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This is an urban fantasy setting in which all manner of fantastical species of humanoid, called monsters, exist under humanity's nose, and in this setting there is a system of magical bloodlines called arcana. The higher a monster's arcana, the more of their species' powers they have access to, and the fewer of their species' weaknesses they are beholden to. For instance, the highest arcana of vampires can more or less shrug off daylight like Dracula could in the original story, whereas the lowest arcana of vampires are so weak to sunlight that merely having the sun in the sky renders them comatose even indoors.

For monsters, arcana is inherited from your parents, which is an important plot point that plays into a lot of the themes of the story. A great many monster species have permanently lost the highest arcana among them with no way to get them back, and there are entire organizations dedicated to preventing this from happening to any more of them.

Humans have arcana too, but unlike with monsters, it doesn't affect them in any noticeable way. Humans have magic, but they have no way to use it, so in practice all that a human's arcana determines is how powerful they would become if a vampire or werewolf or the like were to transform them, or what arcana their children would be if they reproduced with a monster.

Because human society at large doesn't know about arcana, they have absolutely no reason or ability to preserve the rarer, more powerful arcana. Which means that if the highest arcana are as difficult to inherit for humans as I'm planning to make it for monsters, who are actually trying to keep those bloodlines alive, then all logic dictates that the highest arcana will have completely died out of the human population without some massive conspiratorial monster intervention.

As a result, I'm tempted to have arcana follow completely different rules for humans, where parentage is irrelevant and the arcana you get is completely and totally random, essentially a lottery with the lower arcana being more common and the higher arcana being rarer. But this feels like a cop-out, difficult to justify, and potentially excessive, so I want to see if I have a better option here:

How can a trait be both inheritable and rare without being doomed to die out of a population that has no idea the trait exists?

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    $\begingroup$ Blood types are a good example of heritable but invisible trait. What is the fraction of the population in your country having AB− blood type? In my country it's 1%. (And in France it's 0.4%.) $\endgroup$
    – AlexP
    Commented May 30 at 14:40
  • $\begingroup$ @AlexP Is that going down overtime, or staying stable? $\endgroup$ Commented May 30 at 14:42
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    $\begingroup$ On a historical time scale, that is, measured in centuries, it is not dying out. On a geological time scale, that is, measured in hundreds of thousands of years, I don't know. (In general, Rh positive blood types tend to be more common than Rh negative blood types because Rh negative women are at a disadvantage when mating with Rh positive men, whereas Rh positive women are indifferent to the Rh of their partner.) $\endgroup$
    – AlexP
    Commented May 30 at 14:45
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    $\begingroup$ ...and everybody knows, when it comes to blood, vampires suck $\endgroup$
    – g s
    Commented May 30 at 16:16
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    $\begingroup$ I think you have the problem backwards. Rare recessive traits don't typically disappear from a population, they're just masked by the dominant trait. For example, blue eyes aren't going extinct despite being rare and recessive. I see no fundamental problem with a rare trait that isn't doomed to die out - the bigger problem is why it did die out in some monster populations, especially given the fact that arcana seems directly beneficial to monster survival and should be selected for rather than against. $\endgroup$ Commented May 30 at 17:51

8 Answers 8

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You want a trait to be inheritable and rare, but still exist even with no selection pressure favoring it?

Easy short answer: The trait's genes are recessive.

If it's a simple one-gene trait, though, that makes intentionally breeding for it really simple. Just find any compatible pair of individuals who both have the trait, and their children will be guaranteed to all also have the trait.

The arcana trait that you have described cannot be a one-gene trait, however, because that would make it a binary trait - you would either have arcana, or not have arcana - instead of the many-valued range you described. Technically, that could be handled by having many different alleles for the same gene, each giving a different strength of arcana, but that would be poorly suited for the characteristics you want.

So, let's go with a more complex answer: The arcana trait is a cumulative result of many genes, and for most of those genes, if not all of them, the alleles that give greater arcana are recessive.

