Not all deleterious mutations are lethal. Most deleterious mutations are only very slightly deleterious, to the extent where wouldn’t really notice much if you had one. We all have somewhere on othe order of 100 new mutations in our genome that our parents didn’t have, and I think about 2 of those are deleterious, on average. We don’t really notice though - they’re lost in the “noise” that is human diversity. This means they “behave” just like neutral mutations - some will randomly be lost from the population, some will become fixed in the population.
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The story is slightly more complex than given in the article. It seems that human beings can survive on far less vitamin C than other mammals. This is because of a better/more efficient way of distributing vitamin C in the body.
So basically, this system has to evolve before human beings “lost” the ability to make Vitamin C.
Edit: or vice versa
Other mammals that don’t produce vitamin C such as guinea pigs, and bats also have a similar mechanism as per the paper… Shared ancestry perhaps???
Seems like a pretty simple mechanism that is very easy to explain as convergence.
- Produce vitamin C => coexpress stomatin and Glut4, not Glut1 in erythrocytes.
- Don’t produce vitamin C => coexpress stomain and Glut1, not Glut4 in erythrocytes.
I’d be interested in seeing exactly how this regulatory change was acheived in each species though. A new Glut1 enhancer? A deletion of a a Glut4 enhancer? Specific motif changes within existing enhancers?
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This article by the same lead author suggests that Glut1 is expressed in mouse erthyrocytes during the neonatal period, and that they’re only “lost” from the red blood cells in the weeks after birth. Humans, in contrast, basically keep this expression instead of winding it down.
http://www.bloodjournal.org/content/112/12/4729?sso-checked=true
Apparently not much is known about Glut4, but these papers are 10 years old now so I’m sure there’s much more out there now.
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What if there is more specific changes required?
I wouldnt be surprised if there are more specific changes required to increase the efficiency with which vitamin C is used/ as well as reducing the overall requirement for the vitamin compared to mammals that produce it.
Edit: There seem to be differences between human SVCT1 and 2 vis a vis other mammals which also contribute to better absorption of vitamin C.There are complex mechansism involved in absorbng Vit C in the central nervous system, lungs etc also.
The problem with popular science articles like the one we read is that - they present an evolutionary story while missing out on all the facts + make arguments from ignorance.
Why not common ancestry or HGT? why assume convergence? is it a matter of convenience?
HGT is very unlikely to be responsible for specific specific regulatory pathways between several independent lineages of mammals. There’s apparently no other signatures of HGT that would lead anyone to that conclusion.
“Common ancestry” in the way you’re suggesting would necessitate that fruit bats, guinea pigs, and the Haplorrhini are more closely related to each other than to, say, other bats, other rodents, and other primates, respectively. Do I really have to articulate why that’s extremely unlikely to be the explanation?
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i have to admit i was pulling your leg there. However, there is no reason it could not be explained by common ancestry other than the fact that such an explanation would throw the mammalian tree of life for a toss… You are a making a value decision here on interpreting data…
As to convergence. It is a conundrum… Usually, convergence indicates there is selection pressure for a trait… So before you write it off as convergence, you will have to justify it with evidence of some kind.
Unless convergence is a magic wand…
Now lets recap the just so story in the article –
Humans ate a lot of fruit with vitamin C. Human lost the ability to synthesize vitamin C… But hey, who cares… we have oranges…
Really scientific.
Edit: I forgot to mention, we still dont have a complete list of what all needed to “converge” before getting rid of the ability to synthesize vitamin C.
Is there anything wrong with that? Yes, I’m noting that we have a truckload of evidence on one side of the scales saying “X is the topology of the mammal tree”, then along comes a speck that whispers “Y is the topology of the mammal tree”. The decision is obvious.
Convergence isn’t a “conundrum” at all, and in this case there is an obvious selection pressure - in your own words the trait provides a better way of utilising the vitamin C.
No… the ancestor of Haplorrhini ate a lot of fruit with vitamin C, lost the ability to synthesise their own vitamin C, and this loss was tolerated.
