Historic population statistics & ancestry

[Kato]

Since it seems to be athing that pops up once in a while I decided to do some number crunching on numbers of ancestors and conclusions drawn from that. And I'm not entirely sure about my results :smalltongue:

So, assuming we have had a new generation every 25 years, thus four per century (though, I guess 20 years or five per century might be a better estimate?) if I'd have a perfect lineage where none of my ancestors are related/identical I could estimate the number of ancestors I have by just doing something like 2^(years in the past/25), right?

If we go back say, 1000 years/40 generations this way, we'd get almost 1.1 trillion (1.1 10^12) ancestors. Since no estimate I can find puts the number of people alive back then even close to that (more in the area of 300 mio) and I'm probably not related to too many Mayans (though, I can't be sure but I guess my central European origin has been rather weakly impacted by people from too far away) this is probably way off.
Okay, let's assume there were some relations between my ancestors (instead of turning down the number of generations) I tried substituting the 2 for a number between 1 and 2, getting something like 5 billion for 1.75 (which wasn't reached until the 20th century) or 11 mio ancestors for 1.5. So, assuming the population of central Europe back in the day, the last seems... reasonable-ish? Maybe a few more, but certainly not as much as I'd get for 1.75.

So... does that imply as large an amount of "inbreeding" as it suggests? 1.5 on a first rough interpretation seems to mean two in three ancestors were identical, thus that's a pretty large part of my family history that overlaps. I mean, as far as I can tell I'm a pretty healthy adult, so it didn't cause any effect (yes, I know, short term inbreeding is far more dangerous), but it also kind of puts in perspective the question of how closely related people are and how many ancestory two different people share. Obviously, this strongly depends on family history and region etc but I think it to some degree applies to everyone.
Or did I screw up somewhere? (Yes, it's a simplified calculation but so much it would give drastically wrong results?)

[CarpeGuitarrem]

Mainly, it has to do with the family size you're assuming. Your initial estimate placed 1000 AD world population at a trillion because if each family only has one child who survives and perpetuates the family line, the total world population will decrease, since each family means two family lines converging into one.

In reality, you should be making a higher estimate. I think that 2 is still too conservative, because many families were historically much larger than that. Even today, 1-child and 2-child families are only the norm in certain parts of the world.

[Spiryt]

Much more 'inbreeding' certainly has to be in effect.

In small societies most people would live in, those lines would cross multiple times, without no one knowing/remembering.

Much greater death rate from about everything too - modern population is likely coming from relatively few lines of most successful reproductions, while millions of lines sadly made it all the way since Precambrian only to be halted anyway.

[Lord Torath]

"Inbreeding" is actually pretty common. My wife found out that her parents are 5th cousins (located 5 or 6 generations back), which makes her 6th cousins with each of her parents.

I saw a comic in a genealogical library to the effect that you are a descendent of William the Conqueror, whether you like it or not, because if you follow the same "each of my ancestors had 2 distinct parents" back to 1000 AD, you end up with a number bigger than the world population at the time. Again, they were poking fun at this idea, not proving that he's everyone's Great Great .... Grandfather.

[cobaltstarfire]

I am of the general assumption that inbreeding has happened in everyone's ancestry on multiple different occasions. Just because it has happened, even within the past 2-3 generations doesn't mean you're going to end up with unhealthy individuals, the chances of defects just increases as more and more recessive genes begin to build up.


To use an example, all the little country towns off to the north/west of where I live are full of people who are kin to each other. They all migrated from about the same area of Tennessee to the same area here in Texas in waves some time ago, which is part of what facilitated inbreeding for them back then.

[Tvtyrant]

Remember that most people did not travel very far in their lives before about 1900, so it is fairly natural that you would marry someone related to you. As marriage always acted as an alliance system between tribes, there would be a kind of amorphous continuum of kin radiating out from you into the the surrounding area.

[Zyzzyva]

"Inbreeding" is actually pretty common. My wife found out that her parents are 5th cousins (located 5 or 6 generations back), which makes her 6th cousins with each of her parents.