Let's run down my understanding of the characteristics you want and how this would explain each of them:

  1. Arcana strength is inheritable.
    • It's genetic. That's almost the definition of inheritable.
  2. For humans, even with no selection pressure of any kind, high arcana does not die out completely.
    • Since the alleles that increase arcana are recessive, they can each have a large number of low-arcana carriers who pass it on to the next generation but do not have its arcana boost themselves. With no selection pressure, the prevalence of these recessive alleles would just drift slowly, with practically no chance of ever dying out.
    • With matchmaking being effectively random, a much smaller portion of humans will naturally have two copies of any given arcana-boosting allele just by how the probabilities and statistics work out.
    • Again from the natural results of statistics, some people will have double-recessive high-arcana alleles for multiple of the arcana genes, and the more of those genes they have it for, the rarer the combination will be.
    • In short, the recessive potential for various degrees of high arcana will be ubiquitous in most of humanity, and a rare few people will win the genetic arcana lottery by inheriting a large number of the same high-arcana alleles from both parents, and this is all a natural and true-to-reality way for it to work.
  3. For monsters, really high arcana is typically inherited from parents through well-known bloodlines.
    • This inheritance happens via intentional breeding to preserve the bloodline. Any even remotely worthwhile degree of success in such breeding will produce vastly greater numbers of high-arcana individuals than the random genetic lottery will, to the point where even the possibility of someone randomly having extremely high arcana might be completely unknown. Even if monsters get the same rate of random genetic lottery winners as humans do, it would be very plausible if everyone just assumes that every such unexplained lucky bastard is secretly a literal bastard, springing from one of the known bloodlines via an illicit affair, even though no one can determine which bloodline.
    • Because the high-arcana alleles are recessive, every member of any given bloodline will have a double set of each of the bloodline's high-arcana alleles. This guarantees that any breeding between two members of the same bloodline will always inherit all of those high-arcana alleles from both parents, resulting in the offspring also having the same combination of double sets of high-arcana alleles, and thus the same exceptionally high arcana. This explains why the breeding efforts are highly effective and reliable.
  4. Even outside of those bloodlines, monster arcana is inherited from the parents.
    • Parents with higher arcana have more high-arcana alleles, and will thus give their children more high-arcana alleles.
    • While only the high-arcana genes shared in common between the parents will be guaranteed to manifest in their children, each parent is also a recessive carrier for a number of high-arcana alleles. There is a chance of one parent's high-arcana gene combining with the other parent's suppressed recessive allele for the same gene to make that gene manifest in the child. There is even a chance that both parents are recessive carriers for the same gene, and that the child will get the 1/4 chance of inheriting the recessive allele from both parents to pair it up and make it manifest, resulting in a child that has an active high-arcana gene that neither parent has active.
    • Parents with more high-arcana active genes will statistically tend to also have more suppressed recessive high-arcana alleles.
    • There's a major random factor to the whole thing, but overall by far the most common result will naturally be that a child's arcana is close to the average of their parents' arcanas.
  5. The really high-arcana monster bloodlines are difficult to maintain and continue.
    • The full extent of high arcana of a bloodline is inherited only by full purebreds. If a bloodline member breeds with a normal monster, a large percentage of the high-arcana alleles from the bloodline member will get paired in the offspring with dominant low-arcana alleles from the other parent, significantly reducing the strength of the offspring's arcana.
    • Even crossbreeding one high-arcana bloodline with another high-arcana bloodline will produce weaker offspring, because the different bloodlines have different sets of high-arcana genes. Any child of such a union will still have higher than normal arcana from whatever portion of the genes is shared by both bloodlines, but most of the non-overlapped arcana genes from the different bloodlines will put the high-arcana alleles into recessive carrier status, overridden by the dominant lower-arcana alleles for those genes from the other parent. The statistics of what happens with the non-shared arcana genes are substantially different than for typical monster pairings because inbreeding of bloodlines tends to reduce how many genes have non-matching allele pairs.
  6. High-arcana monster bloodlines even tend to outright die out despite active efforts to maintain them, and the greatest heights of arcana have been permanently lost already.
    • Because of the requirement of full-up purebred breeding to get the next generation to inherit the full arcana strength of the bloodline, such bloodlines will quickly get very heavily inbred. The heavy inbreeding will cause increasingly high rates of deformities and other genetic defects, eventually making the bloodline outright nonviable.
    • The highest-arcana individuals would be the rare long-ago genetic lottery winners who founded each of the bloodlines. Each of those individuals was initially the only member of their bloodline (their parents and siblings don't fully count because they didn't have the full combination of the bloodline's entire set of high-arcana genes), and thus had no other bloodline member to breed with. The very first generation of each bloodline after its founder would thus necessarily have already suffered from the weakening of bloodline dilution.
  7. Potential bonus implication, maybe? I'm not sure if this part would be suitable for you to use in your story. It's completely optional, however, so you can use it or ignore it without changing anything else. There are true-to-reality ways that the monsters could go beyond merely preserving bloodlines, and actually breed new bloodlines that are even stronger than the original bloodlines, even with no knowledge of the underlying mechanics of genes.
    • Crossbreeding different bloodlines may initially produce weaker offspring, but the first generation of the crossbreeding will have at least one copy of the recessive high-arcana allele for each arcana gene that either parent bloodline has.
    • Further selective inbreeding of the crossbreeds can, over generations, get more and more of the suppressed recessive alleles to pair up. This would make more of the high-arcana alleles take effect, increasing the arcana strength of successive generations. This would happen via the same mechanics of how inbreeding causes recessive genetic defects to manifest.
    • A portion of the high-arcana alleles would likely be lost in the process, but if such a crossbreeding project is done competently, the portion of high-arcana alleles made to pair up and manifest should be considerably larger than the portion lost.
    • After strengthening the crossbred bloodline's arcana for a number of generations by the above approach, crossbreeding it with the parent bloodlines again would re-introduce all of the lost high-arcana alleles, and would start off with a much higher portion of overlap between the bloodlines, and thus a much smaller initial loss in arcana strength.
    • Repeating these steps over time, and eventually crossbreeding with additional bloodlines, could ultimately produce a bloodline that combines nearly every high-arcana gene, reaching the highest possible arcana.
    • To be consistent with the point of the highest-arcana bloodlines having been lost, you could simply have this be a project that is still in progress and is not yet showing the hoped-for eventual results. This breeding plan would take a great many generations to bring to full fruition. It would be very plausible that it took a long time for someone to come along with enough ambition, ingenuity, vision, and patience to even begin the project, and that it hasn't been long enough since then to reach the goal. You could even have the mastermind behind the project be an outcast mad scientist, scorned and ridiculed by most for putting so much effort into something that most others think can't possibly ever work.
    • I am very much not an expert on the topic, but my understanding is that the process I described is a real-world breeding strategy that people have historically used on various types of livestock to great long-term success, and that its first successful use happened long before Mendel pioneered the beginnings of the study of modern genetics. Writing your monsters as attempting it would therefore be completely realistic, regardless of how much or how little they know about genetics.
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The monsters have a Hapsburg problem

