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Mitochondrial Eve, Y-Chromosome Adam, and Reasons to Believe

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October 28, 2011 Tags: Human Origins

Today's entry was written by Dennis Venema. You can read more about what we believe here.

Mitochondrial Eve, Y-Chromosome Adam, and Reasons to Believe

One of the challenges for discussing evolution within evangelical Christian circles is that there is widespread confusion about how evolution actually works. In this (intermittent) series, I discuss aspects of evolution that are commonly misunderstood in the Christian community. In this post, we tackle the issue of why “Mitochondrial Eve” and “Y-chromosome Adam” are not an ancestral couple from whom all humans descend, as claimed by the Old-Earth Creationist organization Reasons to Believe.

It is reasonably well known among evangelical Christians that all living humans trace their mitochondrial DNA back to a single woman (a so-called “mitochondrial Eve”) and that all living males similarly trace their Y-chromosome DNA back to a single male (a so-called “Y-chromosome Adam”). These individuals are commonly assumed by evangelicals to be the Biblical Adam and Eve, the first humans alive and the progenitors of the entire human race. While most young-earth and old-earth creationist organizations make this claim, perhaps one of the best-known organizations to do so is the old-earth creationist / anti-evolution organization Reasons to Believe, who have produced numerous articles, podcasts, and even entire books on the subject.

In contrast to this common evangelical understanding, the scientific picture is rather different. Mitochondrial Eve, though the most recent common matrilineal ancestor of all humans, was but one of a large population living about 180,000 years ago. So too for Y-chromosome Adam: he was also a member of a large population, and he lived about 50,000 years ago. As has been discussed several times here at BioLogos, there are multiple lines of evidence that indicate the human population has never been below around 10,000 members at any time in its history: we branched off as a large population to form our own species.

When presented with the evidence for human population sizes over our evolutionary history, a common point of confusion for evangelicals is how this evidence fits with Mitochondrial Eve. How can we all come from one woman (and one man) but also come from a large population of 10,000 individuals? Aren’t these two observations in conflict?

The answer is no, these lines of evidence fit together. Humans do come from a large population, and all present-day humans do inherit mitochondrial and Y-chromosome DNA from specific individuals in the past. The reason for the apparent discrepancy lies in how mitochondrial and Y-chromosome DNA are inherited, as we shall see below.

Mitochondria are organelles responsible for energy conversion, and they contain their own small, circular chromosome that they replicate apart from regular chromosomes in the cell nucleus. Mitochondria are not passed on to progeny through sperm, but only through the egg: as such, mitochondrial DNA is passed on solely through the maternal line. Consider a small pedigree (family tree) below. Circles represent females, males are represented with squares. In this family, one grandmother (the woman at the top right of the pedigree) has passed on her mitochondrial DNA to her sons and daughter, but only her daughter passes it on to the next generation. All individuals who have this grandmother’s mitochondrial DNA are shown in blue:

Conversely, if we examine Y-chromosome inheritance in this same family, we would see that (obviously) women cannot pass it on to their children. Here, the red lines show all males who have descended from a grandfather of the family (the male at the top left of the pedigree):

Now we are ready to examine how these types of DNA are inherited in a larger group, and compare their modes of inheritance with regular chromosomal DNA. While it is not possible to draw out a pedigree for a population of 10,000 individuals, let’s examine a smaller group to see how a specific mitochondrial sequence can “take over” a population of organisms (note that this effect applies to other organisms besides humans that use an XX – XY system of sex chromosomes).

In the family tree below, three mitochondrial DNA variants are present in the first generation (the top row of the pedigree) and a represented with different colors (green, blue and red). Tracing the inheritance of these mitochondrial DNA versions through the family tree shows that all living members of this population (the bottom two rows) have inherited the red version only. The blue and green versions eventually hit a dead end where they were not passed on (either through females who did not have children, or males). As such, all living individuals can trace their mitochondrial DNA back to this group’s “mitochondrial Eve”, the woman at the top right of the tree with the “Mito 3” variant.