I saw a comic in a genealogical library to the effect that you are a descendent of William the Conqueror, whether you like it or not, because if you follow the same "each of my ancestors had 2 distinct parents" back to 1000 AD, you end up with a number bigger than the world population at the time. Again, they were poking fun at this idea, not proving that he's everyone's Great Great .... Grandfather.

That's not really true, though. If you're English, sure. If you're West European, probably. But if you're, say, Japanese, no. Big global population shuffling didn't really start until the Columbian Exchange (there was always small population shuffling, but the fact that there existed a person in France in 1000 CE who had a descendant in Bengal in 1500 CE, which is probably true, doesn't really say much about most people's lines of descent, which were generally fairly static).

[crayzz]

So... does that imply as large an amount of "inbreeding" as it suggests?

Depends on what you mean by "inbreeding." Usually, people don't consider more than 5 or 6 degrees of separation, and when they do, it's usually direct ancestors. I can't even name any 3rd cousins (I'll have some when my 2nd cousins have kids, probably within a decade).

But yes, the prediction of 1.1 trillion distinct ancestors is wrong because it necessarily requires totally separate lineages. That assumption works for a couple of generations, but going back 10 or 20 generations that assumption breaks down. Some of that is what we refer to as "inbreeding," but I would guess a lot of it is stuff like "two people with a common great-great-great-great grandfather marry and have kids," and that sort of scenario doesn't fit the connotations of "inbreeding."

[Lord Torath]

That's not really true, though. If you're English, sure. If you're West European, probably. But if you're, say, Japanese, no. Big global population shuffling didn't really start until the Columbian Exchange (there was always small population shuffling, but the fact that there existed a person in France in 1000 CE who had a descendant in Bengal in 1500 CE, which is probably true, doesn't really say much about most people's lines of descent, which were generally fairly static).This was exactly the point of the comic. The mathematical model used to calculate the number of ancestors you have doesn't match reality, which is what Kato is running into.

[Zyzzyva]

This was exactly the point of the comic. The mathematical model used to calculate the number of ancestors you have doesn't match reality, which is what Kato is running into.

Sorry. Missed the point. :smallredface: I have seen the "you're all descended from Julius Caesar!" business propounded seriously before, so I kinda jumped to assumptions.

[razorback]

"you're all descended from Julius Caesar!"

Are you implying I'm really not Julius Caesar CIII???

[Zyzzyva]

Are you implying I'm really not Julius Caesar CIII???

Given he had no grandkids... yes, I'm implying that. :smalltongue:

[tomandtish]

"Inbreeding" is actually pretty common. My wife found out that her parents are 5th cousins (located 5 or 6 generations back), which makes her 6th cousins with each of her parents.

I saw a comic in a genealogical library to the effect that you are a descendent of William the Conqueror, whether you like it or not, because if you follow the same "each of my ancestors had 2 distinct parents" back to 1000 AD, you end up with a number bigger than the world population at the time. Again, they were poking fun at this idea, not proving that he's everyone's Great Great .... Grandfather.

There's a possibility that my wife and I are 7th half cousins on the maternal side. Unfortunately because of the circumstances, we'll probably never be able to determine one way or the other.

[Aedilred]

Once you get out beyond first cousins (or double second cousins) inbreeding becomes largely irrelevant on a one-off basis. The degree of genetic overlap is likely to be relatively small. And research suggests there may actually be advantages to breeding with first, second or third cousins: such unions tend to be more fertile.

The problems tend to come as a result of very close or repeated inbreeding: parent-child, sibling, or first or second cousins repeated generation after generation. This can lead to harmful recessive alleles becoming more prominent, and exacerbate conditions affected by genetic predisposition. There are a few well-known examples in European history where a degree of inbreeding was common: the madness that found its way into the Valois from the Bourgons, and ultimately into the Habsburgs; Habsburg jaw; the supposed porphyria and haemophilia in the house of Hanover, and so on.

