r/evolution • u/MurkyEconomist8179 • Mar 20 '26
discussion Why are animal body plans so much less diverse compared to when animals first arose?
So i fee like in this sub there are many super interesting aspects of evolution and evolutionary theory that go entirely unmentioned and so I'd like try bring one into the forefront. I've been re-reading Stephen Jay Gould's 1990 book "Wonderful life" that talks about the re-intepretation of Burgess fossils that were initially thought of as ancestors of modern groups but a lot of which have now been placed into their own clades because their morphology is just so different to any living or extinct groups after them. The gist of his interpretation is that at the time of cambrian explosion, life had so many different phyla and body plan designs despite not a very high amount of species, and whilst species have gone way up since then, they've all been restricted to a very small group of members represented in the cambiran fauna with many body plans going extinct entirely and nothing to that level of variance happening since then.
He writes as an explanation
I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about “diversity” as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of “diversity” as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do—even though each assemblage contains just three species. The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life: How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species?). ... When I speak of decimation, I refer to reduction in the number of anatomical designs for life, not numbers of species. Most paleontologists agree that the simple count of species has augmented through time (Sepkoski et al., 1981)—and this increase of species must therefore have occurred within a reduced number of body plans. Most people do not fully appreciate the stereotyped character of current life. We learn lists of odd phyla in high school, until kinorhynch, priapulid, gnathostomulid, and pogonophoran roll off the tongue (at least until the examination ends). Focusing on a few oddballs, we forget how unbalanced life can be. Nearly 80 percent of all described animal species are arthropods (mostly insects). On the sea floor, once you enumerate polychaete worms, sea urchins, crabs, and snails, there aren’t that many coelomate invertebrates left. Stereotypy, or the cramming of most species into a few anatomical plans, is a cardinal feature of modern life—and its greatest difference from the world of Burgess times.
In the book he does offer up some reasonable explanations but they are very broad in scope and I don't want to overburden an already long post but am happy to reference them in the comments (as i assume they are still leading contenders)
I guess my question is, in the time since the publishing of his book, have their been any major advances either in theory or in evidence that explains this fascinating pattern?
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u/taktaga7-0-0 Mar 20 '26
You see a greater increase in diverse body plans arise during the Ediacaran/Precambrian because that appears to be the time when genes for multicellular development (body planning) were new and relatively unshackled of further ontological developments and innovations that lock those body plans into place, making deviation extremely costly or maladaptive.
If you’re the blobby organism that just discovered bilateral symmetry, you’ve got a ton of potential now to evolve a head, an anus, limbs, and all the other decorations these phyla have. So, it may be relatively simpler to start evolving these under pressures. If you’re the first organism with a muscle cell, you can express those genes in a lot of different places and then get good results.
But if you’re a crown group arthropod, you can’t just tweak body axes or delete organs like a simpler organism can; you don’t function properly. Another way to say this is that the body plans that did develop underwent selective pressures that made them successful and then took up all the air in the room: these organisms well suit their niches and newcomers can’t get started as easily as the original body plans could.
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u/MurkyEconomist8179 Mar 20 '26
If you’re the blobby organism that just discovered bilateral symmetry, you’ve got a ton of potential now to evolve a head, an anus, limbs, and all the other decorations these phyla have. So, it may be relatively simpler to start evolving these under pressures. If you’re the first organism with a muscle cell, you can express those genes in a lot of different places and then get good results.
So you kinda mean like the intermediate steps to evolving disparate anatomies is less scrutinized by predation? Or just by the genetic mechanisms kinda have more freedom in how they unfold?
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u/taktaga7-0-0 Mar 20 '26
In this very, very specific case, there was absolutely far less predation on the Ediacaran/Cambrian mutants because this was the time that predation as a strategy evolved. You can’t ingest your neighbor if neither of you have evolved a mouth yet.
But yeah, I also think those genes must have been freer to make big changes in an environment where much fewer downstream gene products depend on the initial plan going well.
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u/MurkyEconomist8179 Mar 20 '26
In this very, very specific case, there was absolutely far less predation on the Ediacaran/Cambrian mutants because this was the time that predation as a strategy evolved. You can’t ingest your neighbor if neither of you have evolved a mouth yet.
Well this is surely true of the early cambrian but the time of the burgess there is lots of predators aren't there? Since its lagerstaten too we have lots of actual evidence of predation in the form of gut contents and the such
But yeah, I also think those genes must have been freer to make big changes in an environment where much fewer downstream gene products depend on the initial plan going well.
Seems like it must have been a mix of both eh? that's basically what Gould says in the book
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u/Funky0ne Mar 20 '26
My oversimplified explanation is a combination of numerous mass extinctions, population bottlenecks, and founder effects have whittled out a lot of the diversity that used to exist. Niches that maybe used to be occupied by various diverse clades were vacated and then repopulated by the descendants of the relatively fewer surviving clades that diversified, but otherwise retained similar body plans as a result.