Let’s now examine Y-chromosome inheritance patterns in the exact same family tree. Suppose there are three Y chromosome variants present in the first generations:

Here we can see that the current population has inherited its Y-chromosome DNA from one individual as well (variant 1, the red lines) and that the other Y-chromosome variants (blue and green) hit dead ends through males that did not reproduce or men who only had daughters. All living members of the population trace their Y chromosome DNA back to an individual (filled in with yellow) who lived two generations after their most recent matrilineal common ancestor (the woman at the top right).

Now we are ready to examine regular chromosomal inheritance in this same family tree. Genetic variation on chromosomes other than the Y can be passed through either gender without problem, and individuals can have two variants at a time (one on the chromosome inherited from mom, the other on the chromosome inherited from dad). These key differences (compared to how mitochondrial DNA and Y chromosomes are inherited) produce a very different effect. In this same family, numerous variants (represented by the different colors) have been transmitted to the present generation without loss:

Notice the middle couple in the first generation in the pedigree. This man’s Y chromosome did not make it to the present day, and similarly his wife’s mitochondrial DNA did not make it either (scroll up to see this if you need to refresh your memory). So, they contributed nothing to the current generation, right? Not at all: both of them have passed on regular chromosomal variation to the present day (traced as blue and black lines).

In other words, it would be incorrect to examine this population, determine (correctly) that they share common mitochondrial DNA and Y-chromosome ancestors, and then go on to conclude that these two individuals were an ancestral pair that started this entire family. We know that this group descends from a larger population, because genetic variation in the present population is too large to explain as coming from one pair (there are five colors, or genetic variants in this population, and the max any one pair could carry is four, with two each).

While this example examines a small family, the same principles apply to larger groups: mitochondrial and Y-chromosome lineages, though interesting, cannot be used to estimate population sizes over time. For that type of work, regular chromosomal variation should be examined. Present day human genetic variation indicates that though we all share a common mitochondrial DNA and Y-chromosome source, these individuals came from a population of at least 10,000 individuals, and that they lived over 100,000 years apart. If you are interested in examining the evidence for human population sizes, Darrel Falk and I have discussed it previously.

In summary, anti-evolutionary groups, such as Reasons to Believe, that claim that the evidence for Mitochondrial Eve and Y-chromosome Adam supports an ancestral couple for the entire human race are not interpreting the data correctly. They have failed to account for the unique pattern of inheritance these types of DNA have in populations.

Photo courtesy of Lewis Schofield.


Dennis Venema is professor of biology at Trinity Western University in Langley, British Columbia. He holds a B.Sc. (with Honors) from the University of British Columbia (1996), and received his Ph.D. from the University of British Columbia in 2003. His research is focused on the genetics of pattern formation and signaling using the common fruit fly Drosophila melanogaster as a model organism. Dennis is a gifted thinker and writer on matters of science and faith, but also an award-winning biology teacher—he won the 2008 College Biology Teaching Award from the National Association of Biology Teachers. He and his family enjoy numerous outdoor activities that the Canadian Pacific coast region has to offer. Dennis writes regularly for the BioLogos Forum about the biological evidence for evolution.

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Jimpithecus - #65847

October 28th 2011

“Mitochondrial Eve, though the most recent common matrilineal ancestor of
all humans, was but one of a large population living about 180,000
years ago. So too for Y-chromosome Adam: he was also a member of a large
population, and he lived about 50,000 years ago. “

Yes that would be the hominins from Herto or something like them.  These are the first hominins that are reasonably modern in appearance.  They date to between 170 and 180 ky BP.


Terrance - #65848

October 28th 2011

Nice article, but by now a headline of ‘RTB mangles science’ is about as unsurprising as ‘dog bites man’ or ‘Pope is a Catholic’. Anybody who takes even a cursory glance at the way they present scientific research, and compares the claims they make with the actual scientific articles they cite, will know that they long ago gave up any pretence to scientific credibility. I would rank Hugh Ross as probably the single largest source of bogus information, specious factoids, and misquoted proof texts that I have encountered. Most of the prominent YEC leaders are paragons of scientific scholarship and probity in comparison.


Jimpithecus - #65850

October 28th 2011

The astounding thing is that, once upon a time, when he dealt only with astrophysics, he was a shining light in Old Earth Creation.  While it is true that he never accepted evolution, that was only a small aspect of his overall message and was always subjugated to his attempts to get people to understand the evidence for and old universe.  When he came to the University of Tennessee in the late 1980s, his entire message was astrophysical in nature. 