Eventually you will end up with local genetic clusters and people who might well share some similar characteristics (range of hair colours, range of heights, and so on) but not to the extent they'd be considered inbred (although it doesn't stop jokes about, say, Norfolk). This is in fact pretty obvious if you look at the difference in appearance from people around the world. Some characteristics might be emphasised or de-emphasised, such as resistance to certain diseases, or tolerance for some foodstuffs (like lactose) and these might be advantages or disadvantages depending on perspective and location, but are unlikely to cause problems for the population in its "native" area since the tendency will be to select for beneficial characteristics, as with evolution and selective breeding in general.

[malloyd]

So, assuming we have had a new generation every 25 years, thus four per century (though, I guess 20 years or five per century might be a better estimate?) if I'd have a perfect lineage where none of my ancestors are related/identical I could estimate the number of ancestors I have by just doing something like 2^(years in the past/25), right?

That's correct. It's just the assumption that none of your ancestors are related that's wrong. And yes, this does pretty much mean that everybody in the world more than 7 or 8 centuries ago is either an ancestor of everybody alive today (often many times over) or has no living descendants at all. If anybody at all is descended from William the Conqueror, almost everybody alive is. This is true even for highly isolated populations if you go back a little further. If *one* person managed to survive the trip from Tasmania (the most genetically isolated place on Earth) to anywhere else before 1300 AD and managed to have descendants, then everybody is descended from him to. Since everybody on Tasmania 6 or so centuries before that was either his ancestor or had no descendants in 1300, that's all it takes to include all of them in the ancestry of everybody too.

The inbreeding though, that's different. The thing is that relationship is continuous, but genes aren't. You only have 46 chromosomes. Neglecting crossovers, this means that you did not inherit any genes from at least 18 of your 64 great great great great grandparents. It could be more of them, indeed it's mathematically possible, if vanishingly unlikely, for you to share none with two of your grandparents, but at least 18 of them are not genetic relatives even though they are your ancestors.

So yes everybody alive in 1000 AD who has descendants is your ancestor, but no more than 46 of them passed on a chromosome to you. Some number of the rest of them will be close enough relatives to those 46 to share some chromosome with you too (though you didn't inherit them from them), but the vast majority of them will not. Parenthetically, this is why services that claim to determine your ancestry by genetic testing are selling something close to snake oil. Once you go back more than 6 generations, you inherited no genes at all from the majority of your ancestors, and no test can determine if they are related to you or not.

[malloyd]

So, assuming we have had a new generation every 25 years, thus four per century (though, I guess 20 years or five per century might be a better estimate?) if I'd have a perfect lineage where none of my ancestors are related/identical I could estimate the number of ancestors I have by just doing something like 2^(years in the past/25), right?

That's correct. It's just the assumption that none of your ancestors are related that's wrong. And yes, this does pretty much mean that everybody in the world more than 7 or 8 centuries ago is either an ancestor of everybody alive today (often many times over) or has no living descendants at all. If anybody at all is descended from William the Conqueror, almost everybody alive is. This is true even for highly isolated populations if you go back a little further. If *one* person managed to survive the trip from Tasmania (the most genetically isolated place on Earth) to anywhere else before 1300 AD and managed to have descendants, then everybody is descended from him to. Since everybody on Tasmania 6 or so centuries before that was either his ancestor or had no descendants in 1300, that's all it takes to include all of them in the ancestry of everybody too.

The inbreeding though, that's different. The thing is that relationship is continuous, but genes aren't. You only have 46 chromosomes. Neglecting crossovers, this means that you *cannot* have any genes at all in common with at least 18 of your 64 great great great great grandparents. It could be more of them, indeed it's mathematically possible, if vanishingly unlikely, for you to have share none with two of your grandparents, but at least 18 of them are not genetic relatives even though the are your ancestors.

