A reader pointed me to this fascinating, if tragic, article about the rise of rare recessive diseases among a schismatic Mormon sect which dominates Colorado City. This group has been in the news since the their "prophet" was just arrested. The article points out that because of the inbred nature of the community, and its small size, one particular rare disease, Fumerase Deficiency, has now become rather common. I have talked about inbreeding before. Most of us know the problems that crop up intuitively from experience, rare traits begin to spread in an inbred population. But, what needs to be emphasized is the greater problem from long term customary inbreeding, as is common in much of the Muslim world (and now in the Muslim Diaspora in the West), and in isolated cases as above.
First cousins have a coefficient of relatedness of 1/8, that means you can expect 1 out of 8 genes to be identical by descent from the same ancestor. But as a small community marries only among itself soon everyone is a first cousin in many different ways. In the United States most first cousin marriages are between individuals who share only one pathway of near genetic relationship, via one of their parents. In places like Colorado City this is not so, the family tree is reticulated and twisted back into itself multiple times. This results in a reduced long term effective population, a self-induced bottleneck as a few individuals population what should be a more diverse constellation of ancestors (e.g., your great-great-grand-father is actually your greater-great-grather-father multiple times). Low effective population increases stochastic effects, random genetic drift, and so you have deleterious alleles which can rise in frequency rather quickly. Because of low effective population selection is swamped.
To calculate an individual's inbreeding coefficient, you take this equation:
FI = sum over all common ancestors[(1/2)i * (1 + FA)]2
You are summing over the paths to each common ancestor, FA, with i being the number of individuals in each path (obviously inbreeding via a common great-great-great grandparent is weighted far less than that via a grandparent). The key for the Colorado City Mormons, and many Muslim groups, is that tightly knit clans share many, many, recent common ancestors, so even individuals who are not technically first cousins may share more genetical similarities than the typical first cousin.
In the article above the anger and outrage at the "benign" neglect of the government at what was happening in Colorado City, and the refusal of the community to cease their inbreeding, is palpable. The Colorado City Mormons are dependent on the state for their well being, and their own actions have resulted in the generation of a dependent class of children who will never participate in society, as such. Though the case of Pakistani origin Muslims in Northern England is much milder per capita (due to the larger size of the community, and its more recent provenance1), the size of this community is going to have implications for the National Health Service.
1 - In many Muslim communities it seems that inbreeding has increased with "modernization" for a variety of reasons.
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This makes me wonder about the long term viability of a small population that inmarries. From what I know about the remnant Samaritan population of Israel and Palestine, they've had a tiny population as far back as they can remember (currently ~500) with strict laws about intermarriage. Apparently they do have some genetic defects from this practice, most noticably blindness, but what I find striking is that they haven't yet withered away from inbreeding.
Granted, I'm just some religion scholar with an interest in biology, so I don't know anything more than the most basic of details, but I really do wonder how they've managed to not inbreed themselves out of existence.
they've had a tiny population as far back as they can remember
my understanding is that their effective pop size was pretty large during the classical era. you are right that they've gone through a bottleneck. 500 is probably the limit of genetic viability over the long term.
from wikipedia:
Apart from that [intermarriages], all weddings within the Samaritan community are first approved by a geneticist at Israel's Tel HaShomer Hospital, in order to prevent the spread of genetic diseases.
so one reason they aren't withering is that have a proactive campaign of selective breeding.
Deranged fundamentalist ideology... crippling diseases as a result of those tenets... ain't it great when problems solve themselves?
i'm amazed that they can even live without fumarase. jeez
I see three important factors here.
1) Outbreeding produces a greater genetic vigor
2) Socially/medically a greater contrast in outbreeding produces greater stress, e.g cultural, biological-families of parasites etc-natural flora ,environmental accomodations to natural elements
3)Social-economic pressures, factors of wage earning etc-in a larger more diverse urban setting-personal freedoms with respect to new social conformities.
... as possible factors determining the potential success of outbreeding. It appears to me, that outbreeds would require a nuturing, and would not be as simple as ,"marry someone new." Evolution requires for its' increased diversity a periods of isolation to acquire a footing, where upon the simple suggestion to stop inbreeding would requires in addition a nurturng setting that I am not sure would be available in the same environements , cities towns, or in the modern society urban areas they had not seriosly mingled. I think in addition to this request, a new setting giving the means for a high probability of survival is also demanded. Personally also believe, that such a setting should be free of our prefabricated social medical services -dwelling problem -and economic solution council, for the courses of such VOLUNTEERS to find an original path. I think in such a situation our data banks and reseasrch solutions start clicking too quickly -mathematically as to aseert constrictions on potential variation similiar to the growing "know it all-ism " Nietzsche found rampant in Europe-of its' industrialism's and attitudes of one for the other-a blinding mechanical warming of higher population densities. if a disease potential grows through all of us, the attainment of a full natural lifetime would , first, not be within our natural wisdoms from experience, second, nor can it be assumed that the potential to overcome it is not also present if free in happenstance to find its' way.
