Isn’t science wonderful? 
No, it’s not a silly semantic argument; it’s the definition of what “related” means in this context. All the hand waving you want to do doesn’t change that fact. Genetic similarity is something different than relatedness.
I’m done here, as far as trying to explain this to you any further.
Of course the answer is unknown, but these “reasonable guesses” derived from genetic similarity can still serve as an approximation. In the digital humanities edit distance techniques are often used to try to trace the lineage of historical texts. Back when books were manually transcribed, errors and deliberate changes always crept in and were therefore propagated to subsequent versions. Given only, say, 50 different ancient copies of a gospel and their approximate dates, then by measuring edit distance it’s possible to construct an ancestral tree of texts which shows with a certain degree of confidence who copied from whom, even though the intermediate copies may have been lost. The accuracy of this tree can be further improved when combined with other historical evidence and their associated analysis techniques. It must be similar in evolutionary biology, no? Genetic edit distance, along with many other factors, can be used as some proxy for the relatedness, or similarity, between any given pair of organisms. It may not be a particularly good or useful factor on its own, but it’s a piece of contributing evidence nonetheless.
I absolutely agree. You just have convince others who believe that genetic lineage magically disappears at some point.
No, it isn’t evidence at all. Such a metric would be not tell you anything at all about the degree of relatedness or about the degree of similarity.
You are arguing that the simililarity of lengths between between any given pair of organisms must be a proxy for the relatedness between those organisms. Your argument is literally that much of a non sequitur. The two factors are totally and utterly unrelated.
Others have already expounded at length on why factors such as generation time, chromosomal fusing and other genetic modification makes such a technique utterly useless in determining relatedness between clades.
As far as similarity goes, I only have to ask you to look at the similarities between a tuatara an iguana and a mosoasaur, and then tell me which of those organisms will have the shortest “genetic edit distance”. It’s just a total nonsense. Between clades genetic distance simply isn’t any indicator at all of similarity.
To put this into a literary context, what doing is looking at a copy of “Journey to the West”, a copy of “Zen and the Art of Motorcycle Maintenance”, a copy of a script for “The Matrix” and a copy of “The Art of Happiness”. You know that all these books derive from a common ancestral source, so you therefore conclude that you must be able to determine which of them is most closely related to Buddhist scripture by calculating an edit distance between these works and the “Pali Canon”. Whichever has the shortest edit distance to the “Pali Canon” must be the most closely related to Bhuddist scripture
Now tell me, do you honestly think that is a valid technique to use? Do you think it will tell you a damn thing about the relatedness of these works? Because that is basically what you are doing when you attempt to calculate relatedness between organism in distinct clades based upon genetic distance. You are trying to compare organisms that have been “interpreted” in their own way by the selection pressure unique to that clade just as oral tradition, movie scripts and popular press are all have very different interpretation standards to one another.
There is just no validity at all in attempting to calculate relatedness based on edit distance when you are dealing with print works produced in totally different genre and media. There is no doubt that you *could *do such an analysis, but obviously it won’t tell you a damn thing.
Sigh.
No, genetic lineage never disappears. And nobody has ever suggested such a thing.
Consider this hypothetical Tripolar. I think it will help you understand.
A brother an sister produce offspring. Because of the nature of chromosomal assortment and crossing over, the offspring can potentially inherit anything from 100% to 0% of the genes of either grandparent. Literally, the offspring can be genetically identical to either grandparent, or they can have no genetic similarity whatsoever, or any value in between. That is an indisputable fact.
So, the male offspring of this couple is 70% genetically identical to his grandfather. The female offspring is 7% similar.
So tell us, do you really think that two siblings can have an order of magnitude difference in there degree of relationship to their own ancestor?
Let;s assume you do believe that. The male offspring has a daughter with a totally unrelated woman. That daughter will be on average 35% genetically identical to here grandfather. So now the grandaughter more genetically similar than the daughter. Do you agree with this?
Then how about the great granddaughter? She will be on average 17% identical to her great, great grandfather, which is still much higher than the woman who is that man’s own daughter?