So yes everybody alive in 1000 AD who has descendants is your ancestor, but no more than 46 of them passed on a chromosome to you. Some number of the rest of them will be close enough relatives to those 46 to share some chromosome with you too (though you didn't inherit them from them), but the vast majority of them will not. Parenthetically, this is why services that claim to determine your ancestry by genetic testing are selling something close to snake oil. Once you go back more than 6 generations, you inherited no genes at all from the majority of your ancestors, and no test can determine if they are related to you or not.

[BannedInSchool]

Just to add, I don't think anyone's mentioned half-siblings yet, which increases the number of relations without really being inbreeding if two people have only one of a pair of ancestors in common.

[Zyzzyva]

That's correct. It's just the assumption that none of your ancestors are related that's wrong. And yes, this does pretty much mean that everybody in the world more than 7 or 8 centuries ago is either an ancestor of everybody alive today (often many times over) or has no living descendants at all. If anybody at all is descended from William the Conqueror, almost everybody alive is. This is true even for highly isolated populations if you go back a little further. If *one* person managed to survive the trip from Tasmania (the most genetically isolated place on Earth) to anywhere else before 1300 AD and managed to have descendants, then everybody is descended from him to. Since everybody on Tasmania 6 or so centuries before that was either his ancestor or had no descendants in 1300, that's all it takes to include all of them in the ancestry of everybody too.

The inbreeding though, that's different. The thing is that relationship is continuous, but genes aren't. You only have 46 chromosomes. Neglecting crossovers, this means that you did not inherit any genes from at least 18 of your 64 great great great great grandparents. It could be more of them, indeed it's mathematically possible, if vanishingly unlikely, for you to share none with two of your grandparents, but at least 18 of them are not genetic relatives even though they are your ancestors.

So yes everybody alive in 1000 AD who has descendants is your ancestor, but no more than 46 of them passed on a chromosome to you. Some number of the rest of them will be close enough relatives to those 46 to share some chromosome with you too (though you didn't inherit them from them), but the vast majority of them will not. Parenthetically, this is why services that claim to determine your ancestry by genetic testing are selling something close to snake oil. Once you go back more than 6 generations, you inherited no genes at all from the majority of your ancestors, and no test can determine if they are related to you or not.


I have seen the "you're all descended from Julius Caesar!" business propounded seriously before, so I kinda jumped to assumptions.

And here it is!

The raw ancestor V global population math isn't the only factor because people don't breed in some uniform pool. 1000 years - or 700 years? Really? - is nowhere near enough to get one person's descendants everywhere, especially given that Eurasia, sub-saharan Africa, Australia, the Americas, were all more-or-less perfectly isolated gene pools for the first half of that time.

Also, I like "neglecting crossovers". It's not like sexual reproduction is a thing, right? Joffrey is a genetic clone of Tywin because that's how genes work!

[malloyd]

And here it is!

The raw ancestor V global population math isn't the only factor because people don't breed in some uniform pool. 1000 years - or 700 years? Really? - is nowhere near enough to get one person's descendants everywhere, especially given that Eurasia, sub-saharan Africa, Australia, the Americas, were all more-or-less perfectly isolated gene pools for the first half of that time.

I don't believe in perfectly isolated gene pools, and you shouldn't either. As I pointed out, you only need one person to cross that boundary and in under a millennium....

Seriously the mathematics of this is well studied, because until the discovery of genes is was a major challenge to the theory of evolution - every member of the species reached either maximum of minimum fitness (as either the ancestor of everyone or having no descendants at all) in a few tens of generations. That simply isn't enough time for small differences in fitness to make enough difference for evolution to work. The solution turns out to be that same quantitization, the relevant number isn't how many descendants the individual has, but how many the gene does.



Also, I like "neglecting crossovers". It's not like sexual reproduction is a thing, right? Joffrey is a genetic clone of Tywin because that's how genes work!

Er, do you know what crossovers are? It's a process in which genes are swapped between paired chromosomes. I can't tell from your comment what you think it means, something nonsensical apparently.

[Flickerdart]

So, in true Mythbusters style, we've proven that nobody has everybody as their descendants...

But now that we have global travel, hypothetically, could one man become everybody's ancestor, and how long would it take to do so?