Arcana, as a trait, is massively polygenic, being affected by hundreds of genes, and is poorly understood.

By chance alone, and due to a small population of monsters, several of the most important genes were lost. Monster populations essentially hit a genetic bottleneck. The fewer individuals, the more likely traits are to be lost by random chance.

However, their response made it much, much worse. They decided this loss was from "blood purity" or some bit of fantastic racism, and started excluding humans from the more important bloodlines, focussing solely on breeding from within their own bloodlines.

Unfortunately, this has done little to slow the decline, but instead reduced the size of their effective population, and increased the rate of loss of alleles. In some bloodlines, this has fixed a good number of positive arcana alleles (unfortunately, with some negative traits, but we won't talk about the Von Hapsenvurg chin, the count is extremely sensitive), but with the downside that the population shows very low variation, so no real improvement is possible. Birth rates are also plummeting, and many vampires are not healthy when born.

However, humans are not selective - so an indivdual, essentially, gets a random selection of these alleles. Many will be below the average of one of the inbred vampire clans, but there's a chance for an individual to far surpass them, if they inherit more of the beneficial alleles.

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I think your issue is a tempest in a teapot

Name a single human trait that has "died out." I don't mean through evolutionary time scales (although I'd still posit it would be difficult to answer), but through "modern" times — let's say the last 100,000 years. Anything?