The problem is that he has attempted to branch out into an area of which he knows nothing.  Worse, because he does not know the first thing about evolution, he has hired someone to do the genetics pieces who is quite willing to twist the information to suit the purposes of RTB, and Dr. Ross doesn’t know better.


sfmatheson - #65853

October 28th 2011

i>The problem is that he has attempted to branch out into an area of which he knows nothing.  Worse, because he does not know the first thing about evolution, he has hired someone to do the genetics pieces who is quite willing to twist the information to suit the purposes of RTB, and Dr. Ross doesn’t know better.

i>
I think the problem at RTB is much worse than that. It’s true that Ross knows next to nothing about evolution, but that doesn’t explain why he regularly resorts to distortion and fabrication. His story about junk DNA in Creation as Science is a disturbing fabrication, one that I find difficult to explain away as mere ignorance or carelessness. Something went wrong at RTB at some point, and Rana’s work is an outgrowth of the larger problem. It’s a very serious issue, and it is my opinion that the continuing attempts by RTB to mislead Christians about evolution should be a much bigger concern among evangelicals.
br>
I’ll add that those who would seek to understand how such things can happen among evangelicals, and how they can indeed thrive, should look again at what Karl Giberson and Randall Stephens wrote recently. The key concept, I think, is “anointing,” and recent critiques of their work here at BioLogos seem to miss that important point.

mjblyth - #65860

October 29th 2011

Steve, what do you mean by the key concept being anointing?


beaglelady - #65866

October 29th 2011

Steve was probably referring to the article written by Giberson and Stephens, The Evangelical Rejection of Reason, which you can read here:

http://www.nytimes.com/2011/10/18/opinion/the-evangelical-rejection-of-reason.html?_r=2


sfmatheson - #65872

October 29th 2011

Summarized in this HuffPo piece. I haven’t read the book yet.

http://www.huffingtonpost.com/karl-giberson-phd/the-anointed-leaders-of-t_b_859673.html

Bilbo - #65868

October 29th 2011

Dennis,

When criticizing or commenting on somebody else’s articles or opinion, it is proper to give a reference to their work.   You haven’t.  For those interested:

http://www.reasons.org/were-they-real-scientific-case-adam-and-eve

I’m curious about an article that is referred to by RTB:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1766376/

It’s not clear from the article exactly how large they would have estimated the founder population of sheep to be, but they admit that it was larger than the actual number (which was two).  I would be curious if this would have some relevance to the estimate of the founder population of the human species.


John - #65951

November 1st 2011

Bilbo:
“When criticizing or commenting on somebody else’s articles or opinion, it is proper to give a reference to their work.”

What work, Bilbo? How does misrepresenting the results of the hard work of others without adding a speck of work of your own count? You seem to have a Bizarro definition of “work” in the context of biology.

In turnabout, I notice that you can’t resist the painfully shallow ID practice of quoting passages from papers instead of making an attempt to discuss the actual data from them in your own words.

If you think that your practice represents scientific practice, what would you estimate the ratio of quotes I’ve inserted to papers I’ve published (including reviews), Bilbo?

Bilbo - #65869

October 29th 2011

Steve,

Given how quickly you misinterpreted my questions at your own blog, I doubt whatever interpretations you give of other people whose views you object to.


Bilbo - #65870

October 29th 2011

RTB also has an interesting, relevant article here:

http://www.reasons.org/human-origins/adam-and-eve/orangutan-genetic-diversity-sheds-light-humanitys-origin


HornSpiel - #65871

October 29th 2011

Mathesson’s comment may be referring to this from the NY Time article:

Charismatic leaders like these project a winsome personal testimony as brothers in Christ…. They pepper their presentations with so many Bible verses that their messages appear to be straight out of Scripture; to many, they seem like prophets, anointed by God.
a href=“http://www.nytimes.com/2011/10/18/opinion/the-evangelical-rejection-of-reason.htm”>The evangelical rejection of reason

