Question about evolution, genetic bottlenecks, and inbreeding

This is true. Here's an explanation from Wikipedia:

quote:

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The existence of an MRCA does not imply existence of a population bottleneck or first couple. The MRCA of everyone alive today could have co-existed with a large human population, most of whom either have no living descendants today or else are ancestors of a subset of people alive today. This seemingly paradoxical phenomenon can be easily explained, if the effect of pedigree collapse is taken into account.
When tracing human lineage back in time, most people look at parents, grandparents, great-grandparents and so on. The same approach is often taken when tracing descendants via children and grandchildren. This naive approach calculates a number of ancestors and descendants that grows exponentially. Thus, over 30 generations, or in the time since about the High Middle Ages, a single individual alive today would have 230 or roughly a billion ancestors, more than the total world population at the time.
This simple calculation does not take into account the fact that every fertilisation is really a fertilisation between distant cousins, which leads to the effect of "pruning" or collapsing the individual's family tree. The ancestry tree is thus not strictly a tree, but a directed, mostly-acyclic graph. People with no descendants are placed at the bottom of the graph and ancestors are placed above their descendants. Starting with the people with no descendants one can connect their parents above. (Half-)siblings thus become connected via one or two common ancestors, their parent(s). As each generation of ancestors is added at the top of the graph, more and more people become related to one another (first cousins, second cousin, third cousins and so on). Eventually a generation is reached where one or more of the many top-level ancestors is an MRCA from whom it is possible to trace a path of direct descendants all the way down to every living person at the bottom generations of the graph.
It is incorrect to assume that the MRCA passed all of his or her genes (or indeed any single gene) down to every person alive today. Because of sexual reproduction, at every generation, an ancestor only passes half of his or her genes to the next generation. The percentage of genes inherited from the MRCA becomes smaller and smaller at every successive generation, eventually decreasing to zero (at which point the Ship of Theseus paradox arises), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.

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What this is saying is that if you trace a family tree back far enough, you can find an individual who everybody is related to in some way. This does NOT mean that this individual is responsible for "creating" the whole human species. So, yes, there is probably an individual to which all humans and chimpanzees can trace back to. However, this does not mean that this individual had offspring that were either human or chimpanzee. The evolution still occurred at the population level.Edited by Stagamancer, : No reason given.

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"There are three kinds of lies: lies, damned lies, and statistics."

It's not that there's a single individual that we are descended from. Harry, let's use you and I as an example. At some level, we are related. We may be 3rd, 12th, or even 50th cousins, but we are related to some degree. So, if we go back far enough, we can find at least one person who is in both of our family trees. However, this does not mean they were the only person to contribute to our individual gene pools. What it means it that we have 1 person who has contributed some DNA to both of us. So, if you do that for the whole human species, you can find 1 person who has contributed DNA to everyone alive. BUT, there is not necessarily an inbreeding problem, because billions of other people have contributed to the worldwide gene pool as well. Does that help?

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"There are three kinds of lies: lies, damned lies, and statistics."

Yes, well, the term "common ancestor" is mostly used to talk about a species, the term "most recent common ancestor" often refers to a person. Is this confusing? Yes, but what can ya do. I should, however, also point out, that with each new generation, the fraction of the genome inherited by each person from the MRCA approaches zero. So, if you go back far enough, you actually WON'T be able to find an individual MRCA. Now, I don't know how many generations have occurred since the split between humans and chimpanzees, but I'd be willing to guess it's enough that we wouldn't find an individual MRCA. Therefore, it is probably more accurate to talk about the MRCA of chimps and humans as being at least a population. As for humans and great white sharks, for example, there's just no way that there is an individual who is the MRCA, so in that case you WOULD have to use a whole species as the MRCA.

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"There are three kinds of lies: lies, damned lies, and statistics."

Yes, you can find an individual MRCA for specific genes, this is true. What I meant was that there's probably no single MRCA that has contributed DNA to every living human and great white shark as there IS probably a single human who has contributed DNA to every living person.

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"There are three kinds of lies: lies, damned lies, and statistics."

This actually not at all impossible. Common ancestor does not mean progenitor, it just means if you go back far enough, you can find an individual that is a part of everyone's family tree. Humans are not that diverse genetically, either, which makes it even more probable. Of course, because of the relatively large numbers of alleles we have, trying to figure out when such a person existed is very time consuming because we'd have to take all of the histories of all our genes into account. This doesn't mean that this common ancestor is responsible for all our genes, simply that he/she has contributed genetic material (or genetic material derived from his/hers) to everyone. So it may be that my gene A is derived from the MRCA and your gene B is derived from that same individual. Doing all this work is probably not even worth it, which is whywe search for mitochondrial-Eve and Y-Adam instead. This cuts down on time and effort and still gives us the information we're searching for, such as where in the world Homo sapiens originated.

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"There are three kinds of lies: lies, damned lies, and statistics."

