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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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Bilbo - #65948

November 1st 2011

Jimpithecus, thanks for clearing that up for me.

Steve, yes I misspoke (or miswrote).  Our mtDNA and Y-chromosome DNA is not all the same, but can be traced to common ancestors.  Yes, the difference in dates (Eve earlier than Adam, I believe) should be a problem for RTB.  I would like to see a response from them on that, unless they want to use the same, “Genetics is an inexact science when determing exact history or population sizes.”

But once we correct my imprecision, my point (or Fuz’s point) still stands:  If the founder population was two people, then the probability that our mtDNA and Y-chromosome DNA can be traced back to two common ancestors is 100%.  And I agree that Fuz still has the problem of explaining why the rest of our genomes does not show the appropriate amount of diversity.  But once again, Fuz would say that Genetics is an inexact science when determining population sizes. 

Argon, I didn’t fully understand your point, but I think it’s mistaken.  10,000 years from now we should still be able to trace our mtDNA and Y-chromosome DNA back to the same two common ancestors, who either won out in the competition, or who were the first two humans, whichever interpretation is correct.


Bilbo - #65949

November 1st 2011

Darrel,

I’m disappointed that you haven’t responded to Fuz’s arguments.  In a thread where you and Dennis were determined to make RTB look bad, you’re not doing so well, so far.


Ashe - #65957

November 2nd 2011

With respect to Fuz’s arguments with regard to Orangutans,  it’s true all else being equal, that we expect larger populations to have more genetic variation so it is a surprise that Sumatra orangutans have such a high genetic diversity. However, I think it’s obvious that the small present day population size is relatively recent. If they have dramatically decreased in population size over the last 1000 years, from a larger population, then they would not have lost much of their original diversity yet.  


The studies that I’ve read regarding early Homo is to look at very deep coalescence events along the genome and get a sense of the distribution of these events, and form that we can learn about early human diversity. The pattern to look for there would be how large the variance in coalescence times is, since the larger the population the larger the variation. This method would not be fooled by changes in recent history, since it would explicitly look at both the time of coalescence events and the variation in them. 



Bilbo - #65950

November 1st 2011

Argon,

I forgot to say that it is not at all clear to me why only one female and one male will win out as common ancestors.  It seems at least as likely, if not much more likely, that if there were many different mtDNA and Y-chromosome DNA among the founder population, that we would be able to trace our lineage back to many of them.


John - #65952

November 1st 2011

Bilbo:
“I forgot to say that it is not at all clear to me why only one female and one male will win out as common ancestors.”

No need to tell us! It’s perfectly clear to us that it is unclear to you. So the question becomes, since you lack a clear understanding, why are you so bellicose in your defense of RTB?

“It seems at least as likely, if not much more likely, that if there were many different mtDNA and Y-chromosome DNA among the founder population, that we would be able to trace our lineage back to many of them.”

Then I would suggest drawing out the pedigrees. Start with 3 breeding pairs with different mtDNAs and Y chromosomes and randomly mate a subset of their offspring through several generations.

That would do far more than months of quote mining would to increase your understanding.

sfmatheson - #65958

November 2nd 2011

Bilbo writes: “it is not at all clear to me why only one female and one male will win out as common ancestors.  It seems at least as likely, if not much more likely, that if there were many different mtDNA and Y-chromosome DNA among the founder population, that we would be able to trace our lineage back to many of them.”

This means that Dennis and I and others have failed to communicate some very basic concepts to you. Let me try a different tack to see if we can make some progress.

You appear to picture the scenario as involving individuals “winning out.” This is wrong, and that was one of the plain themes of Dennis’ review. I strongly recommend that you re-read it. What Dennis is describing is NOT one individual winning out. Look at the pedigrees. What is “winning out” is not a person but a piece of DNA.

Here’s another way to look at this, adapted from a blog post I recently wrote on the subject. Suppose that one individual in a small population (of, say 1000 breeding individuals) bears a new mutation that confers a reproductive advantage. Let’s call this person Adam. Suppose that mutation, as a result, becomes more common in the population as its advantage is felt, and that 20 generations later, most of the individuals in the population have that form of the gene. And then, perhaps 100 generations later (about 2500 years), that form of the gene is “fixed,” meaning that essentially every individual in the population carries it.