Does this actually make sense to you? Do you really think there is any validity in claiming that Tom’s great, great, great granddaughter is much more closely related to Tom then Tom’s own daughter is?
If you don’t believe that, then you can now see why genetic distance between clades is worthless as a measure of relationship to the common ancestor.
And if you do agree, then it should be quite obvious that you are using the term “related” in a manner that nobody else in the whole world uses it. Nobody but you would consider that a fourth generation descendant to be more closely related to the ancestor than a first generation.
let’s see, without reading the replies here, just deduction (in short, no dope):
it would be any reptile descended from the branch-off with mammals. in this case, that “closely” related reptile would look, act, and feel 100% reptile with absolutely no physical similarities to mammals that one can easily find.
Which is all of them.
qualify it to the most recent branch off, if that’s possible. so you’re saying it was a non-reptile that branched (the only possibility i see.)
Still all of them.
Precisely.
If it had been a reptile then the branching must, by definition, already have occurred, and the grouping “reptile” must be polyphyletic, and thus cladistically invalid.
But do you admit that such a technique is useful to determine relatedness within clades?
Edit distance to what?
The Matrix did not arise from Buddhist scripture through an iterative process of rote copying with modification, whereas I thought evolutionary descent actually is such a process. Apart from microorganisms which may freely swap DNA with utterly unrelated species, do not most organisms reproduce by making a mostly faithful copy of themselves (or rather, constructing the offspring out of mostly faithful copies of portions of their and their mate’s DNA, in the case of sexually reproducing organisms)?
From what I understand, there are no complete fossils of basal amniotes like Casineria; however, they were probably descended from Reptilomorphs living some 340 million years ago. The basal amniotes - the first fully land-based vertabrates - did not exists for more than a few million years before splitting into the ancestors of mamals and the ancestors of reptiles.
Well of course it is.
To whatever you like. Sponges, bactrian camels, bottle gourds. It doesn’t matter.
If what you are saying is correct, it should make no difference what you calculate the distance to. The degree of similarity of those organisms is fixed, it can’t change, regardless of whether you measure their relatedness relative to an amoeba or an asparagus. If the similarity *between *organisms changes depending upon what you measure the distance *too *then that is all the proof we need that the technique is useless. There is a fixed, objective degree of difference between an iguana, a mosasaur and a tuatara. If your technique tells us that the difference is plastic and subjective, then you have proven conclusively that your technique does not work.
Only *within *lineages.
I suspect this is the cause of much of your confusion. Mammalian DNA wasn’t produced through copying of reptilian DNA or vice versa. Mammalian DNA and reptilian DNA are both derived from a common third-party source, an evolutionary “Q document” if you will. One isn’t a copy of the other. As such you can no more infer the relationships between mammals and reptiles based on edit distance than you can divine the relationship between “The Matrix” and “Journey to the West” based on edit distance.
That’s true enough that we can let it slide.
But the point to realise is that it only true within reproductive lineages. Reptiles don’t derive their DNA by copying mammalian DNA, nor do mammals derive their DNA by copying reptilian DNA. They never have and they never will. As such you can’t tell a damn thing about the relatedness of a reptile and a mammal by comparing them genetically because their genetics are not copied from each other and never have been. They are derived from a common source, but they aren’t modifications of each other, they are both modifications of the common source.
As such they are only related to each other in the same way that “The Matrix” is related to “Journey to the West” and “Zen and the Art of Motorcycle Maintenance”: through a common source. So unless you believe that an edit distance can validly divine whether “The Matrix” or "“Zen and the Art of Motorcycle Maintenance” is more closely related to “Journey to the West”, you likewise can not apply it to this problem, for precisely the same reasons.
How do you define “lineage”? In the ordinary sense of the word, a lineage is descent in a line from a common progenitor; depending on which progenitor you choose, the lineage may contain just one organism or all organisms currently existing on the planet. Mammals and reptiles therefore share a lineage rooted at their most recent common ancestors. Like “relatedness”, I suspect you’re using this term in a technical sense with which the non-biologists here, such as myself, are not familiar.