[BannedInSchool]

But now that we have global travel, hypothetically, could one man become everybody's ancestor, and how long would it take to do so?
I'm off to a late start, but I'll get right on that experiment... :smalltongue:

[Spacewolf]

Well everybody living today do have common ancestors they're called the Mitochondrial Adam and Eve. So the theory isn't wrong it's just an incorrect time frame.

[cobaltstarfire]

And here it is!

The raw ancestor V global population math isn't the only factor because people don't breed in some uniform pool. 1000 years - or 700 years? Really? - is nowhere near enough to get one person's descendants everywhere, especially given that Eurasia, sub-saharan Africa, Australia, the Americas, were all more-or-less perfectly isolated gene pools for the first half of that time.


I know that personally I can say that I have ancestors nearly everywhere. My grandfather? Filipino (his ancestry was Chinese, Spaniard, and Filipino blood). My grandmother Hispanic (Spaniard+Native Blood). My Fathers side is a mix of French and English. The only continent I'm missing is Africa, but thanks to England deciding to go down and colonize I may still share some genes with people there now.


Looking at something other than my personal history, humanity has been engaging in trade, conquest, and whatever else for a very very long time. Look at the Indus Valley Civilization for example, this civilization is one of the oldest known to have existed, and they were engaging in trade, and sprawled across at least Pakistan and parts of India.

We get to slightly younger civilizations (Egypt, Rome, Babylon, China) and many of them were trading, and spreading all over Eurasia, into the near east, and into Africa.

Get even newer, the age of sail and colonization, we've got Dutch and English men taking over South Africa. England took many countries in Asia, India, Malaysia, Singapore. All kinds of shenanigans going on between China/Korea/Japan.

You can trace certain art and cultural aspects from South America back to South East Asia, and on top of that there is evidence that Central/South American natives were engaging in trade as far north as the American Midwest before Europeans set foot in the Americas. (where there is trade in goods there is also trade in genes)

Humans have been getting around quite a bit for literally thousands of years. There are cases of people being isolated genetically, but I honestly wouldn't call it the rule.

[Kato]

Cretainly a bunch of things to think about, thanks guys :smallbiggrin: Just a few notes on two points in particular, mostly the "isolated gene pools".


Seriously the mathematics of this is well studied, because until the discovery of genes is was a major challenge to the theory of evolution - every member of the species reached either maximum of minimum fitness (as either the ancestor of everyone or having no descendants at all) in a few tens of generations.


If *one* person managed to survive the trip from Tasmania (the most genetically isolated place on Earth) to anywhere else before 1300 AD and managed to have descendants, then everybody is descended from him to. Since everybody on Tasmania 6 or so centuries before that was either his ancestor or had no descendants in 1300, that's all it takes to include all of them in the ancestry of everybody too.


"A few tens of generations" seems pretty vague to me. I mean, depending on the number of (surviving) descendants and the amount of isolation. I guess it's hard to make a profounded guess at the numbers but considering even if the theoretical escapee from Tasmania has a large number of living offspring, some of them would "inbreed" unless he really went on a wrodl tour spreading his genes and the chance of them reaching another isolated region are... well, whatever the chance of them including an outsider in their gene pool. (I guess one also has to consider cultural aspects. Some are really iffy when it comes to people from different races/cultures)
And also in relation to the theoretical individual, he and his ancestors might become my ancestors but the other not-escaping Tasmanians aren't (at least not directly). They might be relatives but reproduce happily further and have many descendants but they aren't part of my ancestry (until another person escapes and has enough time for his world tournee)


btw, in how far would the same logic still hold up for the future, considering the far larger number of people nowadays. With 7 billion people you can go thrity generations with no inbreeding and still not use all of them in your ancestry. Given, assuming some people's lines will die out at some point, the number might not be quite so high but the large population increase should have an effect, shouldn't it?

[JustWantedToSay]

"Inbreeding" is actually pretty common. My wife found out that her parents are 5th cousins (located 5 or 6 generations back), which makes her 6th cousins with each of her parents.