This is how gene forms get fixed. The numbers are arbitrary, and my scenario involves selection instead of drift. But my scenario is the basic outline of how evolution of gene forms happens, whether in fish or finches or sunflowers or people. In my opinion, if someone were then to express skepticism regarding whether that one founding individual could have “won out,” I would conclude that they were missing something very, very basic. And I would encourage them to read more science, more basic genetics, before they read anything written by RTB or the DI. That’s my recommendation to you.

BTW, regarding my scenario above, you might wonder how it can be that the gene in Adam can become fixed without everyone in the population being essentially identical to him. In other words, everyone in the population has Adam as a common ancestor, and yet the population is still diverse. How can this be? As you consider the answer to the question, you will approach a better understanding of how population genetics is misused by Rana and others to create the illusion that the 2-founder hypothesis is reasonable.


Jon Garvey - #65976

November 3rd 2011

Steve - a slight quibble: it’s a shame you call your hypothetical mutation-donor “Adam”, rather than Sydney or Wendy. It seems to me that the heat (and lack of illumination) in this discussion comes from correlating our mitochondrial donor with “Eve” and our Y donor with “Adam”.

This gets Creationists looking to map Genesis as history to the palaeolithic, TEs looking to map Genesis as myth to the palaeolithic and everybody talking about biology rather than covenant or salvation history, as if the former were what Genesis cares about most.

It was, after all, the scientists who first confused the issue for lay-people by giving the impression, via the terms “Y-Adam” and “Mitochondrial Eve”, that the findings had anything to do with human origins rather than the merely fortuitous (or adaptive) survival of the few big chunks of DNA that happen to be traceable,  from random points in the race’s pre-history.

And maybe the Creationists are right to suspect that some of that confusion was sown deliberately to further a secularist agenda. “You have your fairy-tale orgins stories - but see here - genetics reveals their falsity. Adam is just  one chromosome, and Eve just one mitochondrium amongst many.” The field, after all, is hardly free from polemic.


Dennis Venema - #65959

November 2nd 2011

For anyone local to the Vancouver / Lower Mainland area, tomorrow evening (Thursday) Fazale Rana of RTB and I will be having a discussion at 7pm at Regent College (RTB is renting the space, it’s not an official Regent event). This session will be on the evidence for evolution, human evolution, etc. Then, on Saturday we are having a second discussion discussing population genetics / Adam & Eve, etc - this one is at 10th Avenue Alliance Church, also at 7pm. All and sundry are welcome. 


Bilbo - #65962

November 2nd 2011

Steve,

If you want to make the claim that there was some sort of selective advantage for the mtDNA or Y-chromosome DNA that all of us have a descended form of, fine then I get the argument.  But then I think it would be up to someone who made that claim to provide some sort of supporting evidence for it.

 If you want to say that it was just a matter of genetic drift, fine I get that argument, also.  But if it is just a matter of genetic drift, then the likelihood that only one kind of mtDNA and one kind of Y-chromosome DNA became the sole ancestors of every human being is rather small.  If you disagree with this, please explain why. 


sfmatheson - #65967

November 2nd 2011

No, Bilbo, there’s no need to explain either of those mechanisms. The point is not whether selection or drift is more or less likely to have caused one form of mtDNA or Y-chromosomal DNA to have been fixed. Both mechanisms are well established, and so basic that I will direct you to some basic readings on the topic if you think that would help. There is no need to provide “supporting evidence” for the ability of selection to drive alleles to fixation. Ditto for drift, which is not at all unlikely to contribute to fixation in small populations. (Of course, both can contribute.) The point is that no knowledgeable biologist would find any of this to be remarkable, and no competent biologist would make the mistakes that RTB makes. Again, if you really do want references for the effects of drift and/or selection, let me know.


Perhaps it isn’t clear enough that mitochondrial genomes and Y chromosomes (in humans) are unique in that they are largely free of recombination. Thus, during sexual reproduction, they are inherited as whole units. This is quite unlike the rest of the genome, which is subject to crossing over. This is why I was trying to get you to focus on the inheritance of pieces of DNA; you simply cannot understand the concepts here if you don’t have a basic grasp of the mechanisms of inheritance. Whether driven by selection or drift, a mitochondrial genome or a Y chromosome can go to fixation, and when/if that happens, the whole thing will be fixed. This appears to have happened most recently in the human lineage at some particular points in the past, and unsurprisingly at completely different times for the mtDNA and the Y chromosome.

It’s basic. It’s uncontroversial. But it’s complicated enough that if a reasonably intelligent but uninformed person is given just part of the story, they can become very confused. I remain convinced that those who take the time to understand inheritance, population genetics, and some evolutionary biology will be quick to conclude that the RTB position is indefensible.

