He’s a Mike Gene clone their vague “hunch” has been so for close to a decade.
True, with non-essential genes, a lot depends on the environment. A vitamin C-deficient primate may be perfectly fine, because its diet contains sufficient vitamin C. Or it may find itself without access to its usual diet, due to climate change, forest fires, etc.
Same with a white-bodied Biston betularia. Sharing the hue of light-colored trees and lichens works great for camouflage. Until the Industrial Revolution arrives, leaving the trees covered in soot and your carbon-colored conspecifics at a selective advantage.
Not so with essential genes. Knock out bicoid in D. melanogaster, and you don’t need to take the environment into account to know you’re not going to end up with a viable organism when the larvae fails to develop a head.
That’s another of your subjective opinions. As I wrote, immediately after what you quoted:
Yet this is an incomplete picture. After all, there are no bacterial analogues to animals, plants, fungi or algaea. Multicellularity may be a succesful strategy, but not every type of organism is equally equipped to acces it.
Two points:
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In noting that conventional evolutionary biology failed to predict the deep homology of the genetic toolkit and in fact predicted the opposite, I am not stretching the conventional view beyond its theoretical limits, akin to demanding weather forecasts into the far future. In fact, the claim that the standard synthesis could predict the homology, or lack thereof, between distantly related species came from Ernst Mayr himself, one of its architects.
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I am responding to your own claim that the predictions flowing from front-loading are exactly the same as those flowing from conventional evolutionary biology. It’s quite jarring to see you move from “the conventional view makes the exact same predictions on this subject” to “how can you even demand predictions on this subject” so quickly.
Yes, that was the state of affairs in 2001: The genetic toolkit had been discovered to have “originated even before the divergence of these basal animals“, i.e. at the base of Metazoa. I was well aware of these findings and wrote about them at the time.
But my prediction went further: I predicted that the genetic toolkit would date back even further, all the way back to the first eukaryotes.
I’ll say the intervening decade has been very kind to front-loading, having shown the genetic toolkit and other multicellularity-related genes to be much older than expected by the conventional view, and that reductive evolution made us underestimate the complexity of the first lifeforms.
Still, my hunch hasn’t turned into a conviction, just a better-supported hunch. Extraordinary claims require extraordinary evidence, and I do not have sufficient evidence to convince myself (let alone others) of the extraordinary claim that the first life on Earth was the product of intelligent design.
That’s what your own post was arguing for, that mulicellularity would evolve on its own.
You are stretching the theory beyond what it is meant to do. Evolution doesn’t predict which of many possible evolutionary pathways a lineage will take. That’s not how it works. Evolution only describes the pathways that were taken as part of natural history.
Then let’s see how your predictions work. Since birds and bats both fly, then they should have evolved the same adaptations. They didn’t. Bats have a membrane stretched between fingers while birds have feathers coming off of a thin arm with no stretched out fingers. Front-loading has been falsified, right?
How do you determine when front-loading predicts deep homologies and when it doesn’t?
That’s wrong. It said they originated before the divergence of basal animals. That would be below the base of Metazoa. It was already predicted that these homologies predated animals.
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Never say you agree with me and follow it with a statement that contradicts what I said. It causes confusion. So you define “essential gene” as one that’s necessary in all possible environments. I consider that a bad definition. And we haven’t gone into the question of whether any of these homeodomain genes is essential in any unicellular eukaryotes. I would claim that if they weren’t essential they wouldn’t have been maintained over hundreds of millions of years.
Yes, given that it possessed the ancient genetic toolkit present in LECA.
Your argument is with Ernst Mayr, not with me. I’m perfectly willing to accept the neo-darwinian view on evolutionary biology in those areas in which it works, even though it failed to predict the deep homology of the shared genetic toolkit. I’m just pointing out that one doesn’t get to claim that the predictions of front-loading match those of neo-darwinism when in fact they don’t.
This is a good question. The predictions generated by front-loading depend on which features of life one expects to have been front-loaded. At the moment, I’m not comfortable extending my claims about front-loading further than the origin of multicellularity, but let’s play around with the idea of more specific features (like flight) having been front-loaded.
On the surface, the idea does have some merit. Powered flight has evolved multiple times, in at least birds, bats, pterosaurs, and insects. Of course, this could also be a case of flight being a good strategy that is best achieved with some sort of wing-like structure that natural selection can be expected to arrive at multiple times, regardles of the starting point. Let’s call this the “Many Roads to Rome” hypothesis and consider it a null hypothesis to be rejected for front-loading to be of any interest.
Now, one observation that could favor front-loading over our “Many Roads to Rome” null hypothesis would be that organisms engaged in powered flight in some way all used the same molecular material or structure. Maybe feathers, insect wings, and the skin in bat membranes all consisted of materials made up of the same protein, modified in different ways. Or maybe they all used some sort of navigational system which turned out to be based on the same molecular machinery. This would indicate that there was in fact only one “Road to Rome”, and that the evolution of flight depended crucially on the same “flight-prepared structure”, re-tooled in various ways.
Furthermore, we would expect this “flight-prepared structure” to be ancient, dating back to the first eukaryotes. If the structure turned out to have arisen de novo, say, right before the Protostomia/Deuterostomia split, front-loading wouldn’t be a viable option.
Now, none of this seems to be the case for the origin of powered flight. The various flying organisms seem to have taken multiple “Roads to Rome”, building their wings of whatever materials were available to them. You might even say that the front-loading of flight has been falsified. I certainly don’t consider it a viable conjecture.
But my prediction wasn’t just that they would predate animals, but that they would also predate the animal/fungi split and the plant/alagae split, all the way back to the Last Eukaryotic Common Ancestor (LECA).