5th cousins share great*4-grandparents.

A person has 64 great*4-grandparents, but only 46 chromosomes. If counting mitochondrial inheritance to make 47, that still leaves a guaranteed 17 ancestors that did not pass on any genetic material to the subject. On average it's more like 64-(47*[(1-e)/e])= ~34.

I calculate a 61% chance that those 5th cousins cannot be both traced genetically to the same pair of grandparents. (ignoring the chance of getting the same genes through different lines).

The chance of them both having any one gene would be [(1/2)^6]^2 = 1/4096. And so the chance of them being genetically related would be 1-(4095/4096)^(47*2) = 2.27%

[Lord Torath]

The Genealogy's been done to show they are both descended from the same man. I think he had two wives during his life, and her father goes through one wife, and her mother through the other.

[Brother Oni]

Once you go back more than 6 generations, you inherited no genes at all from the majority of your ancestors, and no test can determine if they are related to you or not.

Well there is, but only for certain characteristics (an individual's mother and a male can trace his paternal ancestors).


Well everybody living today do have common ancestors they're called the Mitochondrial Adam and Eve. So the theory isn't wrong it's just an incorrect time frame.

It's Mitochondrial Eve (http://en.wikipedia.org/wiki/Mitochondrial_Eve) (from whom everybody's mtDNA can be traced back to) and Y-chromosomal Adam (http://en.wikipedia.org/wiki/Y-chromosomal_Adam), where males can trace their Y chromosome from.

Depending on where you draw the line, there are various dates of when everybody had the same ancestor, from Mitochonrial Eve (100,000 to 200,000 years ago) and Y-Chromosomal Adam (200,000-300,000 years ago) to the more later Identical Ancestors Point (http://en.wikipedia.org/wiki/Identical_ancestors_point) (5,000 to 15,000 years ago) to the Most Recent Common Ancestor (http://en.wikipedia.org/wiki/Most_recent_common_ancestor) (a mere 2,000 to 5,000 years ago).

[Zyzzyva]

I don't believe in perfectly isolated gene pools, and you shouldn't either. As I pointed out, you only need one person to cross that boundary and in under a millennium....


I know that personally I can say that I have ancestors nearly everywhere. My grandfather? Filipino (his ancestry was Chinese, Spaniard, and Filipino blood). My grandmother Hispanic (Spaniard+Native Blood). My Fathers side is a mix of French and English. The only continent I'm missing is Africa, but thanks to England deciding to go down and colonize I may still share some genes with people there now.

GAH! Everyone is arguing this backwards!

I have genes from a lot of places. You have genes from a lot of places. That person over there has genes from a lot of places. That's a true fact. That has no bearing on how dispersed the descendants from some random person in 1000 CE are! Take your Tasmanian. He ended up in, let's say Indonesia, somehow, and from there his genes went all over the place. 60% of Thailand is now descended from a Tasmanian! Consider now, his sister. She also lived c 1000CE. She also has descendants in the present day. She does not possess, as descendants, the whole world. She probably has most people of Tasmanian descent, and that's it. I'm not arguing that no European ever went to China or vice versa in the 13th C, or that the Sahara was an absolutely impassable barrier before de Gama, or even that no Polynesian ever washed up in Peru. I'm saying that these things weren't large-scale enough to meaningfully affect the dispersal of descendants of most people who lived back in the day.


Er, do you know what crossovers are? It's a process in which genes are swapped between paired chromosomes. I can't tell from your comment what you think it means, something nonsensical apparently.

I know! You're the one who's claiming that every one of Joffrey's chromosomes is identical to one of Tywin or Joanna's, and that this is a reasonable simplification.

[Kato]

I know! You're the one who's claiming that every one of Joffrey's chromosomes is identical to one of Tywin or Joanna's, and that this is a reasonable simplification.
Huh? :smallconfused: a) Uhm, mostly yeah? I'm not a biologist or genetics expert but I think genetic crossovers hardly have such a large impact. Sure, you could probably find a few differences but in large, yes, all of his genes are from one or the other. b) This still has nothing to do with your earlier claim about Joffrey being Tywin's clone. THAT is not how sex works, with our without crossover effects.