Here's a list of ten vestigial organs that humans still have despite no real reason to have them and certainly no breeding intent to keep them:

  1. The Appendix
  2. Arrector Pili and Body Hair
  3. Tonsils
  4. Paranasal Sinuses
  5. The Plica Semilunaris / Nictitating Membrane
  6. The Palmer Grasp Reflex
  7. Parts of the Ear
  8. Male Nipples and Breast Tissue
  9. The Tailbone
  10. Wisdom Teeth

The truth is, it's nearly impossible to eradicate a genetic trait. A particular trait may diminish (notably in a specific group of people), but even in complete isolation where the trait is recessive and there's no outside influence reintroducing the trait, it'll pop up from time to time.

Then there's the bloodlines where traits pop up over and over because they're not recessive or because it keeps getting reinforced as kids are born. Here's a list of common traits.

  1. Tongue rolling
  2. Earlobe attachment
  3. Dimples
  4. Curly hair
  5. Freckles
  6. Handedness
  7. Hairline shape
  8. Green/Red Colour blindness
  9. Hand clasping

That link would be a good read as it introduces the ideas of what makes traits inheritable and how inheritance happens. However, it boils down to "they exist" and "people like having sex."

If you think about it, the U.S. has been arguing your question since 2013 when the same-sex marriage debate heated up. To get it out of the way, the "simple truth" is that government services in a "free society" should not be limited to people who share the government's ideologies. That debate, however, was overshadowed by the idea of how does one know if they're homosexual? There's no definitive scientific test for it, even now, and if we look at the propagation trends of various known genetic traits (everything from the items on both of the above lists to things like dwarfism, pulmonary weaknesses, etc.) we discover that homosexuality should be a rare trait — but there are more people claiming homosexuality than science supports. I am NOT trying to stir up those old debates. I'm merely pointing out (using a brutally simplistic and probably unfair assertion) that if you consider the tendency to not breed on its own merits, it should have "died out" shortly after the tendency to not eat. And yet here it is, a fundamental and defining attribute of our modern society. Which leads me to an important point:

Your real problem is how to rationalize the trait as rare

Although "nobody thinks about it so nobody breeds for it" should be good enough for government work to rationalize why a fictional trait is rare.

Explaining why a rare trait pops up in groups or families is even simpler, like the preponderance of Irish red hair among a population that lives on an island or using an idea like "the relationship between traits A and B are so strong that one almost never sees one without the other:" and then make trait B something that makes large families difficult.

To more specifically answer your question

  • All genetic traits are inheritable. That's what makes them genetic traits.
  • It takes millions of years for genetic traits to actually vanish — if they ever do. When they do, it's probably<citation required> because something else has taken their place.
  • Rarity probably<citation required> has more to do with a trait not being sexually desirable than anything else. Although the only reason off the top of my head that would justify a tendency to magic as not sexually desirable would be the tendency for humanity to go mad with power and become something that needs swift destruction. You know... like politicians. (Remember remember the fifth of November...)

One more thing... People don't like hearing the phrase, "it's your world, do what you want," but you're talking about a genetic tendency toward magic. Magic! You've already hand waved the existence of a scientifically impossible trait but you're worrying about its inheritable viability. Why? In a world where readers happily consume book after book about vampires and magicians, why are you worried about justifying any of this? The idea that the tendency toward (e.g.) magic is a latent ability in humans that sometimes manifests in that one individual or group of individuals with strength isn't a new idea. In fact it's a very old idea. This is one of those questions where good worldbuilding is expressed as, "set the rule as your narrative requires and move on."

