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Author Topic:   The Common Ancestor?
Jon
Inactive Member


Message 31 of 341 (583044)
09-24-2010 12:40 PM
Reply to: Message 28 by AZPaul3
09-24-2010 11:35 AM


Re: Bad Definition = Bad Argument
First of all, Jon, "common ancestor" is not an analogy for some other concept. It is the direct concept itself. The concept of direct lineage.

When we talk of the evolution of species, direct lineage (which you claim to use synonymously with 'common ancestor') simply does not exist. If this is your understanding of 'common ancestor', then it is flawed. You imply a neatness to the process that it does not merit; decent is messy, hazy, half-assed, and indirect.

Second, in the case of my family, we have a direct demonstrable link between the various later individuals and a single specific earlier individual from which we stem. That is our common ancestor.

Third, as we proceed deeper into time the specific information of exact genealogy, between say you and I, may be lost and the best we could hope for is to identify a population subset from which we both stem. European? French? Mediterranean coast? Marseille region? The "common ancestor" concept is as valid and meaningful in this instance as in the above and is not analogous to any other concept but is a concept of direct lineage, though, in this case, can only get as specific as to identify a sub-population, a founder population, of common origin.

When we get to bears, cats and dogs we find that the best we can get, the last direct link between these that can be identified, is not an individual or even as fine as a sub-population, but the more granular overall population of an early carnivore we call miacis about 60 million years ago. The concept still holds and is valid. Miacis is the founder population, the earliest "common ancestor," to these lineages.

This is the standard definition of "common ancestor" used in the field.

This? There are three things being described here—all different, and only the first an example of direct lineage. My grandparents are dead; our ethnicity is tied to an arbitrary regional distinction; miacis is an entire genus. These concepts are related, but not identical. If you think them representative subsets of a single cohesive concept, then describe that concept instead. Trying to get by on poor analogies isn't helping to clarify your position.

Jon


"Can we say the chair on the cat, for example? Or the basket in the person? No, we can't..." - Harriet J. Ottenheimer

"Dim bulbs save on energy..." - jar


This message is a reply to:
 Message 28 by AZPaul3, posted 09-24-2010 11:35 AM AZPaul3 has responded

Replies to this message:
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Jon
Inactive Member


Message 32 of 341 (583048)
09-24-2010 12:50 PM
Reply to: Message 30 by AZPaul3
09-24-2010 12:34 PM


Re: Bad Analogies = Bad Science
Not when the 'common ancestor' being discussed is of the "idealised ... with no further genetic flow" type.

No such thing.

I said that just one sentence after. You're quote-mining now, and misrepresenting my position. I don't do dirty debating.

quote:
Jon in Message 29:
When this is the meaning folk begin putting on to the term, it becomes necessary to point out that their notion of 'common ancestor' is a fantasized non-existent entity/species/generation.

Jon


"Can we say the chair on the cat, for example? Or the basket in the person? No, we can't..." - Harriet J. Ottenheimer

"Dim bulbs save on energy..." - jar


This message is a reply to:
 Message 30 by AZPaul3, posted 09-24-2010 12:34 PM AZPaul3 has responded

Replies to this message:
 Message 33 by AZPaul3, posted 09-24-2010 1:06 PM Jon has not yet responded

  
AZPaul3
Member
Posts: 4415
From: Phoenix
Joined: 11-06-2006
Member Rating: 3.3


Message 33 of 341 (583050)
09-24-2010 1:06 PM
Reply to: Message 32 by Jon
09-24-2010 12:50 PM


Re: Bad Analogies = Bad Science
I said that just one sentence after. You're quote-mining now, and misrepresenting my position. I don't do dirty debating.

As the communicator I obviously did not make my intent clear to the receiver. I will attempt to do better.

When this is the meaning folk begin putting on to the term, it becomes necessary to point out that their notion of 'common ancestor' is a fantasized non-existent entity/species/generation.

A "common ancestor" of the "idealised ... with no further genetic flow" type does not exist. That you perceive this definition as the one being used in this thread is strictly within your own head and is fallacious.


This message is a reply to:
 Message 32 by Jon, posted 09-24-2010 12:50 PM Jon has not yet responded

  
Taq
Member
Posts: 7997
Joined: 03-06-2009
Member Rating: 3.8


Message 34 of 341 (583051)
09-24-2010 1:06 PM
Reply to: Message 29 by Jon
09-24-2010 11:59 AM


Re: Bad Analogies = Bad Science
I think every time we go back we're going to undoubtedly find input again from even earlier generations, requiring that we go back to that generation as the 'common ancestor'. But even this generation will have input from earlier ones, requiring that we again step back further in time.