[Zyzzyva]

Huh? :smallconfused: a) Uhm, mostly yeah? I'm not a biologist or genetics expert but I think genetic crossovers hardly have such a large impact. Sure, you could probably find a few differences but in large, yes, all of his genes are from one or the other. b) This still has nothing to do with your earlier claim about Joffrey being Tywin's clone. THAT is not how sex works, with our without crossover effects.

But no: they're all blends of Tywin and Joanna's chromosomes. Individual genes are traceable (probably, as long as you don't crossover one in half), but the claim I was arguing against was this:


The thing is that relationship is continuous, but genes aren't. You only have 46 chromosomes. Neglecting crossovers, this means that you did not inherit any genes from at least 18 of your 64 great great great great grandparents. It could be more of them, indeed it's mathematically possible, if vanishingly unlikely, for you to share none with two of your grandparents, but at least 18 of them are not genetic relatives even though they are your ancestors.

So yes everybody alive in 1000 AD who has descendants is your ancestor, but no more than 46 of them passed on a chromosome to you. Some number of the rest of them will be close enough relatives to those 46 to share some chromosome with you too (though you didn't inherit them from them), but the vast majority of them will not. Parenthetically, this is why services that claim to determine your ancestry by genetic testing are selling something close to snake oil. Once you go back more than 6 generations, you inherited no genes at all from the majority of your ancestors, and no test can determine if they are related to you or not.

Which is ridiculous. "Neglecting crossovers" indeed. You didn't inherit anything from at least 18 of your great great great great grandparents! I can one up that: neglecting the negligible mass of the sperm, you didn't inherit anything from 63 of your great great great great grandparents! Mendelian genetics is a lie and a fraud!

...Seriously, the whole point of meiosis and, arguably, sexual reproduction, is the crossover phase.

And yes, the clone thing was late at night and I wasn't thinking about random assortment of "whole" chromosomes. :smallredface: My bad. I should have just hit the ludicrous claims where they're most silly directly.

[Avilan the Grey]

"Inbreeding" is actually pretty common. My wife found out that her parents are 5th cousins (located 5 or 6 generations back), which makes her 6th cousins with each of her parents.

5th cousin is pretty damn far apart though. Seriously. The "inbreeding" at that point would only be a problem if you are equally close to every parent involved (which COULD happen in very small isolated populations, of course) and you continued to "work the circle" so that next generation would be the 5th cousins to a different set of the same family and so on.


Remember that most people did not travel very far in their lives before about 1900, so it is fairly natural that you would marry someone related to you. As marriage always acted as an alliance system between tribes, there would be a kind of amorphous continuum of kin radiating out from you into the the surrounding area.

People traveled far more than we give them credit for. Even if you weren't a sailor, you usually travelled to markets, or to get seasonal jobs (many farmers from up north in Sweden walked down to Stockholm to work over the winters, for example. Many young women choose to stay in Stockholm, and other young women followed back north).
Plus, kids will be kids. Several traditions help curb inbreeding, like the "Lusse" mischief we had up until the 20th century, where teenagers from one village would form groups, dress up a bit then rumble over to another village, drink, fraternize and quite possibly end up well... not-inbreeding. Adults often tried to stop them, but it was from a genetics point of view a good thing they didn't always succeed in prohibiting this.

[Eldan]

Africa certainly is much less isolated than people think. The Indian Ocean Trade was quite flourishing for a long time, from Africa all the way to South-East Asia.

There's always been some connection. But no, not everyone has ancestors from every population. Reproductive isolation is a thing, even with some limited gene flow.

[Avilan the Grey]

Africa certainly is much less isolated than people think. The Indian Ocean Trade was quite flourishing for a long time, from Africa all the way to South-East Asia.

There's always been some connection. But no, not everyone has ancestors from every population. Reproductive isolation is a thing, even with some limited gene flow.