Also Giberson and  Stephens are the authors of “The Anointed: Evangelical Truth in a Secular Agefont class=“Apple-style-span” face=“georgia, ‘times new roman’, times, serif”>” where they elaborate on the issue  
font class=“Apple-style-span” face=“georgia, ‘times new roman’, times, serif”>br>
font class=“Apple-style-span” face=“georgia, ‘times new roman’, times, serif”>My question is why does Mattheson say Biologos has missed that point? On what basis does he say that and what would they do different if they “got it?”
br>
Certainly, reading the NY Times article I was disappointed to read that they were inappropriately applying social-science findings to a critique of Evangelical stances on child rearing and homosexuality. Which means, I think, that even Christian scientists can over step.
br>
What  find refreshing about Biologos is the mixture of reason, faith, humility and conviction. Biologos  does not come across as “anointed” in the  Giberson-font class=“Apple-style-span” face=“georgia, ‘times new roman’, times, serif”>Stephens sense, but I think it is.

sfmatheson - #65873

October 29th 2011

Hi Hornspiel, I did not claim that BioLogos in general missed that point. I referred to recent critiques, and was thinking of Mark Mann’s recent post which focused in on a distinction between “secular knowledge” and Christian faith. Mann suggests that Giberson and Stephens make such a distinction, but I think he may be misinterpreting them. I suspect that Giberson and Stephens are pointing to the overwhelming evangelical tendency to make such (false) distinctions. And that tendency helps to undergird the anointing of particular evangelical “experts” who are empowered to feed evangelicals a steady and ruinous diet of pseudoscholarship.


My intent was not to urge BioLogos to enter this particular fray. But I do think that Mann’s piece makes two mistakes. First, it assumes that Giberson and Stephens meant to reinforce any distinction between “secular knowledge” and Christian thought or scholarship. (Mann claims that this is one of their “central assumptions.”) Second, by harping on that theme, he obscures the point that Giberson and Stephens are trying to make. I would say he undermines it.

This is not to say that I disagree with Mann’s basic theme (that there is no “separation” between “secular knowledge” and Christian belief); in fact, I very strongly affirm it. And if Mann were to discuss those themes in a different context, I’d be very enthusiastically behind him. But instead, the context was a misguided critique of two scholars who are saying something very important, very provocative, and in my view, distressingly accurate.

HornSpiel - #65877

October 29th 2011

Thank you Steve for your reply. After rereading Mann’s post of Oct 26 I see what you mean. There is only one sentence in Giberson and Stephens’ NT Times piece that even uses the words secular and integrated:

div>i>Within the evangelical world, tensions have emerged between those who deny secular knowledge, and those who have kept up with it and integrated it with their faith. 


HornSpiel - #65879

October 29th 2011

There is something screwy with the editor here. It chopped of the end of my comment above.All I wanted to say to finish is:


Certainly, although we may end up realizing there is not ultimate difference between theological and scientific truth, most of us must go through a process to reconcile them. I think that is what Giberson and Stephens are referring to here. Your point is well taken.

Darrel Falk - #65883

October 29th 2011

Okay, guys, perhaps we need to have been more clear.   You are correct.  The point of Karl and Randal’s piece in the NY Times was not centered on the theme that Mark Mann is developing in his current series.  Mark was not doing a critique of the article.


Rather they had raised one interesting point that Mark saw a little differently. That provided a great opportunity for us to begin a series.  It was nothing more than a jumping off point.



sfmatheson - #65887

October 29th 2011

Darrel, thanks for the response. I think that Mark’s theme is a very important one, and I reiterate my enthusiastic support for his thesis. The choice of a jumping-off point was a bit unfortunate, but perhaps when/if it becomes clear that Karl and Randall are on the same page on all of this, we can get the focus back onto the dangerous secular/religious dichotomy that Mark seeks to break down. (If I’m reading him right.)


Bilbo - #65888

October 29th 2011

From the second RTB article:

“The orangutan species Pongo abelii and Pongo pygmaeus are found on Sumatra and Borneo, respectively. Conservationists have counted around 7,000 to 7,500 P. abelii individuals on Sumatra and around 40,000 to 50,000 P. pygmaeus on Borneo. Both species are endangered, P. abelii critically so. Evolutionary biologists believe that these two species diverged from a common ancestor around 1 million years ago.