No, I am not labouring under that particular misapprehension.
Again, that analogy is not appropriate, because mammals and reptiles were derived from a common ancestor through an iterative process of reproduction with modification, whereas The Matrix and Journey to the West were not. (There was reproduction involved, and there was modification involved, but it wasn’t through a direct line of descent.) I’m not saying that your conclusion is incorrect; I’m just pointing out a flaw in the argument you’re using to justify it.
No, he’s not. He’s using the ordinary definitionof the term. You are insisting on using another definition used by scientists in your field. Seeing as that does not include the OP, everything you say about the word having to be defined the way scientists define it is completely irrelevant. It’s like saying that I didn’t do any work when I programmed something because I didn’t exert any force in the same direction as my movement.
I also agree with him. I can’t see how you can have the concept of ancestry unless you also have the concept of parent. For there to be an ancestor, there must be direct decent. There must be a species that is the metaphorical parent of any given species. The parent must also have parent, and so on. And, within that structure, the concept of relatedness takes two variables: the number of generations between the first species and the ancestor, and the number of generations between the second species and the ancestor. The first is constant as the first species is always human. The second cannot be constant unless it was specifically designed to be. And the smaller that second number, the more closely related it is to humans.
If scientists define the word related differently, then I say they are being disingenuous in using a common term. Don’t use a family tree metaphor if it doesn’t apply.
EDIT: You aren’t saying that neither of two second cousins is more closely related to you. You are saying that neither of two cousins that are once removed can be more closely related to you. And this doesn’t make sense, as my first cousin once removed is clearly more closely related to me than my second cousin once removed.
That’s a bit unfair. All fields have their own specialized vocabulary, and it’s unfair to insist that their users come up with completely novel word forms instead of co-opting existing ones. The point now is to find out who’s using which terms in which sense and then to agree on a common vocabulary so that the topic can be discussed without ambiguity or misinterpretation.
This is a science question. It makes sense to use scientific terms. Scientists are not being disingenuous, they are being accurate.
Eww. Moshe Katzav.
Hi Blake, glad to have you join the fray.
Let me restate my case here. I’ll preface it by saying I do understand what you and Colibri are saying, and there is validity to your position. I’m not attempting to deny that.
Let’s start with the assumption that all modern humans are equally related to the our common ancestor with reptiles. We may as well because we have no possible way of counting generations or accounting for incest along the way, and we’re only talking about one species on the mammalian side.
That leaves us with the reptilian side. The assertion made implies that all reptiles are equally related to that common ancestor. In other words, a bird, a dinosaur, a croc, a snake, and an iguana are equally related to the common ancestor.
I find that defeats the entire purpose of cladistics (though I have only very minimal basic knowledge of that field). There isn’t much point that I see in claiming that a bird and an iguana are equally related to that common ancestor. One of them is quite likely to have had a greater number of genetic changes from the common ancestor. We may not be able to count that number of changes precisely, but we can make reasonable guesses based on the available knowledge. The term ‘related’ is used that way quite often.
The distraction here seems to come from comparing across clades. But because the question doesn’t require measurement on the mammalian side, we are only doing comparisons within a single clade now, (maybe, if the reptilian side is significantly more complicated than that, let me know). But the framework of the question doesn’t require comparing humans to reptile across clades, only measuring the distance between modern reptiles to the common ancestor (or our best guess at what that is). I don’t think people in any field consider an ostrich to have the same relationship to the common ancestor as a dinosaur does. And I think it’s clear to most everybody the answer to the OP comes from the evaluation of how greatly changed some modern reptile is to the common ancestor. Had the question been formed as ‘what modern mammal and modern reptile are most closely related’ would have increased the complexity, but the same technique could be used by measuring the number of genetic changes from each side of that common ancestor. If you got to the point of asking which modern mammal is most closely related to the giant clam, then I can see your point. But that was not the question here.
I wouldn’t characterize it as disingenuous. This kind of thing often arise when the terms aren’t defined up front.
They are being no more accurate than anyone else. This is a question of semantics, not syntax.