Again:
Before the arrival of modern transportation, water was the connection. Not the barrier. It is a modern myth and misconception. An island*, or a coastal country, or a country with a large river... was NOT isolated. The water made it the opposite of that.

*Unless you were living on the Easter Island or something.

[Kato]

Which is ridiculous. "Neglecting crossovers" indeed. You didn't inherit anything from at least 18 of your great great great great grandparents! I can one up that: neglecting the negligible mass of the sperm, you didn't inherit anything from 63 of your great great great great grandparents! Mendelian genetics is a lie and a fraud!

...Seriously, the whole point of meiosis and, arguably, sexual reproduction, is the crossover phase.

I'm still not sure I completely follow/agree. Yes, Mendelian genetics are simplified but they're not "a fraud". People use it constantly in agriculture, husbandry... and it works fine enough.
Again, I'm not an expert but what is your background? Did you study the field? Because from everything I know crossover is not "the point", it certainly helps with keeping evolution going, but the chromosome mixing is still an efficient process to support good genes and eradicate bad ones. Isn't crossover something pretty rare and also something which is entirely random, i.e. has a decent chance to cut up an (essential) gene? It seems to me that would seriously reduce the number of viable offspring if crossovers were that common. Also, it seems like paternity (and other heritage) tests would be null and void if you can't recognize a parent chromose anymore.



Again:
Before the arrival of modern transportation, water was the connection. Not the barrier. It is a modern myth and misconception. An island*, or a coastal country, or a country with a large river... was NOT isolated. The water made it the opposite of that.

*Unless you were living on the Easter Island or something.
I think Eldan was refering not to rivers or coastlines but like, oceans. You know, like the Atlantic which people pre-15th century passed once in a while when a crazy viking wanted to know what was to the far west? Euro-American trade was kind of hindered by that large body of water in the way, I think :smallwink:

[Brother Oni]

Again:
Before the arrival of modern transportation, water was the connection. Not the barrier. It is a modern myth and misconception. An island*, or a coastal country, or a country with a large river... was NOT isolated. The water made it the opposite of that.

Well yes and no. At its closest point, Japan was ~100 miles from China, which the Japanese viewed as close enough for easy trade but far enough to keep out unwanted ideas.

[Zyzzyva]

I'm still not sure I completely follow/agree. Yes, Mendelian genetics are simplified but they're not "a fraud". People use it constantly in agriculture, husbandry... and it works fine enough.

Maybe that line should have been in blue? :smallredface: It was a sarcastic ad absurdum. I don't actually think Mendellian genetics are wrong.


Again, I'm not an expert but what is your background? Did you study the field? Because from everything I know crossover is not "the point", it certainly helps with keeping evolution going, but the chromosome mixing is still an efficient process to support good genes and eradicate bad ones. Isn't crossover something pretty rare and also something which is entirely random, i.e. has a decent chance to cut up an (essential) gene? It seems to me that would seriously reduce the number of viable offspring if crossovers were that common. Also, it seems like paternity (and other heritage) tests would be null and void if you can't recognize a parent chromose anymore.

I am not; I just think malloyd is much more wrong. I'm not sure how often crossover occurs, actually: I'd be delighted to hear from a biologist on this. Certainly I just in the last few hours discovered a wide variety of recombination activities that meiosis apparently engages in.

Since viable genes don't have a huge range of variation, and the chromosomes are supposed to align (thus keeping the genes aligned), cutting a gene in half is unlikely to produce a nonviable gamete.

And heredity tests work because of recombination: as malloyd noted, you'd get no genes from most of your ancestors otherwise.

[Mith]

I know it's already been stated that inbreeding isn't a big deal after so many degrees of separation, but an example I can give is that there is (or at least was) a town that my family could trace back to being related to everyone in the town, since it was basically two family clans that intermarried. There might have been a few other families at the start but they had a generation of only surviving daughters and thus the surname disappeared. It just requires a good memory for who's where on your local branch of the family tree.