As part of the orangutan genome project, the researchers spot-sequenced the genomes of ten orangutans, five from each island, and compared them. To their surprise, they discovered that the orangutans on Sumatra displayed a much greater genetic diversity than did the apes of Borneo. This result is counter-intuitive because generally larger populations display greater diversity

This unexpected result means that geneticists have a limited ability to relate population size to genetic variation within a population. According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.”2


Darrel Falk - #65894

October 29th 2011

Okay, Bilbo, let’s respond to this quote from Fuz’s article:

“This unexpected result means that geneticists have a limited ability to relate population size to genetic variation”  According to William Amos, a geneticist at the University of Cambridge, ‘We don’t fully understand the relationship between genetic diversity and population size.’”

I realize that you are not a geneticist, so you thought Fuz was making a good point here.  This illustrates my concern exactly.  If you go back to the original Nature article, you will see what the author is saying in the article.  He is saying that it is not clear how important it is for a population to have a high degree of genetic diversity in order to be highly viable, (to be large in other words.)   He is concluding that populations of orangutans may well be quite healthy (and large!) even with reduced genetic diversity.

This has nothing to do with how Fuz uses this quote.  Fuz mistakenly takes it out of context and implies that this comment about the health of a population, in some way calls into question whether genetic diversity is a reliable means of estimating population size in ancient time.

I believe these sorts of things that RTB does are genuine mistakes. (I am aware that many of my colleagues think the mistakes are made on purpose.  I don’t.  But either way, its not good.)  Furthermore, and perhaps most important of all,  I also believe that once the mistake is pointed out it is important to go back and pull the article from the web so that people like yourself who don’t have the background to judge are not misled by what has been said.  If one chooses to leave the article up because of other noteworthy parts, then there should, at the very least, be a footnote with an errata. 

This, Bilbo, is my biggest concern.  The errors sit there, the many errors sit there and get pulled up by well-meaning people like you without the background (or time, as the case may be) to go out and determine the scientific validity of the arguments.



Bilbo - #65889

October 29th 2011

And from the first RTB article:

“Even though the genetic data traces humanity’s origin back to a single woman and man, evolutionary biologists are quick to assert that mitochondrial Eve and Y-chromosomal Adam were not the first humans. Rather, according to them, many “Eves” and “Adams” existed.7 Accordingly, mitochondrial Eve and Y-chromosomal Adam were the lucky ones whose genetic material just happened to survive. The genetic lines of the other first humans were lost over time.

While this explanation is not out of the realm of possibility, it is highly contrived. It would work if only a few of the first humans reproduced, or were allowed to reproduce. If the data is simply taken at face value, the biblical model is the more parsimonious explanation.”


Darrel Falk - #65891

October 29th 2011

Bilbo,


This is the very point that Dennis has shown is no longer viable.  

So I’m  going to put my professor hat on and you can pretend you are a student taking my general ed class.  So here’s my question:  in what way do you think Dennis  (and virtually all geneticists, actually) thinks that he has just shown that the position that Fuz outlines here is no longer tenable?   Which statements in Fuz’s article are clearly (in the mind of almost all geneticists) inconsistent with what Dennis has outlined so clearly?

This would be a wonderful exam question, Bilbo!!  

Here’s my next question and it would be the follow-up for extra credit.. Having demonstrated that you clearly understood the basis of the scientific conclusions (must get full points on that question to do this extra credit portion), pretend that you are an ID critic of mainstream science.  What argument would you put forward to illustrate why you see things differently.

sfmatheson - #65897

October 29th 2011

That error is so basic that it should be considered profoundly embarrassing to the person who wrote it. It’s almost rude to post it and to name the author. That’s how bad the error is, and that’s how important it is to encourage RTB to change.


Dennis Venema - #65890

October 29th 2011

Bilbo, that quote from RTB is a good one to compare with my post above: Fuz is claiming that it is “highly contrived” that “the genetic lines of the other first humans were lost over time.” Yet in the post above I show how easy it is to “lose” a mitochondrial or Y-chromosome lineage and have others take over a population. It is not “highly contrived” in any way - it is a natural consequence of how these special types of genetic material are inherited, and every geneticist knows that. It’s not rocket science. 