And I hasten to add that the position I refer to is not a theological or historical conjecture; I refer specifically to the scientific claims made by Fuz Rana as discussed in this thread. Those are indefensible; RTB’s peculiar preferences are not.

Bilbo - #65963

November 2nd 2011

Ashe,

I certainly don’t understand your point.  What makes me think that the orangutan example is a problem for population genetics is the response of William Amos, geneticist at the University of Cambridge, who said (regarding the orangutan example), 
“We don’t fully understand the relationship between genetic diversity
and population size.”


Ashe - #65971

November 2nd 2011

That quote was already explained by Darrel Falk in comment #65894 did you not understand it?


Bilbo - #65964

November 2nd 2011

Steve,

I better be more precise, or you’ll jump on my imprecision as evidence that I don’t understand the topic.  When I said, “...then the likelihood that only one kind of mtDNA and one kind of
Y-chromosome DNA became the sole ancestors of every human being is
rather small….” I meant that only one kind of mtDNA and one kind of Y-chromosome DNA became the sole ancestors of every kind of mtDNA and Y-chromosome DNA that every human being possesses.


sfmatheson - #65969

November 2nd 2011

Bilbo, I do think it’s pretty clear that you don’t understand the topic. I don’t think that’s a bad thing, and I don’t think it’s an insult for me to tell you this. What we’re trying to accomplish (this is my view, anyway) is to explain some fairly demanding genetic concepts in the presence of multiple misunderstandings. At this point, it’s clear that your grasp of the relevant concepts is weak enough that you aren’t able to see the relevance of the original post (by Dennis) or the meaning of the claim that mito Eve and Y-chr Adam were not contemporaries.


So, your added precision here doesn’t change anything; you remain unclear on the basics, or you would know that the mito Eve and Y-chr Adam story cannot be coherently challenged in the way Rana does.

By the way, none of this has anything to do with whether Adam and Eve were real, or whether anyone’s favorite theological/historical narratives are true. To me, anyway, it’s simply about whether certain scientific analyses or claims are complete or coherent. I hope that makes sense.

Bilbo - #65966

November 2nd 2011

Dennis,

Since I won’t be in Vancouver, feel free to explain here why you think Fuz’s arguments are wrong.  Apparently Darrel is too busy to do so.


Dennis Venema - #65968

November 2nd 2011

Bilbo, have you drawn out the pedigrees like John suggested above? That was an excellent suggestion. That and the post above is all about showing why Rana’s view is not tenable. There is a well-known, high-probability mechanism that accounts for mtDNA and Y chromosome coalescence that maintains high levels of autosomal diversity. We have multiple lines of evidence that indicate a population size of ~10,000 or more as far back as we’d like to go. Do you really think one misunderstood quote on Rana’s part changes all that? Rana’s comments don’t even deal with several independent means for estimating population sizes over time (Linkage Disequilibrium studies, which assume NO mutation, for example). 


Ashe - #65973

November 2nd 2011

When are you guys coming to the northeast , I would pay for that, seriously


Menno van Barneveld - #65979

November 3rd 2011

Theology does not include science comletely, for science is in development always to remove wrong visions from imperfect reasoning. Also science is not able to observe the spirit or the concious that together form the soul. But the fear of Jahveh is the beginning of all wisdom.

This is what Jesus Christ our lord taught me as a servant of him:
God created the first man Adam almost one million years ago. In the male fruit of a female man-ape He changed some chromosomal DNA to give man speach. This DNA is not in the X-, Y- or mitochondrial DNA. Also He desided that the human soul had to become indestructible. This is not by changing some DNA, because the holy spirit knits the elemental particles together with bosons for every individual and so the holy spirit makes our soul indestructible.
Then Adam had to get a wife. God planted the speach giving DNA from Adam in the female fruit of another female man-ape and Eve was created.
From then on the human descendants from Adam and Eve spread over the earth.
God is well able to protect people against being whiped out, if needed. It is written that He is well capable of bringing a man to the other side of the sea on a piece of wood.
Mutations in the Y-chromosome and the mitochondrial DNA occured in the evolutionary way..
This teaches us that the coming in existance of the human kind depended on an act of God and could not occur through evolution by chance, because the connection between elemental particles by bosons is not a natural proces, it is done by the holy spirit for every single connection in every single creature.


Bilbo - #65989

November 3rd 2011

Ashe:  “That quote was already explained by Darrel Falk in comment #65894 did you not understand it?”

