacuum of space, to be dehydrated and to go into a suspended animation mode.

Actually there is nothing stopping your aliens from reproducing any way they want to and using genetic material from whatever species you choose.

There are organisms (see below) on earth that can absorb and later use genetic from other unrelated organisms, a process called horizontal or lateral gene transfer. Tardigrades are an amazing example that have apparently stolen genes from fungi, plants and other animals, foreign genetic material make up about one sixth of their DNA. Among the super powers that this DNA has conferred on the water bears are the ability to withstand the vacuum of space, to be dehydrated and to go into a suspended animation mode.

There are no obstacles to aliens using earth DNA, and there are plenty of reasons that such use would be advantageous to them. Its just as well that genes code for proteins and not body parts, it would be useless for an Alien to have a human leg. On the other hand the ability to digest earth foods would be useful as would being able to photosynthesise.

It seems almost a no-brainer that some species would take advantage of the human males's willingness to ejaculate small quantities of conveniently packaged DNA.

By the way the concept of 'species' is arbitrary. There is no real lasting natural boundaries for genetic transfer when you look at it on an evolutionary timescale and for many organisms on any timescale.

here is a link with some interesting info on tardigrades Tardigrade info from the smithsonian

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  • $\begingroup$ Reading through the link. OK, very convincing. As they are engineered the need for a believable evolutionary path is zero, as they evolutionary path occurred maybe inside a computer simulation (form a very advanced civilization). So convincing evolutionary path is not required. Using DNA or being able to benefit from ours is then, and you seem to agree, also possible. In the other hand, having accepted it being possible, we must give it some thought as how may it be done. Even if the actual answer result being completely different. $\endgroup$ – Hatoru Hansou Feb 9 '17 at 4:53
  • $\begingroup$ One more thing. I understood that the concept of species is arbitrary. Something human understanding came with. And when finally faced with complex alien life, except that we share a common origin, those concepts may prove obsolete to explain life in general, and only fitted to explain life on Earth. $\endgroup$ – Hatoru Hansou Feb 9 '17 at 4:55
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No, because sexual incompatibility is a major force in the creation of new species. If two organisms can produce offspring, the offspring will inevitably be a mixture of the genes of the parents. Speciation requires that whole sets of genes separate and only exist in one population or the other. If the two populations are constantly interbreeding, they remain a single species.

The only real exception to this is physical isolation- if two populations are not exchanging genes because they occupy different geographical areas or they are active/mating during different times of the day/year, they could remain genetically compatible for quite a while despite slowly drifting apart in terms of which genes are in which population (look at human populations partially separated on different continents for thousands of years). The problem is that since sexual compatibility between the two populations isn't being selected for in either population, eventually genetic drift will render them entirely incompatible with one another. Could take thousands of generations though.

In practice, we do see breeding between closely related species like coyotes, wolves and dogs or between horses and donkeys, etc. But you get sterile offspring and other problems. Compare that to something that is obviously a slightly different species like a human and a chimp- completely incompatible. The closest you'll probably get to inter-species reproduction between wildly different species is something like Alien, which is probably not what you're aiming for.

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Intelligent (re)design

I’m reading Iain M. Banks’ The Hydrogen Sonata, and came across this line:

“The Gzilt never joined in the great genetic mash-up that the rest of the Culture proper thought appropriate to ensure everybody could breed with everybody else,”…

(Page 316 in the paperback edition)

So, by simply hanging a bell on it Banks makes it belivable enough. Pondering it, I wonder about totally varied niches like aquatic eel-like species breeding with humanoid, but being intelligent rather than nature, the agent that designs the offspring can make sure it has a fully functioning body of one parent, and combine the traits in a more poetic way.

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  • $\begingroup$ I like the concept but, and I haven't read the book, doesn't that forcefully require lab work? Or do you think that the "lab work" can be packaged in a portable solution already built-in into our engineered species? $\endgroup$ – Hatoru Hansou Feb 9 '17 at 4:58
  • $\begingroup$ I think the lab work would be done inside the mother’s body, but the artificial tissues may need to access high-powered servers remotely if it’s a challenging or extreme case. $\endgroup$ – JDługosz Feb 9 '17 at 9:13
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Everyone has been going on and on about why this is impossible. However, plants experience cross-species genetic transfer all the time. So I see no reason why a space faring alien (to us) species couldn't evolve to take advantage of incorporating alien (to them) DNA into their own whenever they discover new life.

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    $\begingroup$ That's nearly the same general idea I'm working with. The other answers tried hard to provide also an explanation of how it may be done, and say "can't be done" when the "how" could not be found. That's OK too. $\endgroup$ – Hatoru Hansou Feb 9 '17 at 5:00
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I've been listening to Sword and Scale and a doctor had a really interesting thought on genetics. If three men have sex with a woman the same night and she then becomes pregnant. Who is the biological father? They all are, although only one of them has his half of the DNA matching the woman's other half of the DNA, there are bits of genetic data / mutations and the like that influence and alter the child's genetic code. Microscopic organisms shape us in the very same way. (http://podbay.fm/show/790487079/e/1389139313?autostart=0)

Perhaps the females are capable of breeding children by themselves (but as someone else said, that would have unwanted consequences) but this is their solution. It would also explain why the resulting infant is always of the mother's species.

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