Fuz also seems to be implying that scientists think the entire genetic material of the other folks from the population has been lost. This is of course not the case - their genetic variation on regular chromosomes is with us to this day, and clearly shows that there has always been at least 10,000 of us as far back into our evolutionary history we would like to go. 

In reply to your other comment, that I should link over to RTB’s take on this, I felt it was well-enough known how they approach this issue. If a link will make you happy, here’s one that lays their view out pretty concisely:

http://www.reasons.org/were-they-real-scientific-case-adam-and-eve

Jimpithecus - #65902

October 29th 2011

Back in the late 1980s, when the mtDNA analyses were coming out in favor of the “out-of-Africa” replacement model of modern human origins, Milford Wolpoff pointed out that researchers were not accounting for stochastic loss of mtDNA lineages and that the trees and resultant dates were too simplistic.  As David Maddison later showed (http://sysbio.oxfordjournals.org/content/40/3/355.extract)  that was, indeed the case. 


Bilbo - #65892

October 29th 2011

Darrel’s questions:

1) “So here’s my question:  in what way do you think Dennis  (and virtually all geneticists, actually) thinks that he has just shown that the position that Fuz outlines here is no longer tenable?”
 
I assume you mean Fuz’s position that Mitochondrial Eve and Y-chromosome Adam are more simply explained by a 2-person origin than by thousands of Eves and Adams (or their offspring, or their offspring’s offspring, etc.) failing to reproduce females or males at some time, and thus failing to pass on their mitochondria or Y-chromosomes.  I think Dennis thinks that he has shown that this is indeed possible, and that since the human population has never fallen below 10,000 people, then this must indeed be the case.

  “Which statements in Fuz’s article are clearly (in the mind of almost all geneticists) inconsistent with what Dennis has outlined so clearly?”

 
That the human population has been lower than 10,000 people.  
 “Here’s my next question and it would be the follow-up for extra credit.. Having demonstrated that you clearly understood the basis of the scientific conclusions (must get full points on that question to do this extra credit portion), pretend that you are an ID critic of mainstream science.  What argument would you put forward to illustrate why you see things differently.”

I would point to the data of the smaller population of orangutans having a larger variation in their genomes than the larger population of orangutans, followed by, ” According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.”

I would also point to the genetic study that estimated that the number of founder sheep on the island was larger than two, when in fact it was two.


Darrel Falk - #65896

October 29th 2011

Oh Bilbo, you are so fortunate that you chose not to major in genetics or worse yet to take a class from me.  :)  Here was your answer:  “I think Dennis thinks he has shown that this is indeed possible, that since the human population has ...etc”


Bilbo,  all you have done is restated the article’s conclusions.  You have not demonstrated your knowledge of how those conclusions were reached.  You get 2 point of 10 for demonstrating that you understand the conclusions.   However, sometimes, I give an opportunity for make-up, especially to philosophy majors (Sorry, just kidding!)  Care to try again?

I didn’t grade your extra credit question because you need to get full points on the preceding question.



John - #65904

October 30th 2011

Darrel:
”...pretend that you are an ID critic of mainstream science.  What argument would you put forward to illustrate why you see things differently.”

Bilbo:
“I would point to the data of the smaller population of orangutans having a larger variation in their genomes than the larger population of orangutans, followed by, ” According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.””

How does that even begin to suggest that evolutionary theory is wrong and ID is right, Bilbo? And why would a quote matter as much as data?

“I would also point to the genetic study that estimated that the number of founder sheep on the island was larger than two, when in fact it was two.”

Wow. You’ve twisted that beyond all recognition to suit your purposes.

Here’s your problem, Bilbo: you’re assuming the model not explaining the data somehow suggests that a whole branch of biology is wrong, when as Steve pointed out, it demonstrates that there is a phenomenon or phenomena that need to be included—in this case selection.

Bilbo - #65893

October 29th 2011

From the article that Dennis so kindly linked to:

In 1957 a male and female yearling were placed onto Haute Island (an island in the Kerguelen Archipelago). These two sheep were taken from a captive population in France. By the beginning of the 1970s, the number had grown to 100 individuals and peaked at 700 sheep in 1977. Since that time the population has fluctuated in a cyclical manner between 250 and 700 members. Given that the population began with only two individuals (the founder effect), has experienced cyclical changes in the population size, and was isolated on an island, the researchers expected very low genetic diversity (measured as heterozygosity).