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    $\begingroup$ The appendix is believed to have a purpose (maintaining gut microbiome); the tonsils have a purpose (immunological). It would be eminently unsurprising to me if some of the others in your list were also not vestigal. $\endgroup$ Commented May 31 at 4:47
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    $\begingroup$ @Spitemaster I don't know the genetics of red/green, but as someone with some red/green impairment I can see it being a trait that benefits society if some of it's members have it. Growing up I had a problem with understanding the concept of camouflage--the totally obvious tiger "hiding" in the grass?? Having someone in your hunting party that's unlikely to be fooled by such things might compensate for the fact they're not as good at some other color tasks. And for the occasional tetrachromat woman I don't think there is any downside other than not agreeing what color something is. $\endgroup$ Commented May 31 at 5:42
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    $\begingroup$ @Spitemaster it would be eminently surprising to me if any of the 10 listed organs were vestigial. $\endgroup$ Commented May 31 at 11:28
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    $\begingroup$ To add to the not vestigial organs list: Arrector Pili tighten the skin and reduce bleeding when wounded. Body hair helps us feel things like tics and and flees so that we can better respond to parasites. Paranasal Sinuses offer thermal insulation and function like spaced armor against facial injuries. The Plica Semilunaris / Nictitating Membrane allow greater rotation of eye because of how they affect muscle attachments. The Palmer Grasp Reflex creates is a learning mechanism for voluntary grasping. "Parts of the Ear" is rather nonspecific, so I'm not sure what you mean, but in general... $\endgroup$
    – Nosajimiki
    Commented May 31 at 16:44
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    $\begingroup$ the whole structure of the ear functions to direct and amplify sound. Male nipples are not directly needed, but they actually make a lot of sense because they are so important for females to have. If women with slightly off hormones did not develop nipples at all, then that would significantly impact thier fitness; so, its better for men to have what we don't need than for women to sometimes fail to develop them at all. The Tailbone is used as an anchor point for muscles used in walking. When an adolescent loses an adult tooth, the wisdom teeth are often allowed to come in to replace it. $\endgroup$
    – Nosajimiki
    Commented May 31 at 16:44
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Arcana makes humans more resistant to magic.

While they can't access the magic, having a high arcana rating gives you substantial resistance to magical attacks. Mind control, being turned, magical attacks. Such attacks are more common than most people know, and people with rare bloodlines are notably more resistant.

So, bog standard humans who have encounters with the arcana tend to die. The vampire mesmerizes them, the voodoo priest zombiefies them, the wizard curses them. Ones with stronger bloodlines can resist these effects and survive. They ignore curses, break illusions, and can resist being turned.

This means that stronger human families will tend to have stronger arcana. Political leaders without arcana are prone to being mind controlled or killed by magic, while political families with strong bloodlines survive. Likewise, the rare humans who do believe in magic and form hunter communities will tend to have magical bloodlines.

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From the comments, blood type is a good place to start. Blood type is heritable, but like most genetic traits you get one from each parent, and it's the mix that determines your type. So, having a father or a grandfather with AB blood doesn't mean you will, and in fact both the A and B alleles in your grandfather's genes could be totally absent from your own, because the O allele is by far the most prevalent in human bloodlines. So if your father's father was AB, and your father's mother was A with an O, your father could also be A with an O. If your mother has an O allele as well, you could have type O blood, just two generations removed from someone with AB blood. This is why, despite A and B being relatively common in most world cultures (B is most prevalent in Asia where there's roughly an even split between O, A and B blood types), type AB blood is found in just 3% of the world human population, and once you add in RH-factor, AB-negative is seen in less than 1% of humans.

Now, you can foster a higher percentage of ABs by selective breeding; a parent with AA alleles and a parent with BB alleles are guaranteed to produce an AB child, and two AB parents have a 50% shot at producing an AB child (with a 25% chance each at an A or a B, that if you pair with the opposite type are again guaranteed to produce an AB). So, by simply typing everyone and "encouraging" commingling between ABs and between people you know are "pure" As or Bs, you can increase the AB prevalence to about 75% of the population if you had enough time and enough compliance from your gene pool. But to get there, you have to breed out the O allele from both parents; a parent with type A and a parent with type B blood, each with a recessive O allele, can produce children with any of the four types with equal probability, significantly reducing your AB population percentage.