But we can still state that ALL members of generation B have ancestors in an earlier generation A, can we not?

Let's look at humans and chimps. When we say that humans and chimps share a common ancestor, or more accurately a common ancestral pool, we are saying that at one point in history there existed a single generation of interbreeding individuals. All humans and all chimps can trace their lineages back to members of that single generation.

When this is the meaning folk begin putting on to the term, it becomes necessary to point out that their notion of 'common ancestor' is a fantasized non-existent entity/species/generation.

I think "hypothetical" is a much better term than fantasized or non-existent.


This message is a reply to:
 Message 29 by Jon, posted 09-24-2010 11:59 AM Jon has responded

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AZPaul3
Member
Posts: 4415
From: Phoenix
Joined: 11-06-2006
Member Rating: 3.3


Message 35 of 341 (583061)
09-24-2010 1:32 PM
Reply to: Message 31 by Jon
09-24-2010 12:40 PM


Re: Bad Definition = Bad Argument
There are three things being described here—all different, and only the first an example of direct lineage.

Wrong. They are all examples of direct lineage. The only difference is in the granularity of the result not in any difference in the concept.

If you think them representative subsets of a single cohesive concept, then describe that concept instead.

Message 30 Second paragraph.

Whether the comparison is between two individuals, two closely related sub-populations, two related species, two broadly separated species, makes no difference. Follow the lineages back until they converge. This is the most recent "common ancestor."

Edited by AZPaul3, : clarification

Edited by AZPaul3, : correction

Edited by AZPaul3, : No reason given.


This message is a reply to:
 Message 31 by Jon, posted 09-24-2010 12:40 PM Jon has not yet responded

  
Jon
Inactive Member


Message 36 of 341 (583116)
09-24-2010 5:14 PM
Reply to: Message 34 by Taq
09-24-2010 1:06 PM


Re: Bad Analogies = Bad Science
I like your notion of 'common ancestral pool'. I think it more accurately describes what we see in evolution.

But we can still state that ALL members of generation B have ancestors in an earlier generation A, can we not?

Yes, assuming that they do. I'm not attempting to argue against the processes of common decent or human evolution, nor and I trying to undermine the theories that show humans and chimps to be related by these processes. I am merely trying to point out that modern pop-culture anthropology uses terminologies that imply a degree of simplification to these processes that they merely do not exhibit.

I cannot be 100% certain, but I feel you and I are in somewhat of agreement on this issue.

Jon


"Can we say the chair on the cat, for example? Or the basket in the person? No, we can't..." - Harriet J. Ottenheimer

"Dim bulbs save on energy..." - jar


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 Message 34 by Taq, posted 09-24-2010 1:06 PM Taq has not yet responded

  
Blue Jay
Member (Idle past 927 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 37 of 341 (583231)
09-25-2010 3:04 PM
Reply to: Message 23 by caffeine
09-24-2010 4:39 AM


Re: Bad Analogies = Bad Science
Hi, Caffeine.

caffeine writes:

Surely, even if 'common ancestor' is taken to refer to a single individual, it's still a logical necessity for everything to have a common ancestor with everything else (assuming common descent). There wouldn't be a unique individual, and we could never hope to find or know if we've found such an individual, but how could they not exist?

Well, maybe. But, I don't think so.

If we take a strict gradualistic approach, any new evolutionary development must have occurred within a pool of organisms (or proto-organisms) that was just one step short of the new development. Interactions and exchanges between the organism with the new development and the other organisms in the pool are probably unavoidable.

I think this is probably true of every step in evolutionary history, although the very first true cell might be an exception (I don't think so though: HGT, prions and viruses were probably major contributors around that time).


-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.


This message is a reply to:
 Message 23 by caffeine, posted 09-24-2010 4:39 AM caffeine has responded

Replies to this message:
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barbara
Member (Idle past 3031 days)
Posts: 167
Joined: 07-19-2010


Message 38 of 341 (583405)
09-26-2010 11:18 PM
Reply to: Message 13 by Taq
09-23-2010 4:07 PM


Deletions
How do you know it was deleted if it is no longer there anymore? The same question for insertions, how do you know that it wasn't always there in a genome sequence?
This message is a reply to:
 Message 13 by Taq, posted 09-23-2010 4:07 PM Taq has responded

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Nij
Member (Idle past 3119 days)
Posts: 239
From: New Zealand
Joined: 08-20-2010


Message 39 of 341 (583408)
09-27-2010 1:10 AM
Reply to: Message 38 by barbara
09-26-2010 11:18 PM


Detecting deletions and identifying insertions
How do you know it was deleted if it is no longer there anymore? The same question for insertions, how do you know that it wasn't always there in a genome sequence

By comparing what came first and what happened later, and if that's not possible then by comparing anything we can put our hands on.