What isn’t scientific about that? What part of the story do you think is implausible, other than your denial(?) of primate common ancestry?
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Traits… not trait… or more like a full blown system of handling vitamin C.
If the ability to synthesis vitamin C remained, we dont know if there is an advantage to fitness.The fact that the vast majority of mammals make do without this trait over hundreds of millios of years in the same environment is not very conducive for selection.
I would put how this trait got selected for as an unknown.If you have some actual work on the subject, we can discuss it.
The part where they needed far more vitamin C than supplied by fruits… This is where the need for a new system to handle vitamin C more efficiently comes in. This is the key enabler to do without synthesizing vitamin C as per science… Somehow the story misses it.
The story is a just so story that ignores reality. Read it and compare the impression you go away with vis a vis the truth.
I’m only talking about the Glut1 side of things, since that’s one example that you’ve fleshed out here. If you want to bring up other systems then we’d have to go through them in at least some amount of detail to see if they’re relevant to this question of convergence or not. So far we’ve established that the Glut1 trait is a contender for convergence, not that the entire systems of “handling” vitamin C are.
Why would it have had to be selected prior to the loss of vitamin C synthesis? It’s seems perfectly plausible that GULO was broken first, which was fine since the species got abundant vitamin C from their diet. When this dietary vitamin C waned in later species, there was selection for increased efficiency of its use. We don’t even know if this increased efficiency due to Glut1 is common to all Haplorrhini, for example, just humans. We can’t assume that just because humans have this trait all Haplorrhines do.
I agree though, we don’t know all the details of how this trait might have been selected for. That doesn’t mean we can’t be confident it’s an example of convergence.
The ancestral Happlorine primate didn’t get enough vitamin C from it’s diet? Citation please. Do see my point? The article points out that we humans need our more efficient system of vitamin C usage to survive, how about other Haplorrhines, or for that matter, Strepsirrhines? What are their vitamin C requirements? How much do they get from their diet?
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I atleast have established something… what you need to establish is that there are no other candidates for convergence similar to GLUT 1. And the mechanism by which said convergence happens. Till you do so, all you have is a hypothesis unsupported by evidence.
How do you know this? How much Vitamin C would be required in such a scenario and what kind of diet would suffice. Did the purported ancestor have access to such a diet.
How do you justify making such claims without even a rudimentary analysis if it is possible?
Of course we cant. I am not asking you to.
It only means your confidence is not based on evidence.
This is why i called the entire story a just so story and not science… i could come up with such stories without a degree in science or a lab. So why call it science. Where is the studies, comparison to current literature and empirical data to support the claims made?
If you ask for citations for my assumptions, how is it that you are making them without any evidence whatsoever?My version is as plausible as yours, because there is no evidence.
Can i say we have common ground here?
Why would I need to establish that there are no other candidates? I’m not claiming they don’t exist, I’m just saying that we need to establish that they do exist before claiming there is a whole complicated system that has convergently evolved (or not).
As I’ve said several times, noting that there is overwhelming evidence for a particular mammal tree topology is good evidence for traits that are similar between “distantly related” species on that tree being the result of convergence. This seems to be the core of your objection throughout this thread and in the remainder of your comment. You’re saying that because we don’t know all the selection pressures involved, it’s a “just so story” to say that several lineages lost the ability to synthesise vitamin C and gained a greater efficiency in their usage of vitamin C instead. To make a long story short, the facts (we can’t produce our own vitamin C, we use it more efficiently, and etc in other species) are interpreted in the light of the established phylogeny of mammals. Based on that, we can say “convergence” with great confidence before knowing the first thing about the mechanism that led to it.
I don’t claim to know it, I said it was perfectly plausible, not factual.
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Ya… you are confident without specific evidence. This is what i said. Irrespective of how great the claim of convergence, if it doesn’t fit the tree, you will claim it is due to convergence.
In such cases , the word convergence might as well mean “not due to common ancestry”.
And not supported by evidence. I call a “plausible” story without much details and no evidence to support it… a just so story…
You said “not based on evidence”, not “not based on specific evidence”. If I see a CCTV image of my friend in a shop in the next city, I can say “my friend went to the next city” without having to know what method of transport he used.
