The Common Ancestor?

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. 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

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

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

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

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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.

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Jon

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

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

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

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

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. 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, : clarificationEdited by AZPaul3, : correctionEdited by AZPaul3, : No reason given.

I like your notion of 'common ancestral pool'. I think it more accurately describes what we see in evolution. 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

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

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

Hi, Caffeine. 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).

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-Bluejay (a.k.a. Mantis, Thylacosmilus)Darwin loves you.

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.

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.

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Generally, the notion is that breeding individuals belong to the same species.

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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: Liger - WikipediaEdited by Strongbow, : No reason given.

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

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

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

But this ape also has ancestors. Why stop at the ape? Why not keep going further back? 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 existthere is no such generation.Jon

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

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

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.

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.