For example, say an ancestor had the DNA ABC-AD-BDC-BAC.
If we get 5 current species with DNA that looks like
1 ABC-ADD-BDC-BAC
2 ABBBC-AD-BDC-BAC
3 ABC-AD-B-BAC
4 AB-AD-BDC-BAC
5 ABBBC-AD-BDC-BC
(not in any particular order)

we could tell what had been deleted and what had been inserted based on the idea that if your ancestor had it, it is far more likely to be present now than if they didn't: the probability of something being lost almost completely is much lower than the probability it was never there to begin with but was picked up later by a couple of descendants, and vice versa. For example, almost every species has BAC in the final section so we would guess that the ancestor also had it (unless these species are all more closely related than that, but see next).

By examining what, if any, record exists, we can also narrow down the possibilities and confirm our guesses. In the above example, you would not know whether the DC in the third section had been inserted or deleted from the original ancestor because the ratio is pretty close. But if you also knew that species 2 came before species 3, you could tell the mutation in that case was deletion. Likewise for the BB in the first section, you can tell it was an insertion because (say) species 1 existed before species 2. It's not only the DNA of one thing we look at, it's the DNA of anything and everything plus the records of when what happened.

If you can find a simple evolutionary algorithm that provides you with a record, get hold of the last 3 generations, then see how well you can construct the tree backwards to the original ancestor by comparing what you know came first and what came later, and by how common something is. That's basically how it gets done in the real world.


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caffeine
Member
Posts: 1682
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 2.6


Message 40 of 341 (583431)
09-27-2010 6:45 AM
Reply to: Message 37 by Blue Jay
09-25-2010 3:04 PM


Re: Bad Analogies = Bad Science
Well, maybe. But, I don't think so.

If we take a strict gradualistic approach, any new evolutionary development must have occurred within a pool of organisms (or proto-organisms) that was just one step short of the new development. Interactions and exchanges between the organism with the new development and the other organisms in the pool are probably unavoidable.

I think this is probably true of every step in evolutionary history, although the very first true cell might be an exception (I don't think so though: HGT, prions and viruses were probably major contributors around that time).

There seems to be a bit of confusion here. Of course the organism will be interacting with other organisms in the pool, but so what? If you have some Miocene ape sat somewhere in Africa, who is an ancestor of chimpanzees and an ancestor of humans, then he is a common ancestor of apes and humans, and he is also a direct common ancestor of apes and humans. Yes, he'll be interacting with other apes, and some of those will also be common ancestors of humans and chimps.

I'm not sure exactly where the confusion is arising. It seems to me that objections from both you and Jon seem to be based around the idea that saying there are individuals that are common ancestors means this individual must have been seperated from all individuals at the time, and produce two group of offspring which themselves separate off and don't interact. But all a common ancestor is, with a literal reading of the term, is an ancestor who's common to both or all the populations under consideration. His or her parents would also be common ancestors, many of his or her contemporaries would be common ancestors, some of his descendants may be common ancestors. A common ancestral population is made up of a bunch of individual common ancestors, plus others who had no descendants or whose descendants didn't make it to today.

If we could go back in time and take any population of organisms, and then we took a selection of modern day organisms, each individual in the ancient population would either be the direct common ancestor of all the modern day organisms, the direct ancestor of just some of the modern day organisms, or the ancestor of no modern day organisms. The fact that it's not possible to look at a fossil and place it in any group doesn't change this basic fact.

As for the question of the idealised scenario of one couple or one pregnant individual going off to found a new population, with no further gene flow with the original population, of course this happens! The sheep population of the Kerguelen archipelago was founded by one pair brought by humans in 1957. As long as we don't bring any new sheep to the island or take any away, this population will remain isolated and have no gene flow with the outisde world. Similar things have probably happened on oceanic islands many times in the past, as local populations were founded by individuals or pairs drifted there by chance. This is not the typical method of speciation, but it's wrong to say it doesn't happen.


This message is a reply to:
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Replies to this message:
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Strongbow
Junior Member (Idle past 3139 days)
Posts: 26
Joined: 09-16-2010


Message 41 of 341 (583444)
09-27-2010 9:33 AM
Reply to: Message 18 by Jon
09-23-2010 8:57 PM


Re: Definitions & Feel-good Science
quote:
Generally, the notion is that breeding individuals belong to the same species.

That is one possible definition of species, but not comprehensive, and maybe not even accurate.

