We Finally Get to Talk About the Royals! (Subtitled: Inbreeding *Usually* Bad.)

Buckle up, boys, girls, and non-conforming, we're going for a ride. 

Follow me down the rabbit hole to the good, bad, weird, and fun info on inbreeding and why you should still probably avoid it even if there are beneficial reasons it occurs in within the animal kingdom (and plants too).

The Basics

Inbreeding is a pretty general term, but there a ways to quantify how much inbreeding is occurring. An inbreeding coefficient is the number against breeding and fitness equations in order to determine how much the inbreeding is actually affecting the overall outcome. A marriage with a second cousin in clinical genetics is generally defined as equal to or higher than 0.0156 (getting higher the closer two individuals are related) (Hamamy, 2011). How much a population is already inbred will also affect the coefficient, since a "second cousin" might not genetically be precisely that. In short, more inbreeding means a higher coefficient means a greater effect on an individuals fitness and breeding. 

Where is the Line Drawn and Why is it Not Always There?

Generally, when people think inbreeding, the idea is "ah, yes! That thing is always bad! Nothing good can ever come from that!" And while you're not...wrong...you're also not right. 

Creatures generally have a natural avoidance to inbreeding. The common idea is that if another mate is there then they will choose that one. However,  paper by Hanna Kokko in 2007 has a bit to say to the contrary (and she's not the only one). The paper works in theoreticals, posing the idea that given the ramifications to fitness, which they have chosen to define as number of offspring produced, inbreeding can be more beneficial than outbreeding given the right conditions. For example, if one individual were given a choice between mating now with a relative or waiting to mate later with a non-relative, the more beneficial option would be to mate now. This would be for a few reasons:

1) While the individual would not be in the "non-breeding" stage and still susceptible to other mates, they also might have to wait another breeding cycle until that opportunity occurs

2) Given the lifespan of some, the individual might not survive to or be fertile by the time of another breeding cycle

They also take into account parental care, which plays a significant role in whether or not inbreeding might be the better choice. 

3) Assuming the "choosing" individual is a female: the relative, the female, and then another female would all have the opportunity to reproduce because the female mated with the relative and the relative could move on (and possibly be more desirable)

4) Assuming that there is little parental care on either part, then both parents would have ample opportunity to mate and then find more mates.

Now, this is not to say that inbreeding will just be the better option, as seen in the other scenario where the "choosing" individual is given the simultaneous choice of relative or non-relative. There's also examples where there is absolutely no choosing within a population that led to my favorite quote, 

"Fitness is more strongly determined by how many times the individual can complete a breeding cycle...and individuals accept any mate almost regardless of how valuable the offspring are..."

Oh! It's got at least three legs and most of it's vital organs? Nice! Next!

As much as it's taboo and still not a good idea to go around practicing copious amounts of incest in the human world, we have to remember to not place human morals or stipulations on the animal kingdom (or plants...those too). 

Now for the fun part! 

But...What if I Wanted to Marry My Brother?! What's So Bad About it!

It's time to talk about the birds and the bees! When a mommy and a daddy love each other very much they swap alleles. Then, they go to a cabbage patch, mug a stork, and shove those mixed up alleles into a baby.

...

Okay, but seriously, let's all remember out Punnett squares really quick. If inside me I have some recessive alleles that might make some super terrible heritable disease, but only if it's recessive dominant, and I make a baby with a guy who just so happens to have that same recessive allele code, then the chances of the baby that pops out having that super terrible heritable disease has increased. 

With me so far? Review your Punnett squares. 

If I go out a marry a random dude, the chance of that happening and my dormant allele coming to the front is pretty small. However, if I marry someone (like maybe my brother) who has a very similar gene code and allele make up to mine, then the chance of that happening has now increased a lot. More than that, if down the line our ancestors continue to inbreed, then even little flaws will become more pronounced and defects will begin piling up, one on top of the other. 

One of the most well-known cases attributed to inbreeding was Charles the II of Spain, from the notoriously inbred House Hapsburg. Charles had a number of physical and mental ailments that are presumed to be caused, if not exacerbated by inbreeding. Neither of his marriages produced an heir and there were rumors he was simply infertile. Charles could not speak before the age of four and could not walk before the age of eight. An autopsy at the time of his death indicated he suffered from pituitary hormone deficiency. This can cause hypothyroidism, delayed or lack of puberty causing in an inability to have biological children, and a decreased cortisol that increases the body's chance of infection (to bring it full circle, he's purported to have suffered from malaria, rubella, chickenpox, and number of other ailments possibly because of this). He's also said to have suffered from distal renal tubular acidosis, where the kidneys don't remove acid properly from the blood or urine. Now, without any sort of DNA to test, there's no way to truly "confirm" that most of his problems can be attributed to inbreeding. But, given that both of these conditions are recessive in nature and it's extremely rare to have both of them at the same time, combined with the amount of physical deformities that were passed on through the bloodline and compounded: it's safe to assume that the inbreeding didn't exactly help. 

(Feel free to also join me down the pit into the DNA of King Tut if you so wish, a few articles linked below.)

https://www.nationalgeographic.com/culture/article/100216-king-tut-malaria-bones-inbred-tutankhamun

https://www.smithsonianmag.com/smart-news/newest-king-tut-theory-he-suffered-severe-disorders-due-inbreeding-180953113/

https://www.npr.org/templates/story/story.php?storyId=123781211

The Crossroad: Why is This Something We Need to Talk About?

For conservation purposes, this concept of inbreeding and when to pay more attention to it is incredibly important. It's not just The Hapsburgs that were/will be affected by constant inbreeding. There's a reason that at some point down the line they just...couldn't produce offspring. The gene pool became so small, the chance of mutation increasing so much each generation, that everything finally broke. Sometimes we have the option to shift the alleles around, like when breeding in zoos or transplanting individuals from one population to another. Other times, though, as Hanna Kokko has pointed out, it might be more beneficial for the species and individual to choose a relative that breeding season. The consequences in the natural world are relative and require more information on each individual situation, but having the background information on when to step in and when to let it go will be invaluable when inbreeding occurs. 

A Footnote: The Human World

Generally, the western world is not known for it's incest (and no, you can't just start singing "Sweet Home Alabama" at me). When looking for resources, though, it's hard to not find the information and awareness meant for areas where it's not only more common but expected. In places like Iraq, Iran, Turkey, Saudi Arabia, etc. marriages with family members is so common that most places require counseling to educate the couple before they go through with it. There are a number of reasons that people choose (and I do mean choose) to marry cousins or other, whether it be to maintain assets, knowing the family history, knowing the person since they were young, or whatever else might come up within the culture. There are even areas where the counseling is not effective because it came too late, plans had been arranged and lives were going to change whether they liked it or not. It's not really relevant to inbreeding in "the natural world", but I believe it does shed some light on the cultural differences that might incline people to choose inbreeding over outbreeding, such as the "choosy" individual in Kokko's study. 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3419292/

Links

https://www.bbcearth.com/news/what-are-the-effects-of-inbreeding

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3419292/

https://www.nature.com/articles/s41467-019-11724-6.pdf

https://www.discovermagazine.com/health/what-scientists-found-after-analyzing-cases-of-inbreeding-in-the-uk

https://medlineplus.gov/genetics/condition/combined-pituitary-hormone-deficiency/

https://medlineplus.gov/ency/article/000493.htm

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0014-3820.2006.tb01128.x