Unless the claim of convergence data is largerthen, or at least close to, the dataset for the given tree topology of the groups in question, pretty much. That’s how evidence works - you weigh it to come to conclusions.
That’s why I didn’t claim my idea as factual, because it lacks rigerous support. Remember what you originally called a “just so story” though. You “recapped” these sentences from @NLENTS article, and sarcastically said “really scientific”:
Our ancestors stopped making vitamin C because they suffered a mutation in their GULO gene. This was tolerated because they already had vitamin C in their diet.
Both of these sentences are in fact very scientific, with lots of data supporting them. The only place for some nuance might be in the second sentence which could be amended to say something like:
“This was tolerated because they already had vitamin C in their diet and possibily because some other traits had already evolved to increase that ancestor’s vitamin C use efficiency.”
Considering Nathan was writing for a lay audience and probably wanted to avoid the endless footnotes and appeals to potential mechanisms that might be discovered in the future, what he wrote is completely appropriate.
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We should all thank Nathan for his participation and his insights. I learned a lot and hope everyone did also. Thanks Nathan.
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Why is that all discussions like this end up in analogies. We are discussing something specific.
Your analogy is not valid.
You missed the point. Saying something is convergent doesn’t explain anything other than that you don’t believe it is a feature shared by a common ancestor… and vice versa. It’s a circular explanation which explains nothing. It just an inference from natural causes.
It was appropriate to make his point… not to showcase the facts. We don’t know that the lack of viatmin C could be supplemented by diet without other changes. This is crucial to his point.
The other traits makes the story complicated and works against his argument. It makes the entire process being “accidental” more implausible. There are metaphysical tones to his argument which are supported by his inaccurate presentation.
Communicating to lay people is not an excuse for being inaccurate.
Perhaps Nathan should consider adding a clarification to his post mentioning the actual science…
Vitamin C is just one of his examples I checked as a test case.
Thank you for your kind words. You make a very good point about pop science writing. It cannot possibly cover every caveat, propose every possible mechanism, and cite every source, or else it is not pop science anymore. I’ve written several review articles and book chapters for the scientific audience and this is a totally different kind of writing. My pop science articles are meant to whet the appetite of the scientifically curious. If you want citations and primary data, Google Scholar is there for you.
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Because I’ve tried explaining several times that overwhelming “general evidence” is enough, we don’t need “specific evidence”, to use your terms. I overwhelming general evidence that my friend lives in the same city as me. Now I encounter “general evidence” that he was in a city 50 miles away yesterday. This means that I can say with great confidence “my friend travelled to the other city” - how else would he come to be there considering he doesn’t live there? I don’t need to investigate and see if he rented a car, took a bus or train, which bus or train he might have taken, in order to say “my friend travelled to the other city”. The analogy is valid. If you disagree, explain why.
… And? How is that a rebuttal to anything anyone has said? Of course it’s an inference, and it’s not intended to explain the specific origin of the trait. “Convergent” is a descriptor, not an explanation.
What exactly was “his point”, do you think?
It’s an excuse to simplify. When we tell kids “the mitochondria is the powerhouse of the cell”, do we need to immediately caveat that “well, it produces most of the ATP in your cells, not all of it”? There are obviously many examples of this kind of thing. Popular-level books usually don’t contain endless caveats, they present the major points in broad strokes. There would be little distinction between popular science books and review articles if they did.
@evograd - You are doing such a great job, but it seems your efforts are wasted arguing with someone who has no intention of considering your points honestly. He doesn’t get it because he doesn’t want to. He’s already decided what he believe in this regard. Between the straw man arguments, circular reasoning, deliberate misreading of your reasoning, exaggerations, and false dichotomies, I lost count of how many logical fallacies have been thrown your way. That’s a sign that someone is not arguing in good faith or has no sound reasoning on which to rely (usually both). Your time is better spent doing… just about anything else. No one is so blind as he who refuses to open his eyes.