Evolution (or lack thereof) of microtubule polarity from an ancestor lacking polarity

One could accept common descent, but the reason I’m a creationist is that it seems to require miracles to make it possible.

Microtubules are critical to many living organisms. Yet these microtubules have polarity (as in like a one way highway). The polarity is achieved as described here:

Microtubules are built from subunits—molecules of tubulin—each of which is a dimer composed of two very similar globular proteins called α-tubulin and β-tubulin, bound tightly together by noncovalent interactions. The tubulin dimers stack together, again by noncovalent bonding, to form the wall of the hollow, cylindrical microtubule. This tubelike structure is made of 13 parallel protofilaments, each a linear chain of tubulin dimers with α- and β-tubulin alternating along its length (Figure 17–12). Each protofilament has a structural polarity, with α-tubulin exposed at one end and β-tubulin at the other, and this polarity is the same for all the protofilaments in the microfilament. Thus the microtubule as a whole has a structural polarity: the end with β-tubulin showing is called its plus end, and the opposite end, which contains exposed α-tubulin, is called the minus end.

Alberts, Bruce. Essential Cell Biology (Fifth Edition) (p. 581). W. W. Norton & Company. Kindle Edition.

So, is gene duplication a good explanation for the origin of the alpha and beta paralogs of tublin (the material in making microtubules)?

Well, if the paralog pair is missing, then how is polarity achieved? Seems the paralogs are created, no matter what phylognetic story is told. If we don’t assume creation, there is the mechanistic problem of having some sort of alternate for microtubules with structural polarity.

Such examples make it hard to believe common descent could proceed without miracles. And if one is willing to accept miracles, then how is this different than special creation?

Science can’t explain to Sal’s satisfaction =/= miracle required.

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Even if we were to accept that miracles are needed to explain some data, the data fit miracles in a common descent context and don’t fit miracles in a separate creation context. So the difference is that miracles, with common descent, actually explain what we see while special creation doesn’t. This seems like a significant difference to me.


Have you read this paper Sal? Seems like a good place to start:

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Isn’t this the fallacy if the excluded middle? Why not affirm miracle guided common descent?

Well, for one, it affirms common descent.

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That’s sort of what Michael Behe does. I find that more palatable than mutation plus selection from a mechanistic standpoint regarding this specific example of polarity implemented by paralogs (although I think actin polarity is achieved in a different fashion).

Thanks for your comment.

Well, just off the top of my head, you could have a homodimer that isn’t perfectly symmetrical. Then polymers of that homodimer would have a polarity. I’m not saying that’s what happened, because I have no idea, but the premise of your post seems to be a classic example of question-begging.

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Well, just off the top of my head, you could have a homodimer that isn’t perfectly symmetrical.

Agreed that something could be implemented with a homo-meric architecture.

What caught my eye though is how a paralog implementation can happen. If it started out homo-meric, how did it become hetero-meric? I think the dyenins and kenisins marching to and fro have to sense the polar structure somehow, and going from a homomeric structure to a hetero-meric structure would require co-evolution of the users (dyenin and kinesin) of the microtubule highway.

So I want to make sure I understand your chain of reasoning here. You’d normally be inclined to accept the evidence for common descent, if it weren’t for what you take to be the requirement that miracles occurred to make it happen. Hey, if that was true I’d be with you up to this point.

But then the chain breaks off. Because since you don’t like theories with miracles in them, you discard common descent and opt instead for a theory of nothing but miracles. Instead of species evolving gradually, and occasionally a miracle being performed, you just go straight to miracle upon miracle. One divine intervention piled on top of another.

Tubulin? POOF - out of thin air!
And this is more plausible to you, because that’s … less of a miracle than common descent?


Maybe like FtsZ?

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LOL, I had forgotten about that story when I typed my post, I swear!

I really encourage people to read this paper, it’s extremely comprehensive. Here’s the abstract:

Tubulin, the protein subunit of microtubules (MTs), is an α/β heterodimer. In this chapter, a hypothesis on the evolution of the tubulin molecule is proposed, based in part on recent reports on the structures and functions of different forms of tubulin and its relatives. The concentration is on three main areas. 1) Evolution of the vertebrate β-tubulin isotypes. In addition to providing a clear idea about the relationships among these isotypes, recent data suggest that tubulin may have functions that do not involve being in a MT, namely, that it can function as an isolated α/β dimer or as a non-MT polymer. 2) Examination of the entire tubulin superfamily, which includes not only tubulins α, β, γ, δ, ε, η, and others but also a variety of prokaryotic proteins. The hypothesis is presented that the common ancestor of all these proteins formed a filamentous curving polymer that used the energy of GTP hydrolysis to apply force to nucleic acids and/or membranes and that this common ancestor may have been coeval with the first cells. A variety of chaperones, motors and MT-associated proteins may have coevolved with tubulin and their histories illuminate that of tubulin. The branched, highly negatively charged C-terminal domain present on α- and β-tubulin appears to be a relatively recent addition to tubulin. 3) The hypothesis is presented that the C-terminal domain may have been of prebiotic origin and that it gradually developed into a protein serving particular metabolic functions whose gene eventually became fused with those of α- and β-tubulin. Finally, some experiments are proposed that could illuminate the probability of these hypotheses.

Or you could have a monomer that does so, like actin.

Could be? There are. Actin is the most obvious one.

Quite easily. You’re looking at biology only deep enough to find something to use as ammo in your culture war. If you tried looking at biology in depth, that wouldn’t happen.

No, it wouldn’t.

Tell me, Sal, if your point has any merit whatsoever, how does myosin “sense the polarity” of a homopolymer?

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Tell me, Sal, if your point has any merit whatsoever, how does myosin “sense the polarity” of a homopolymer?

Well, I was hoping you’d tell me since you’re the myosin expert, but I’m not talking about actin filaments, I’m talking microtubules.

And the real point is the paralog architecture, which some say isn’t fundamentally necessary for polarity – but that’s not the issue. The issue is NOT whether polarity can be achieved with a homomeric architecture, the issue is why a heteromeric architecture would evolve using paralogs.

In fact, there are at least 3 paralogs of tubulin: alpha, beta, gamma. The gamma paralog is pretty important:

Tubulin - Wikipedia

γ-Tubulin, another member of the tubulin family, is important in the nucleation and polar orientation of microtubules. It is found primarily in centrosomes and spindle pole bodies, since these are the areas of most abundant microtubule nucleation.

So without the 3-paralogs simultaneously there, microtubules as we know them won’t exist or work.

And here is a relevant diagram showing the importance of the paralogs and the Microtubule binding domain of dynein that attaches to the alpha and the beta paralog.


No sense having am MTBD (microtubule binding domain) in dynein that matches the alpha and beta tubulin paralogs nor the gamma tubulin as nucleation sites and involvement in polar orientation. So the point is the paralogs have to be there from the start, it makes little sense from a mechanical standpoint for them to naturally evolve tubulin paralogs from a common ancestor. Phylogenetic methods fail again to give real mechanistic explanations and only provide non-sequitur assertions totally devoid of considerations of physics, mechanics and chemistry.

The issue isn’t that it could be implemented simply, but that it was implemented extravagantly when a simpler solution was available. Like the Peacock’s tail bother Darwin, the issue isn’t pure survival value, but the extravagance that is not needed but that exists.

So the necessary parts for microtubule function:

alpha tubulin paralog
beta tubulin paralog
gamma tubulin paralog
dynein with a MTBD that ties to both the alpha and beta tubulin paralogs.

I haven’t even covered Kinesin which is itself interesting in that it appears it only walks on the beta paralogs:

Sure we could have, I suppose hypothetically an all-beta type microtubule for Kinesin, but then how do alpha paralogs evolve to insert themselves in between this supposed pure beta tubulin microtubule. Evolutionary scenarios don’t make mechanistic sense, but must make appeals to unknown and unknowable and improbable mechanisms. How is this fundamentally different from creationism?

You still haven’t looked at the paper I posted, have you?


I walk into a room and find a puddle of water in the middle of the room. I can’t immediately explain how it got there. Should I then conclude a supernatural deity created the puddle of water?

I really don’t see the logic or reason of continually using God of the Gaps arguments.

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But then how do… you avoid arguments from ignorance?

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I did tell you. You were asking whether a homopolymer could have polarity. I’m telling you that it can and does.

Why don’t you try going into a bit more depth before pontificating? How many do you think there are?

There aren’t only 3 paralogs.

Have you considered stating your hypotheses as hypotheses, instead of falsely stating them as facts? You’d learn a whole lot more that way.

I don’t see how the diagram is relevant to anything you brought up.

The diagram doesn’t show that.

It makes plenty of sense, but you limit yourself to copying/pasting irrelevant figures.

You’ve misrepresented two hypotheses as facts. Why not try some modesty for a change and test them BEFORE pontificating?

Sal is attracted to bright colors.


? How many do you think there are?

Well I provided a link that showed there are more than 3, but I listed 3 of immediate interest.