From my view point we all have some ingrained negativeism, expressed in a sarcastic attiude, futilistic view point even in our comedy..of some historical bad luck event,, that I think has taken too deep a seed in our behaviors problem solving and living habits ...over confident in some situations from experience yet totally lined on-below the surface with an unnatural inception from a ? experience we cannot a path to stop for a break... the kids are out of the house finally..to school etc...but all the inlaws came by with all their problems, the same oxygen tanks and fertile advice ..there's no fresh air rest or relaxation.
Razib, there countless examples of inbreeding very small communities that have survived for ages and prospered. The whole Maori people in New Zealand is descended from a few families. No island people died off because of inbreeding. The Samaritan population survived before genetic councelling was invented, and anyway, they intermarry with Jews, so it is not strictly inbreeding. BTW the hospital does not grant licence to marry as you seem to suggest, but genetic councelling, which is not the same thing.
Anyway, inbreeding is a powerful way of cleaning the population from bad mutations and set the good ones. All domestic animals are in bred, and in the case of milch cows, most of the cows descend from a few bulls and are almost clones.
No island people died off because of inbreeding.
this is false. polynesia is actually a good case in point of meta-population extinction! (e.g., pitcairn was settled before the mutineers arrived, but it was too isolated and spare to make a go of it, and the descendents of the mutineers themselves have had problems with inbreeding)
BTW the hospital does not grant licence to marry as you seem to suggest, but genetic councelling, which is not the same thing.
i seem to suggest, doesn't mean i did. i didn't say must approve, i just said approve. don't turn your connecting-between-the-lines into a refutation of a point i didn't make.
the maori are not a case of long term inbreeding per se, but founder effect. the effective population was quite large on aotearoa in relative terms to other polynesian islands (the only ones close in size would be samoa or hawaii).
Anyway, inbreeding is a powerful way of cleaning the population from bad mutations and set the good ones. All domestic animals are in bred, and in the case of milch cows, most of the cows descend from a few bulls and are almost clones.
your exposition leaves out the fact that inbreeding is not the raison detre of selecting only a few animals, but selection is. they are looking for rare and specific traits. and humans aren't domestic animals, there's a reason we have to take care of them! (e.g., see the difference between purebred domestics and non-pure breds)
[in the case of selection for domestic animals selection for specific traits often results in a correlated response on other traits which would be deleterious in the wild. domestic animals can give us great insights, an they certainly were important for darwin's understanding of evolution, but we need to be careful about making simplistic analogies]
Are you saying that Pitcairn Polynesian population died off because of inbreeding? Do you have a more documented example? On the other hand, we have many populations founded by a few persons, and prospering mightily. All the millions of American Indians south of the Rio Bravo descend from maybe eight Siberian nomads.
And you know, human genes mutate so fast that inbreeding soon is not among true homocygotes.
And Mormons dont practice inbreeding per se, it is not a religious obligation as among Cohens, they are happy to receive converts.
PS Children are, in more than one way, domestic animals, bred for work, good behaviour, etc. Chinese were more successful breeders, their children are more obedient, intelligent, respectful, productive.
All the millions of American Indians south of the Rio Bravo descend from maybe eight Siberian nomads.
again, this is a population bottleneck. the principles overlap, but are not exactly what i am talking about. also, the amerindian evidence is predicated on single-locus analysis, they did go through a bottleneck, but take "8 indivdiuals" with a grain of salt.
also, part of the native american vulnerability to european diseases (probably on the order of 80% die off within 100 years) is due to lack of HLA polymorphism, which is the result of that bottleneck you mention. this is not totally what i am talking about, but it is not irrelevant.
In the Mormon community the majority of males of each new generation are diriven out, so the bottleneck is even narrower than you'd think. Seemingly at some point the whole population will have the samefive or ten male ancestors.