Because of the dilution that occurs with every single generation, I'm not so sure this is true for every single pairing of species. I can't imagine, for example, that you could find an individual that has contributed genetic information to every human and every Escherichia coli bacterium. But, I'm going to have to research more on what the experts say about this. So, I honestly can't give you a definitive answer now. But yes, I agree, that according to the MRCA is defined, logically, it is possible, though perhaps not inevitable.

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"There are three kinds of lies: lies, damned lies, and statistics."

This is true, and this is what differentiates a MRCA from a progenitor. The MRCA, by definition is really a product of the group that you are looking at. Obviously, the MRCA of me and my siblings is different from you and your siblings, and the MRCA for me, my siblings, you, and your siblings is different too.

There is only one MRCA (or possibly pair, if you using whole individuals) for a given group, but the MRCA will change for how you define the group. So, if we take as a group me and my cousins on my father's side, our MRCA pair is our grandmother and grandfather. This, of course does not mean that all our genes came from those grandparents, simple that those grandparents have contributed something in the DNA of each of us. However, if we start looking at individual genes, e.g. eye color, then that MRCA will mostly like be different for each gene or even each allele. This is why mitochondrial-Eve and Y-Adam probably lived generations apart. So yes, there can be kazillions of MRCAs if you look at it on a gene for gene basis. BUT, if you take each individual as a consensus of genes you can still find 1 single individual (or pair of individuals) that is the MRCA for a whole population.So to sum up: an MRCA for a population of individuals is the individual (or pair) that has contributed SOME DNA (regardless of which gene or set of genes) to every LIVING individual.An MRCA for a gene or set of related alleles is the individual who had an ancestral form of that gene or set of alleles from which all CURRENT (i.e. exist in living individuals) are derived.

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"There are three kinds of lies: lies, damned lies, and statistics."

This doesn't even make sense. MRCA stands for MOST RECENT Common Ancestor. So, for you and your sister, that answer is your parents. So yes, there are two there, but it's a mating pair. Your grandparents don't count because, while common ancestors, they are NOT MOST RECENT. However, think of this. If you have 1/2 siblings because your father has had two marriages (for whatever reason) then only your father is the MRCA for you and your 1/2 siblings. So yes OBVIOUSLY there can be more than one common ancestor, but there is only 1 individual or 1 mating pair that can be the MOST RECENT common ancestor.

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"There are three kinds of lies: lies, damned lies, and statistics."

Why would the two central pairs in the 3rd generation be MRCAs for the 4th generation regardless of whether they moved to islands? The 2 central pairs of generation 3 contribute NO genetic material to either S or Z, therefore they can't be an MRCA for the 4th generation.Though, it's true in that diagram, because of all the inbreeding A-H have equal status as MRCA for S-Z. But, as soon as you start having people mate with individuals not directly descended from the A-H group, that would change.Edited by Stagamancer, : No reason given.

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"There are three kinds of lies: lies, damned lies, and statistics."

Actually, I'd beg to differ on this point. I just came up with this, so hear me out. According to the A-H, S-Z diagram, the 1st generation all have MRCA status BECAUSE of the inbreeding. The less inbreeding there is, the fewer numbers of MRCAs there can be. That is to say, if you only have 1 single individual or pair that is the MRCA for one group, that means their family trees all only touch at that one point. However, if you have multiple MRCAs for a group, that means their genealogies touch multiple times, meaning they are actually more related. I would predict, based on this that while the current Cheetah population has a very low numbers of PROGENITORS (thus the inbreeding problem) they probably have a high number of MRCAs

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"There are three kinds of lies: lies, damned lies, and statistics."

OK, yes. And unfortunately, I must concede that there is no logical inevitability for there always being 1 individual or mating pair that is the MRCA. However, in order to get multiple MCRAs as I've just pointed out in recent posts requires extreme inbreeding between cousins (3rd gen) or where sets of brothers marry sets of sisters (2nd gen). So the diagram proves it's not a inevitability, but I doubt that diagram is an accurate representation of the norm.Edited by Stagamancer, : No reason given.

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"There are three kinds of lies: lies, damned lies, and statistics."

I don't think anyone has argued differently.

Agreed, which is why I don't think it is an accurate representation of the norm, and why you end up with all four couples in the 1st generation being MRCAs for the 4th.

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"There are three kinds of lies: lies, damned lies, and statistics."

No, you are right on the money. That's exactly what geneticists have been looking for to try to determine whether ancient humans interbred with neanderthals. I'm not positive which genes they're looking at, but mt-DNA is always a good one. So far, though, there isn't any evidence of neanderthal genes in the human population. Sure, they may just be so diluted that we just haven't come across them yet, but it's not looking like that's the case. They're currently working on sequencing the neanderthal genome as we speak from some frozen specimens, and hopefully that will make it easier to figure this all out.

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"There are three kinds of lies: lies, damned lies, and statistics."