This is why I hate the “Reply to this comment” system that is used here.  I didn’t even though that Darrel had made that comment until you pointed it out to me.  Thanks.  Unless one has the time (and I certainly do not) to continually go back and find and review all the comments that might be relevant to the discussion, one can miss very important information.  If I were in charge here, I would eliminate this system immediately.


Bilbo - #65990

November 3rd 2011

Darrel,

Thanks for your comment at #65894, which it took me a while to find, since comments here are not necessarily in numerical order.  OK, Fuz was quoting from an article (I think it was Science Daily) that was quoting from Nature.  So it was the Science Daily article that originally took the quote out of context.  I suspect that Fuz never checked the Nature article.  But he should, and I agree with you that he should retract or at least correct his comments on it.  


Bilbo - #65991

November 3rd 2011

Steve,

 it is clear to me that even though I understand this topic, you do not understand me.  I am not surprised.  At your own blog, it was clear that you have an inability to understand anyone with whom you have the least disagreement.  I see no point in us trying to communicate with each other anymore.


Bilbo - #65992

November 3rd 2011

Dennis,

You suggest I follow John’s suggestion:  “Start with 3 breeding pairs with different mtDNAs and Y chromosomes and
randomly mate a subset of their offspring through several generations.”

It appears to me that the problem with this suggestion is that the results would be based on a very small, and perhaps misleading sample size.  For example, I could toss a coin 10 times and get 7 heads.  From this I might incorrectly conclude that if I tossed the coin 10,000 times, I would get 7,000 heads.  Likewise, starting with 3 breeding pairs, instead of starting with 5,000 breeding pairs, might give a false sense of what the probabilities are for 5,000 breeding pairs. 

So let me ask you:  Has anyone done actual probability calculations, based on a population size of 10,000 humans, to see what how likely it would be to get down to one mtDNA ancestor and one Y-chr DNA ancestor? 


Argon - #65998

November 4th 2011

Bilbo, I’ve seen you comments but my connection has been difficult. While it’s up…


Regarding calculations and methodologies, try:

So yes, the numbers are based on actual probability calculations & modelling. The researchers wouldn’t have much to talk about otherwise.

Bilbo - #65993

November 3rd 2011

And finally, to either Dennis or Darrel,

Have either one of you addressed Fuz’s example of the mouflon sheep on the island?  Did I miss that response, also?


sfmatheson - #66000

November 4th 2011

Bilbo, I think that the disconnect is due partly to your failure to follow the threads and not entirely to the causes of your snap judgements regarding my character. But I could be wrong. In any case, I will continue to address your questions for the benefit of those readers seeking to understand the genetic principles that Dennis set out to explain in the original post.

Bilbo asks in comment #65992: “Has anyone done actual probability calculations, based on a population size of 10,000 humans, to see what how likely it would be to get down to one mtDNA ancestor and one Y-chr DNA ancestor?”

This gist of the question has been addressed by others in this conversation (Argon #65946 on page 2, and me (#65958 and #65967, page 3). I will attempt here to illustrate the confusion inherent in the question and then to answer the particular issue it seems to raise.

First, the question depicts a scenario that has not been postulated to occur, namely the simultaneous coalescence of two pieces of DNA (the mitochondrial genome and the Y chromosome) in a single population of 10,000. Instead, the analyses of human genetic variation discussed by Dennis indicate that each of these pieces of DNA can be traced to a common ancestor at completely different times in history. This means that “getting down to one” didn’t happen at the same time for mdDNA and Y-chr DNA, nor did it necessarily occur in a population of a particular size. It is important that the concepts that Dennis illustrated in his original post not be obscured by these misconceptions. In other words, the phrasing of Bilbo’s question introduces significant confusion and presupposes a scenario that was not proposed by scientists and is not consistent with the data. Bilbo is not trying to mislead anyone, and the misconceptions are understandable given the demanding nature of the topic. But it is important to make this clear: the nature of his question reveals significant misunderstanding.

Perhaps what Bilbo is asking is this: “How likely is it that the patterns depicted by Dennis in the original post would actually occur in a real population?” (I think that’s the clear thrust of his comment #65992.)

The answer is: very, very likely. What we’re discussing is the process by which a piece of DNA (a gene, or something bigger like mtDNA or a Y chromosome) can become predominant in a population. This process is called ‘fixation,’ and when a piece of DNA becomes predominant in a population then that piece of DNA is said to be ‘fixed.’ And there are two known forces that can act together or independently to drive a piece of DNA to fixation: selection and drift.