After reading the last few posts I have a couple of notions i had sorted out in my own mind, and after starting out beginning courses in Philosophy and history of ideas. To help get a good perspective on the word evolution and our studies:
The key word to all is "Change" how do we understand it-from ancient times the word had to be defined-the field of science started-what change does a person really see to change in the world but life and death. Who would have viewed the falling of an object and listed in to the catagory of change. Where along come newton, darwin, einstein, all with logical-mathematical description and discovery-Mendel suspicious invisible things -obviously smaller than whole animal behaving in a way to mathematically reveal the expressed traits from matings. This has grown a million fold more complicated--And Plato-Aristotle-Socrates-the world also has form-shape-we have hardly gotten back to-is all mathematics we know so far-nothingof the seeds opf life-just a construction plan-why and what makes a baby cry on birth in life. Our knowledge has gotten so revealing we forget these issues and are hardly a foot a out of the darkness we started with(a little to certain acting-an emotional difficulty we need overcome).
Nature does not select a good gene -put it in a pot- and think it goes along in a treee like way to everything in the pot. The growth of a better branch..etc. Evolution odes not work like that. I thinkit works more like the greater amount of potential diversity(instead of a thinning to grow a new branch-it grows in branches anyways) like a battery that is more fully charged. A maximum lifetime is when the charge is used most efficiently and not wasted.
Second a brancj can runout -a new one grow -and here we are in the midst of this-and part of this-and we want nature to work for us-however of we are going to insist on the shopping center right over there- the road there -the tree and farms over there -refuse suffering alomost tothe pointof refusing inconvenience-nature will eventually slap us in the faces to receive less that what we started with.
The one positive direction is always nuture-and the one problem is we do not always know that direction-a path that averages out that way that we do not induce change (thing change on their own and always know best).
In the case of the mormons the men that migrate spread diversity-the one that inbreded cause suffering-yet they are free to do as they wish in this regard and we are free to educate them as to how we think this suffering comes
about. We can suggest if we want a desert island or an oasis to settle -supply them if they accept-we should know the limits of our knowlegde especially if they do not ours theirs or both and our assesment must hold the truth-in the decision I dont know exactly is probably always true, but an adeventure in anew place is probably more appealing than the stiffling pressures , crowding, of joining with the masses in the urban areas etc. Changeing their breeding habits could cause worse than they have for the efficient use of their evolution energy charge. Concerns of using public dollars on their suffering for their more comfortable living instead of long range scientific plans is ridiculous considering the waste in other pockets.
I also came upon an article in Nature about the application of a pulling force to chromatin-it does stretch like a rubber band-it grows shorter and the twists increase. (I dont believe a pushing force cause the opposite either)-like Mendels discovery of invisible doing affecting things-the assortment of alleles(genes for the same traits(eg blue or brown eyes) in matings. here we are talking of an actual force and change -in a microscopic world we are still hardly aware of and hardly study without considering our own effects in the matter with respect to normal function in "private" of intervention-but may can assume only it(the chromatin) reacts similiarly to stress. But this cannot be mathematised too well but only to add to our intuition and learning-we should not proceed with confidence at any scientificlly acquired data in causing change -our intuitions evolve also and the (invisible-intangeable) forces that move us should remain that way (invisible and intangeable) in our heads before we blow ourselves away from some other endeavors directions inadvance of what or what not we cause to evolution or our better intuitive understanding of it.
For moderately maladaptive genes, inbreeding depression is a major problem because the genes aren't maladaptive enough to so compromise the fitness of an organism so that it can't pass the gene on to the next generation. Heterosis (hybrid vigor) is a good solution to this problem, but is generally only most beneficial in the first few generations (after which you often start to get inbreeding enhancement if the population survives).
Outbreeding depression can and does occur when genes from two heterozygous parents come together to interact in a bad way. Every gene expresses itself in the context of all the other genes the organism possesses. If they get too divergently mixed up, they can actually develop internal incompatibilities. The risk of outbreeding depression is low in early generations, but increases over time.
Inbreeding enhancement can occur when maladaptive genes do more seriously impact fitness, or where recessive genes impart positive impact rather than a negative one. There are lots of examples of this in genetics. The danger, of course, is the increase of homozygosity in an inbred population, which even if it is healthy may become more vulnerable to mutated or invading pathogens.
In practice, organisms chose breeding partners in ways that try to balance the genetic effects I just described. Since the first few generations of inbred populations tend suffer inbreeding depression, non-inbred populations tend to seek out heterosis, but only sufficient enough to avoid inbreeding depression and not court outbreeding depression (choosing mates that are close to their own family tree...but not TOO close). Already-inbred populations can actually get genetic benefit of inbreeding enhancement by continuing to inbreed.
This subject is actually very complex.