tl;dr version: What would be the consequences of population bottleneck/long-term consanguinity in Mythic Europe vs real life?
Super long version: Okay, so, Mythic Europe reality is based (loosely) on a blend of folkloric and academic beliefs held about the world in medieval Europe. The specifics of genetics weren't understood, but while the inner systems weren't comprehended, their outwardly observable manifestations are an obvious and universal part of being alive and in a community with people having kids, so broad things like "people get most or all of their traits from their parents but sometimes something goes wonky" and "incest: probably not a great idea" would be well-known, and thus represented within the Mythic European paradigm.
This awareness can be most obviously demonstrated by the fairly regular conflict occurring within the Catholic Church and its adherents over consanguinity laws - in the 800s they raised it from 4 degrees to 7, and in 1215, a mere five years prior to the canon start date of Ars Magica, they lowered it back to 4, ostensibly in response to marriage rates falling and illegitimate marriages happening more often as it became increasingly difficult for small, distributed communities to arrange enough marriages between adequately unrelated people. Of course, such understanding may not have been universal, particularly among the nobility, who did some mythologizing in their own right to justify the not-infrequent purchasing of papal indulgences to marry relatives due to a paucity of less-related marriage options of sufficient class, arguing on occasion (among other ideas) that the inherent superiority of high-class blood means the benefits of not sullying it by marrying across class outweigh or cancel out the negatives of incest. But still, it's clear that there was a general understanding that the less blood relation the average person had with one's spouse, the healthier the community would be.
What I'm a lot less clear on is how well the propogating effects of incest on a community isolated over a long period of time would be understood.
I'm working on a setting idea for a potential Theban Tribunal campaign I'd like to run. The idea is that when Rome was conquering and consolidating control of the Greek Peninsula in the 140s-80s BCE, a group of powerful Faerie Wizard priests in the major Greek city-states came together on a large-scale supernatural project, initially started as a method of protecting things and people of value and for guerilla warfare and reinforcement between allies against Rome, but which escalated at the culmination of the conflict and the realization that victory was impossible into a last-ditch effort to preserve as much of their cities' culture as possible while biding time for a triumphant return that never happened.
The project started out creating a Faerie regio network between the cities. When all hope was lost, the goal instead became to use these regiones as preservation grounds, taking as much of their irreplacable cultural paraphernalia as could reasonably be transported, as well as whichever citizens were willing to essentially go into mystical exile with them, using Glamour to turn the regiones into replicas of the city-states at the time they were fled from. The original hope was that this would be a temporary solution, and that they would come back when Rome was gone and they could reassert themselves as independent states right where they'd left off. This essentially never happened, with the area still in control of the Byzantine Empire as of the 1220 starting date, and Christianized to the point where they'd likely be targeted heavily if they came back regardless.
There's a bunch of cool ideas I want to explore with the idea of these cities trying to trap themselves in stasis for 1,300 years and how unintentionally alien that process would make them, like having the oldest people there have been around from the start and packing insane Warping scores from the anti-aging effects (and probably having ridiculously swingy Sympathy Traits to keep their Faerie Rank manageable), or different projects each city would undertake to try to retrieve more of their pasts such as making Faerie analogues of their deceased legendary figures, or how the Greek gods would likely be rather active in the regiones as the largest concentrations of their worshipers, and most of all how each place would respond to the inevitability of attempted cultural evolution despite their best efforts, with some evolving willingly because democracy and/or philosophy are themselves part of the cultural traditions they're preserving while others become stagnant parodies of themselves through strictly-policed reenactment of cultural norms and practices that haven't made sense in over a millennium.
But for the purposes of this thread I mainly want to focus on the incest. Within the context of the Mythic European belief framework, how screwed would these guys be if they weren't drawing in new breeding population from the outside world this whole time? Some of the effects could and probably would be mitigated by Faerie immigration, whether arriving autonomously or summoned/created to add to the population (either as citizens or slaves), but it would still be predominantly human, and I need to figure out the consequences of that in the Mythic European framework.
IRL, it looks like it would be at least plausible for a population of 500 humans to repopulate without being seriously messed over by genetic drift. A very quick and dirty Google search suggests that the "average" Greek city-state would have a total human population of about 50,000, of which roughly 10% would be actual citizens. Of course, that's super inexact, Athens would be at least four times bigger and "unimportant" city-states would be like a quarter that size, but that seems like a good ballpark to me with only the major city-states being part of the project, but also doing so during a series of losing wars across the peninsula. Depending on exactly how generously we fudge the "how many people came" and "what proportions of each class of person" numbers, the outcomes are... potentially plausible, but not pretty. Assuming 1% of the full citizens of these city-states went into exile with the project organizers, they'd just barely clear the "theoretically salvageable population" number even if they were insistent on maintaining reproductive class separation from whatever number of non-citizen freemen and slaves came with them. Then again, a sample of citizens from the same city-state might not meet the genetic diversity expectation of that 500-person estimate, because we're already pre-selecting from a shared hereditary class in the same geographic area.
On the flip side, though, I'm not sure how well-understood the compounding effects of generational consanguinous marriage on a large scale would be understood, and therefore reflected, in the Mythic European framework. I mean, Adam and Eve (and later Noah and... does his wife have a name?) are, within the predominant religious frameworks, the shared origin of all living humans, and that's an enormous amount of incest for the entire human population to be founded upon. Without the Mythic European paradigm understanding or reflecting the genetic underpinnings of why incest often has negative outcomes (the loss of genetic variance), it seems plausible that starting with a small genetic variance in the first place wouldn't necessarily matter - as long as the population eventually got big enough that people stopped making babies with their siblings and first cousins, their shared ancestors doing so would cease to have a negative impact on the community. If taken this way, the number of people who initially went into exile in each city would be completely irrelevant by the start date except as a small detail in the regiones' founding myths.
What do you guys think? Is my understanding of the Mythic European view on long-term incestuous community-building missing crucial information that would make the Greeks doing it turn out worse than when Adam & Eve did it, or, hell, am I wrong about the medieval conception of those human origin stories, and maybe they were like "well yeah Adam & Eve made Cain & Abel, but they weren't married to their sisters, God made wives for them out of dust or rib or whatever"? Any input is appreciated.