This kind of genetic mix happens for a lot of other traits, and it's not a difficult handwave to say that a similar genetic lottery is in play among non-monster humans to determine their potential arcana. The more genes and possible alleles for each gene, the rarer the trait, and the more selective breeding you have to do to keep the correct alleles in play wherever you can find them (even if other genes aren't what you want). How rare the "jackpot" is naturally, and how easily you can nudge the dice roll by encouraging selective breeding to increase the chances of a high-arcana child, are variables you can address in your writing. One would think that the world media would start taking notice if seemingly random pairs of men and women were reporting they'd been kidnapped and placed in rooms together, only released once the woman was pregnant by the man. Or, by the same token, as long as the monsters aren't too overt about these activities, you can have your world's mass media discount these people as kooks on a level with alien abductees. Add a dash of your well-known club/date-rape drugs in the mix to confuse memories, and limit how wide the monsters cast their net to find perfect genetic matches, and you'll keep any weirdness in these peoples' stories safely at "morning-after excuse" levels.

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Some quick thoughts on justifying higher arcana in the human population than the monsters possess:

Having inherited arcana in humans have a small but noticeable effect on their personalities such that people with like or complimentary arcana tend to get together would help arcana to last longer in the human population. It may even lead them to concentrate in some small communities. I can see towns, or regions, where a particular arcana is already strong seeing a steady trickle of others with the same trait moving in after they take a short visit or pass through on a roadtrip. They experience a strong feeling of coming home even though they've never been there before, apparently people often feel this way about Cornwall and move there.

The monsters may not try to mess with human breeding but if the magic itself is inherently active in it's own defense/perpetuation then it may manipulate humanity into preserving it.

The monsters may also have the breeding practices needed to preserve/promote higher arcana backwards. While they're busy pairing individuals with highest existing arcana together to try to preserve that strain, with some success, humans aren't trying to preserve anything and their pairings of certain different, but highly compatible, low and medium level arcana produce individuals with the highest, generally extinct, arcanas.

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High arcana doesn't affect humans in any noticeable way, that is, it doesn't confer any physical or mental differences on those who possess it. However, humans' latent magic enables them to sense high arcana individuals. Not in any conscious sense, and they don't recognise it as magic of course. People just get a vague feeling of 'there's just something about this guy/girl' - it works like an odd sort of charisma. Still, it can make a person more attractive and more liked/respected, which makes it a beneficial genetic trait.

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Other answers have pointed out that it's not odd for this trait to survive among humans . . . but then that just inverts your question: how come it's dying out among monsters?

One possible answer is that there are cultural/psychological factors that deter propagating arcana. If monsters with high arcana have elevated status, then:

  • In some monster species, the families with high arcana will want to preserve their own uniqueness, trapping their high arcana in their own small gene pool that shrinks and becomes more inbred over time.
    • Even if they come to recognize that this is happening, they may find it hard to convince themselves to breed with monsters that they see as inferior.
  • Some monster species may grant important offices to monsters with high arcana . . . oh, and did I mention that the holders of these offices are expected to be celibate? Even if that expectation is *cough* somewhat optimistic, it could still sharply curtail the propagation of high arcana. (Especially if monsters have access to effective birth control, or if their biology makes reproduction straightforwardly optional.)
  • Some monster species may practice monster sacrifice, and a monster with high arcana may be considered a more worthy sacrifice.
  • In some monster species, warfare may be extremely common, and either monsters with high arcana are much more likely to be sent off to war and die, or monsters with high arcana are much more likely to be targeted by enemies.
  • Some monster species don't have any of these problems, and simply aren't a concern for the organizations you mention that are dedicated to preventing the loss of more high arcana.
    • These species may serve as convenient foils for the monster species that you are interested in.

Among monsters with truly human-like psychology and reproduction rates, none of these would be likely to be enough to drive a useful trait to extinction. The families or cultures or tribes that don't have these issues will simply continue producing monsters with high arcana. (For example: European royals interbred a lot, with some families completely disappearing; but they also left a lot of descendants who aren't part of the royal family but do carry inherited genes.) But your monsters are probably more interesting if their psychologies are different from ours, anyway, and even more interesting if their psychologies are also different from each other's!

Incidentally, these issues all give good scope for your "entire organizations dedicated to preventing" arcana from being lost. These organizations will work in different ways depending on the issues their species face; in some cases they'll try to change cultures, but in other cases they'll just try to subvert individuals.

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