A primate with a non-functional GLO gene can’t synthesize vitamin C can adapt by including fruit in its diet. A fruit fly larva with a non-functional bicoid gene can’t adapt to never developing a head.
You think this is pointless distinction, and there doesn’t seem to be anything I can say that will convince you of what I consider a rather trivial point. I’m afraid we’re just going to agree to disagree on this.
So how does that work? How do you determine which features will have deep homology and which won’t? It seems that you arbitrarily decide these things.
Predating animals includes the animal/fungi and the plant/algae split.
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Is this “ancient genetic tookit” anything more than a transcription factor with a homeodomain? You understand that transcription factors can bind to anything that evolves a binding site, and so can activate or repress the expression of any gene, as long as its promoter has that binding site. Right? The fact that a few members of one particular class of transcription factor were recruited for developmental regulation in some multicellular organisms is not really all that odd. If not them, then others.
It’s the other way around, you know. A primate with fruit in its diet can lose its functional GLO gene without suffering loss of fitness. Now, as for the homeodomain proteins in various protists, do you know what they do? Do you know they aren’t essential even by your definition? I don’t think so.
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I just wrote a rather long post about the considerations behind labelling a given feature as front-loaded. Why don’t you engage with that before asking me to repeat myself?
Not to be blunt, but no, it doesn’t. The animal/fungi split predates the last common ancestor of animals, and the last common ancestor of animals/fungi/plant/algae predates them both.
Yes. It’s this flexibility which makes it so well-suited for front-loading.
Yes, that’s what the ateleological perspective would lead us to expect. There are many roads to Rome, there’s nothing special about the homeobox transcription factors; if they hadn’t been recruited for developmental regulation, then others would have.
Of course, that leaves one with the surprising finding that every time multicellular organisms with differentiated cell types arose, life employed the same “nothing special” transcription factors, not any of those supposedly equally good alternatives.
Yes, that’s in fact the point I’ve been making for a while.
You understand that transcription factors are universal in life, right? This isn’t front-loading. It’s just control of transcription, which every organism needs.
Actually, there are many different classes of transcription factors used in various ways in different taxa. While plants have homeodomain proteins, if I recall their most prominent family of developmental transcription factors is MADS box proteins. If there were truly this universal front-loading you claim, every taxon would have Hox clusters.
That’s not a point. I was asking a question. Do you know what homeodomain proteins do in various protists? Do you actually know whether they’re essential?
I did read it, and I couldn’t make heads or tails of it. Nowhere is there criteria for determining if something is front-loaded or not.
You just said it yourself. The last common ancestor of animals/fungi/plant/algae predates animals, and the paper I cited from 2001 said that the ancestral genes predated animals.
This thread seems to be experiencing diminishing returns, with the same points being brought up again. I’ll let this be my last post in the thread and let you all have the last words.
If one zooms out far enough, then yes, all life employs transcription regulation, from, say, the bacterial lac operon, to the eukaryotic homeodomain toolkit. But in grouping transient regulation of metabolic pathways with laying down permanent epigenetic pathways under the broad heading of “regulation”, one is conflating very different phenomenon.
True. But MADS-box genes, like homeobox genes, predate the animal/plant split:
Changes in genes encoding transcriptional regulators can alter development and are important components of the molecular mechanisms of morphological evolution. MADS-box genes encode transcriptional regulators of diverse and important biological functions. In plants, MADS-box genes regulate flower, fruit, leaf, and root development. Recent sequencing efforts in Arabidopsis have allowed a nearly complete sampling of the MADS-box gene family from a single plant, something that was lacking in previous phylogenetic studies. To test the long-suspected parallel between the evolution of the MADS-box gene family and the evolution of plant form, a polarized gene phylogeny is necessary. Here we suggest that a gene duplication ancestral to the divergence of plants and animals gave rise to two main lineages of MADS-box genes: TypeI and TypeII. We locate the root of the eukaryotic MADS-box gene family between these two lineages. A novel monophyletic group of plant MADS domains (AGL34 like) seems to be more closely related to previously identified animal SRF-like MADS domains to form TypeI lineage. Most other plant sequences form a clear monophyletic group with animal MEF2-like domains to form TypeII lineage. Only plant TypeII members have a K domain that is downstream of the MADS domain in most plant members previously identified. This suggests that the K domain evolved after the duplication that gave rise to the two lineages. Finally, a group of intermediate plant sequences could be the result of recombination events. These analyses may guide the search for MADS-box sequences in basal eukaryotes and the phylogenetic placement of new genes from other plant species.
Alvarez-Buylla et al., 2000, “An ancestral MADS-box gene duplication occurred before the divergence of plants and animal”
We’ve been over this before. Derelle et al. propose that they’re involved in “higher order processes, e.g., communication between individual cells, or cell modifications along the life cycle”.
These are the key points from my post:
Have you read the review you’re citing? The authors only discuss evidence of homeobox genes in metazoan groups; arthropods, vertebrates, cnidaria, porifera, ctenophores, etc. There is nothing in the article to suggest that those genes would also be found in plants.
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Then I can only conclude that front loading has been falsified because multiple lineages of animals evolved flight through different mechanisms.
The article simply states “before the divergence of basal animals”. That includes all of the other eukaryotic lineages.
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Yes, even humans have a few MADS-box genes. The point is that you were claiming the same “genetic tool kit” was used in the evolution of multicellularity and control of developmen in all groups. But it isn’t true. Plants and animals have genes from different families of transcription factors that control development. The deep homology you claim just isn’t there.
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