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Author Topic:   Common Ancestor Terminology
Taq
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Message 5 of 15 (672911)
09-12-2012 10:45 AM
Reply to: Message 4 by Dr Adequate
09-12-2012 10:01 AM


I'm not sure how this is meant to work. If you and I are related by being descended from two people who were cousins, then those two cousins must have had a common grandfather, and who is our common ancestor.
Each cousin will also carry DNA from a set of grandparents that is NOT shared between the cousins. That DNA can be passed on to the next generation. An ancestral gene pool or population is a much, much better model.
Let's stay with these cousins as our example. Let's say that a mutation occurs in one set of grandparents that is not shared. This mutation would end up in one of the cousins, and this potentially important genetic marker would not be the result of common ancestry between the cousins.
Edited by Taq, : No reason given.

This message is a reply to:
 Message 4 by Dr Adequate, posted 09-12-2012 10:01 AM Dr Adequate has replied

Replies to this message:
 Message 6 by Dr Adequate, posted 09-12-2012 12:17 PM Taq has replied
 Message 7 by Straggler, posted 09-12-2012 4:02 PM Taq has replied

  
Taq
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Posts: 9970
Joined: 03-06-2009
Member Rating: 5.6


Message 8 of 15 (673036)
09-13-2012 11:52 AM
Reply to: Message 6 by Dr Adequate
09-12-2012 12:17 PM


The author of the OP seems to be suggesting otherwise. He says: "The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more." But surely he's wrong --- the closest that two organisms' family trees can get can never be distant cousinship, 'cos if they get that close then there must also actually be an individual common to both.
The author of the OP also said:
"Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature? Is there simply a weakness in the definitions of our language, and we need a new word? Or does the existing terminology need to be readdressed and properly defined to those outside the biological sciences?"
So this is really about how scientists use the term. What scientists are ultimately concerned with is how a species got the DNA that they have. For cousins, they share DNA from multiple common ancestors, not just one. Each mutation and gene will have it's own lineage within the population, and each could coalesce to a different common ancestor. With sexually reproducing species it makes no sense to talk about an individual as a common ancestor, at least from a genetic standpoint. We could also talk about incomplete lineage sorting and the impact it has on determining common ancestry.
Ultimately, evolution comes back to genetics, and in genetics the common ancestor is a gene pool, not an individual.

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 Message 6 by Dr Adequate, posted 09-12-2012 12:17 PM Dr Adequate has replied

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Taq
Member
Posts: 9970
Joined: 03-06-2009
Member Rating: 5.6


Message 9 of 15 (673038)
09-13-2012 11:54 AM
Reply to: Message 7 by Straggler
09-12-2012 4:02 PM


But the cousins in your example still share a common ancestor.
They share multiple common ancestors from multiple generations, each of which made different contributions to their genome.

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Taq
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Posts: 9970
Joined: 03-06-2009
Member Rating: 5.6


(1)
Message 11 of 15 (673060)
09-13-2012 12:51 PM
Reply to: Message 10 by Dr Adequate
09-13-2012 12:11 PM


It would, of course be wrong to suggest that such an individual was unique, which I didn't. But they did exist. If humans and chimps (for example) have a species that is our common ancestor, then necessarily there was at least one individual in that species which was our common ancestor, because two individuals can't be related unless they have at least one common ancestor.
I fully agree that there would be such an individual, and probably more than one.
But is that really what we are talking about when we reference a common ancestor? I would say no. That is not what we mean. When we say that humans and chimps share a common ancestor what are we really trying to describe? Well, we are ultimately talking about a speciation event where a single population split into two populations. The common ancestor would be the ancestral population that spawned the two separate lineages.
When we look at the fossil record we are forced to use the same concept. We have no way of determining direct ancestor/descendant relationships between fossils, barring the discovery of DNA in those fossils. We are forced to describe relationships through common ancestry, and once again these relationships are referencing an ancestral population.
The OP asked, "Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature?". That is a great question. Perhaps we SHOULD stress what we really mean by common ancestry. It is trivially true that we can trace the lineages of any two organisms back to an individual who is a common ancestor, but that is not really what we are describing. I think it also causes confusion when we talk about mitochondrial DNA and y-chromosome MRCA's. It can cause people to think that ALL of our DNA came from those two individuals.
So should we be using something like "common ancestral population" instead of "common ancestor"? Perhaps we should consider it.

This message is a reply to:
 Message 10 by Dr Adequate, posted 09-13-2012 12:11 PM Dr Adequate has replied

Replies to this message:
 Message 12 by Dr Adequate, posted 09-13-2012 2:51 PM Taq has replied

  
Taq
Member
Posts: 9970
Joined: 03-06-2009
Member Rating: 5.6


Message 13 of 15 (673084)
09-13-2012 4:06 PM
Reply to: Message 12 by Dr Adequate
09-13-2012 2:51 PM


Well, we mean both.
My problem is my own biases in how I view it, so I would be happy to agree with that. I tend to stress the population over the individual common ancestor, but that doesn't necessarily have to be the case.
Admit that I'm right, which I clearly am, and we have no quarrel.
You are mostly right.

This message is a reply to:
 Message 12 by Dr Adequate, posted 09-13-2012 2:51 PM Dr Adequate has replied

Replies to this message:
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