Using mathematical models, the heterozygosity of a population can be computed at any point in time from the heterozygosity of the ancestral population (which was known for the original mouflon pair) and the original population size. What the researchers discovered, however, when they measured this quantity directly for the sheep on Haute Island was that it exceeded the predictions made by the models by up to a factor of 4. In other words, the models underestimated the genetic diversity of the actual population.


sfmatheson - #65899

October 29th 2011

I hope I’m not posting answers to the test.


I read the article, and it’s pretty interesting. The unexpectedly high levels of heterozygosity strongly suggest selection against homozygotes (or, conversely, selection for heterozygotes), and that’s what the paper is about. This means, I think, that the heterozygosity is not indicative of increased genetic diversity in terms of numbers of alleles in the population. In other words, the paper seems to be specifically about heterozygosity and its use in population genetic models, and the authors conclude that their case illustrates the difficulty in predicting the relative strength of selection and drift in very small populations. Drift is thought to predominate in such situations, but this example seems to show that selection can predominate at least occasionally.

There has been at least one followup publication regarding the sheep population, and it suggests why heterozygosity alone may not be a good measure of overall genetic diversity in small populations. More interestingly, that paper begins to address the reasons why heterozygosity might be favored by selection. Link below.

http://rsbl.royalsocietypublishing.org/content/4/5/504.long

Bilbo - #65895

October 29th 2011

I’ll tune in tomorrow for my grade.


sy - #65898

October 29th 2011

I dont know if this will help. Mitochondrial Eve is the ancestor of all living women. She lived 50,000 years ago. But if we went back 10,000 years, and checked to see when the last common ancestor of all the women who were living 10,000 years ago lived, it would not be the same woman. It would be someone who lived much earlier. In fact our version of Eve would be one of the descendants of the prior Eve. But over the next 10,000 years, as some lines go extinct (new lines cannot form) the timing of the last common ancestor becomes more and more modern.

Another way to put this is that it is entirely possible that a woman alive today, (like Beaglelady for example) could end up being the Mitochondrial Eve of a future human population, especially if we happen to undergo another major bottleneck.

Population size will affect the timing, but not whether or not the phenomenon occurs.


Bilbo - #65906

October 30th 2011

Darrel:  “Bilbo,  all you have done is restated the article’s conclusions.  You have not demonstrated your knowledge of how those conclusions were reached.”

OK, I’ll try to explain Dennis’s explanation:  The mitochondrial DNA of a woman is passed on only to her female offspring.  So if she has no female offspring, her mitochondrial DNA won’t be passed on.  She may have male offspring, in which case her genomic DNA will be passed on.  Thus, in a population of 10,000 people, assuming half were women, and assuming that each woman had different mitochondrial DNA, it is possible that eventually only one of those women’s mitochondrial DNA will survive in her progenitors, while the mitochondrial DNA of all the other women will cease to exist, since at various places in the geneologies they might have failed to produce female offspring. Dennis gave no indication of how probable such a result would be.

Meanwhile, the Y-chromosome DNA is passed on from the male to his male offspring.  If he has no male offspring, his Y-chromosome DNA will not be passed on, even if he has female offspring.  Thus, in a population of 10,000 people, assuming half were men, and assuming that each man had different Y-chromosome DNA, it is possible that eventually only one of those men’s Y-chromosome DNA will surive in his progenitors, while the Y-chromosom DNA of all the other men will cease to exist, since at various places in the geneologies they might have failed to produce male offspring.  Again, Dennis gave no indication of how probable such a result would be.


John - #65911

October 30th 2011

Bilbo:

“The mitochondrial DNA of a woman is passed on only to her female offspring.”

Really? If I’m a male, where did my mitochondria come from?

Jimpithecus - #65912

October 30th 2011

Bilbo, the MtDNA is passed on to both her male and female offspring but her male offspring do not pass it on to their children.  However, her female offspring pass it on to their children. 


Argon - #65945

November 1st 2011

div>“Again, Dennis gave no indication of how probable such a result would be.” 