The pedigrees in the original post illustrate how it is that a population can be traced to a single ancestor without that individual ancestor being the only person around. What those pedigrees don’t do is specify how the other lineages disappeared. It’s clear that when an individual fails to reproduce (or, in the special cases of mtDNA and Y chromosomes, fails to have children of one gender), that branch of the lineage dies. But why might that individual fail to reproduce? One possibility is that the individual was “unsuccessful” due to low fitness caused by the DNA in question. (She/he was relatively unhealthy or relatively unattractive.) Even if the fitness differential is somewhat small, it will influence the population’s genetics to a measurable extent. This is selection. The other possibility is that the individual was “unsuccessful” for reasons unrelated to the DNA in question. (She/he was unlucky, or she/he was unhealthy/unattractive for reasons unrelated to the DNA in question.) This is the basis of drift.

[to be continued]

 


sfmatheson - #66001

November 4th 2011

[continued from previous comment]


Note that selection can be positive or negative in its effect. Some variants are removed by selection because they reduce fitness; perhaps they’re harmful in some way. Others are favored by selection because they enhance fitness somehow; in such cases, the competing variants experience reduced fitness without changing in any way. Note that drift will be particularly prevalent in relatively small populations; a rare variant is always at risk of being lost accidentally by drift. And note that a variant can “hitchhike” with other variants; for example, some mtDNA could have become more/less prevalent in a population by fortuitous association with some other piece of DNA that enhanced/reduced fitness.

I know those are very basic concepts, but the question of whether and how a mtDNA or Y-chr variant could “get down to one” in a population of any size is a question about fixation, and the answers are very basic and uncontroversial.

Finally, Bilbo asked about “probability calculations.” It’s hard to know which probability he refers to, but perhaps he’s wondering how likely it is that fixation of, say, a mtDNA variant would occur given a starting population of 10,000 and a certain number of variants. The way to explore this is by simulation, and there are some very nice online simulators designed for this purpose. One very good one is at pandasthumb.org/sims. It enables the user to set various parameters, including strength of selection, then watch the population’s behavior. You’ll learn a lot by running the same simulation repeatedly, especially if the population size is relatively small (say, around 1200 breeding individuals, which is the current estimate of the most significant human population bottleneck). Try it!

sfmatheson - #66002

November 4th 2011

In comment #65993, Bilbo asks, “Have either one of you addressed Fuz’s example of the mouflon sheep on the island?  Did I miss that response, also?”


I addressed those questions in comment #65899 on page 2. Here’s what I wrote:

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

Jimpithecus - #66212

November 23rd 2011

We see this kind of “balanced polymorphism” in humans as well, with the sickle cell anemia/malaria problem.  There is selection for the heterozygote because it has the best chance of beating sickle cell anemia and yet not contracting malaria. 


Bilbo - #66035

November 8th 2011

I’m not a Young Earth Creationist, but I was curious whether any of them had written anything on this topic.  Robert Carter has:

http://creation.com/historical-adam-biologos

I don’t know if his views are at all valid, but they were at least interesting.


Ashe - #66041

November 9th 2011

The heart of his argument seems to be  figure 2. Genetic drift does tend to result in differences between populations, but human populations are, despite what many people think, extremely similar to each other at the genetic level. One measure of differentiation, called Fst, which measures the amount of ‘total’ genetic variation that is due to between-population differences, is about 10% in humans, i.e. pretty small. So we don’t expect big differences between populations, except where there has been local selection for different genetic variants - e.g. lactase or the Duffy blood group protein. 


ChrisMuriel - #66514

December 13th 2011

I doubt its as easy as that. There is evidence of extra-terrestrial seeding of life, even cohabiting with humans which science is yet to investigate. Recently there is also some speculation that junk dna is ET in origin. I’m not sure about the reliability of that story. But there are enough smoking guns in the legends of various lands of ET activity in the past. The Dropa people in china is one such example. Indian legends speak of many kinds of humanoids who interbred with humans, viz. the asuras, the devas, the nagas, the apsaras, rakshasas, etc. etc. The lakshman rekha was a psychic boundary drawn by lakshman to protect his brother’s wife seetha, who was desired by the rakshasa  named Ravanna. The rakshasa way of marriage was by taking away desirable women by force. These are not myths. There are people who still follow these customs. Unless all these cases are investigated, I don’t think it is going to present a clear picture of our descent. 


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