I assume you'd agree that Lions and Tigers are separate species, yet Ligers do exist:

http://en.wikipedia.org/wiki/Liger

Edited by Strongbow, : No reason given.


This message is a reply to:
 Message 18 by Jon, posted 09-23-2010 8:57 PM Jon has responded

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Jon
Inactive Member


Message 42 of 341 (583455)
09-27-2010 12:05 PM
Reply to: Message 41 by Strongbow
09-27-2010 9:33 AM


Re: Definitions & Feel-good Science
quote:
Generally, the notion is that breeding individuals belong to the same species.

That is on possible definition of species, but not comprehensive, and maybe not even accurate.

I assume you'd agree that Lions and Tigers are separate species, yet Ligers do exist:

http://en.wikipedia.org/wiki/Liger

I can agree, for purposes of discussion that lions and tigers are separate species, but if the scientific evidence says otherwise, then who am I to disagree? There are many instances in the history of science where long-held preconceptions have been thrown out by scientific evidence. Just because we arbitrarily agree to describe and label two things as distinct species does not make them distinct species.

Furthermore, ligers are often sterile, and I must apologize for my forgetful mind, because I neglected to include an important caveat to the general understanding, which is that the offspring must be fertile. Nevertheless, it appears that the occasional liger is fertile, which I must admit is interesting. This pulls us to question our classificatory methods and current classification system.

Before I bring this too far off topic, however, I want to add that these hard-to-tell areas are to be expected given the mussy nature of evolution. Speciation exists on a continuum, where some distinctions are cut and clear and other things are gray. If evolution were cut and clear and species boundaries distinct and precise, then things like the liger would be impossible. This was part of my point early on regarding evolution - it is not only too messy to find the point of common ancestry, but it is too messy for there to even be a point of common ancestry.

Jon


"Can we say the chair on the cat, for example? Or the basket in the person? No, we can't..." - Harriet J. Ottenheimer

"Dim bulbs save on energy..." - jar


This message is a reply to:
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Jon
Inactive Member


Message 43 of 341 (583461)
09-27-2010 12:49 PM
Reply to: Message 40 by caffeine
09-27-2010 6:45 AM


Re: Bad Analogies = Bad Science
But this ape also has ancestors. Why stop at the ape? Why not keep going further back?

I'm not sure exactly where the confusion is arising. It seems to me that objections from both you and Jon seem to be based around the idea that saying there are individuals that are common ancestors means this individual must have been seperated from all individuals at the time, and produce two group of offspring which themselves separate off and don't interact.

I have not taken this position. My apologies if my posts implied that I had. If we go really far back, of course we can find a common ancestral species. But we do not want to go that far back, at least I assumed that the OP was not looking for something that far back. My issue was that we cannot get too close (to the time of the split), because the closer we get to the split, the messier things get, and, of course, the point at which the final grouping stopped interbreeding and could thus be categorized as distinct and separated becomes muddled. And not just muddled in that it is difficult to pinpoint the last generation or group of interbreeders, but in that such a point does not exist—there is no such generation.

Jon


"Can we say the chair on the cat, for example? Or the basket in the person? No, we can't..." - Harriet J. Ottenheimer

"Dim bulbs save on energy..." - jar


This message is a reply to:
 Message 40 by caffeine, posted 09-27-2010 6:45 AM caffeine has acknowledged this reply

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barbara
Member (Idle past 3031 days)
Posts: 167
Joined: 07-19-2010


Message 44 of 341 (583464)
09-27-2010 2:22 PM
Reply to: Message 43 by Jon
09-27-2010 12:49 PM


Re: Bad Analogies = Bad Science
I agree that "common ancestry" should not be used since it is too messy and it is based on speculation. The geographical environmental gene pool that defines a specific ecosystem is the primary initiator for evolution to take over for changes in appearances of the biota over time. That is a better explanation.
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Strongbow
Junior Member (Idle past 3139 days)
Posts: 26
Joined: 09-16-2010


Message 45 of 341 (583465)
09-27-2010 2:31 PM
Reply to: Message 44 by barbara
09-27-2010 2:22 PM


Re: Bad Analogies = Bad Science
It's important to remember that the concept of a "species" is one created by human beings for our own convenience to describe an observed phenomenon. Exactly what constitutes a species is going to be a bit elusive, because as we learn more, we learn that as a concept, it's more descriptive than restrictive. And when it fails to describe the phenomena you are looking to describe, then it must change, or even disappear all together. A species is a bit like pornography in that it may be hard to define precisely, but most people knpow it when they see it. Drawing firm lines between species is anohter thing... and in many ways, not as important as it used to be.
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