Argon - #65946

November 1st 2011

Again, Dennis gave no indication of how probable such a result would be. 

Pretty much 100% in a finite population. Mitochondrial Eve is simply the most recent winner of the “unbroken lineage of girls lottery”. Someone must win. And the next winner of the Mitochondria Even sweepstakes might be one of the last winner’s daughters, perhaps born before today. Unfortunately, it’s a retroactive award: Winners are seldom around to receive their prize.

Now, if Eve and Adam were indeed the sole founding pair of humans you’d expect to see more genetic coalescence than the mitochondrial and y-chromosomes; *all* genes should display roughly the same time frames for coalescence. But that’s not seen. I think that the genes for mitochondrial proteins that reside on the nuclear chromosomes also don’t display the same frames of coalescence.

So Rana’s notion may make sense, but only if you ignore all other data that simply doesn’t fit.

Bilbo - #65907

October 30th 2011

Fuz’s point was that a simpler explanation for the fact that there is only one type of mitochondrial DNA and Y-chromosome DNA in the human population is that originally there were only two human beings, one female and one male, each with their respective mitochondrial and Y-chromosome DNA.  Given such a scenario, the probability that we would all have the same mitochondrial or Y-chromosomal DNA is 100%.

What would be interesting is to know what the probability is that we would all have the same mitochondrial or Y-chromosomal DNA, given an original human population of 10,000.   If the probability is still relatively high, then Fuz’s argument doesn’t have much cogency.  If, however, the probability is very low, then Fuz’s argument would be rather strong. 

Fuz would still need to explain why our genetic diversity shows that the human population has never dropped below 10,000.  From the evidence that he already gave, it sounds like his answer is that genetics is not that exact a science, especially over very long time periods.  


Bilbo - #65908

October 30th 2011

Darrel,

Regardless of whether or not you flunk me, I’m curious what your response is to my extra credit answer, since you would, in effect, be answering Fuz’s points, not mine.


sfmatheson - #65914

October 30th 2011

Before we move forward in answering Bilbo’s questions, I think we should address some of the misconceptions that he has understandably taken on board. Here is an important one that I see.


Bilbo seems to think that all humans share the same mtDNA and the same Y chromsomal DNA. I surmise that by this he means that all human mtDNA is identical, and that all Y-chromosomal DNA is identical. This is incorrect. Like all parts of the human genome, mtDNA and Y-chromosomes vary (due to mutation, which is quite common in mtDNA). We do not have all the same DNA, even in those two unique repositories.

On the contrary, the data discussed in Dennis’ post involves analysis of the variation in those two repositories. That analysis suggests that each repository descends from a common ancestor at a particular point in the past. Notably, the analysis suggests the opposite of what RTB wants people to believe: it suggests that the two common ancestors were not contemporaries. This is unsurprising to anyone with a basic understanding of population genetics.

Bilbo points to one way we can know that the RTB position is currently contradicted by the data. Bilbo writes that if RTB is right, “the probability that we would all have the same mitochondrial or Y-chromosomal DNA is 100%.” I’ve already explained why this is wrong: even if RTB’s model were correct, we would most certainly not all have the same DNA in those places. But more pertinently, the RTB model makes a specific prediction: the amount of variation in those genomic sections—and indeed in all parts of the genome—should be consistent with recent expansion of the human population from two founders. Whatever the amount of variation in the mtDNA or the Y chromosome or anywhere else, it should be the same, according to Rana’s reasoning as approved by Bilbo. But it’s not.

SeanEphraimRice - #65933

November 1st 2011

testing… 


SeanEphraimRice - #65934

November 1st 2011

i>”We know that this group descends from a larger population, because genetic variation in the present population is too large to explain as coming from one pair (there are five colors, or genetic variants in this population, and the max any one pair could carry is four, with two each).”

br>

Interesting stuff. Quick question: could this genetic variation have a different source, say, (1) as a random trait that evolved over time, or  (2) as extra genetic material picked up in interbreeding with other early humanoid species (Neandertals)? From a non-expert point of view, it seems like these two options could explain genetic variation while still providing for a literal Adam and Eve. The question has been on my mind for a while, so feedback would be welcome.

SeanEphraimRice - #65935

November 1st 2011

*evolved over time within the existing human population


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