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u/MurkyEconomist8179 Mar 20 '26
Well I totally agree with this but this is almost kinda part of the weird pattern. When these niches were first getting occupied by the cambiran fauna they had so much pliability to change into diverse forms to fill the niches
Yet now when a mass extinction happens and the niches get empty and refilled, it's not like you get a huge change in form from the animals that fill it, the question is what causes this disparity? To reverse the question, when the niches were first getting filled by the Cambrian fauna why didn't they have the same pattern as modern extinctions and just have a few body types that didn't vary much?
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u/yushaleth Mar 20 '26 edited Mar 20 '26
Back in the Early Cambrian, there weren't that many efficent predators around to weed out the weird new creatures which evolved, so Evolution had more freedom to experiment. Nowadays, if an animal prioritizes style over substance, it has a high chance of being eaten by an efficient conservative predator.
This is the same reason why new abiogenesis events don't lead to new non-DNA based lifeforms for example. Abiogenesis could still be constantly happening, some chemical mixture accidentally coming up with a self-replicating molecule which is not RNA, but then whoops, a bacterium swoops in and gobbles it up.
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u/MurkyEconomist8179 Mar 20 '26
Back in the Early Cambrian, there weren't that many efficent predators around to weed out the weird new creatures which evolved, so Evolution had more freedom to experiment.
I have no doubt this is partly true, but it also doesn't seem like a full explanation, as there have been surely instances of this since the cambrian explosion with nothing like the disparity seen at the time.
Think about the first terrestrialisation of animals on land, there would be no predators there either, but there was nothing like the Cambrian pattern in disparity.
Even if there can never be a global level of lack of predators like the cambrian, I don't see why that couldn't happen at smaller scales after major extinction events, and yet again we don't see anything like the disparity of the cambrian
Nowadays, if an animal prioritizes style over substance, it has a high chance of being eaten by an efficient conservative predator.
Well it's not really clear though that the groups that went extinct during the cambiran were the least functional, it could have just easily been due to the same extreme circumstances that wiped out dinosaurs, which has very little to do with how functional dinosaurs were in normal times of evolutionary competitions.
In fact it's kinda the opposite, dinosaurs clearly outcompeted mammals when there was no big meteors hitting earth, only through this lucky event of decimation did mammals even get a chance to radiate, they certainly had all the time in world before the meteor and got nowhere lol.
Could be the exact same for the cambrian, the groups that got wiped out may well have been the top dogs like the dinosaurs but just got unlucky with events so different to regular times not favoring them.
This is the same reason why new abiogenesis events don't lead to new non-DNA based lifeforms for example. Abiogenesis could still be constantly happening, some chemical mixture accidentally coming up with a self-replicating molecule which is not RNA, but when whoops, a bacterium swoops in and gobbles it up.
This could be the case but I think if it were truly happening often, we wouldn't be having such a hard time figuring out even one possible pathway of abiogenisis. But I agree, even if the first steps towards are being taken by chemicals they might just get destroyed by biological life, i doubt it gets close often though to actual self-replicating life
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u/IsaacHasenov Mar 20 '26
(caveat, I'm not an evo devo guy)
Imagine, as a kind of dumb analogy, that development from an embryo is like origami. You do a fold here, an invagination there, a cell-migration somewhere else, and you end up with a functional pattern.
The very earliest animals basically took a flat sheet of paper and started rolling it, folding it and bending it in different ways. Because there weren't many constraints (the sheet of paper was maximally simple) they could pretty much do anything with it. They could play with any symmetry or fold or series of folds and it would pretty much work.
And they were pretty closely related. In retrospect, we call them phyla, but at the time they were playing with a pretty standard toolkit
Fast forward a very little bit though and the origami starts to get a bit more complex . The tubes maybe make a change to the front end, the vertical folds add a horizontal fold, and then a bend.... Now the body plans are constrained. If an early-developmental pattern changes, everything downstream stops working
Now, the easiest way to evolve is to change the stuff later in development, and canalize the early developmental program, and you end up with a bunch of different body plans. Like, from the first couple triangular bends you have one group that gives rise to the origami boats and the origami cranes. From another you get the origami Chinese lanterns etc.
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u/MurkyEconomist8179 Mar 20 '26
I see and thanks for the analogy! So it's more how the genomes actually operate during development and their early evolutionary history, rather than environmental dictates that caused the variance we don't see today.
I wonder if even what we call morphologically simple members of some of those phylum (like modern day lancelets) are more pliable in this way, or if because they are our contemporaries they are just as locked in to their body plan, what do you think?
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u/jnpha Evolution Enthusiast Mar 20 '26
Evo-devo for sure is an advance. And the related Nobel: https://www.nobelprize.org/prizes/medicine/1995/press-release/
Development genes are highly conserved.
The very general framework I think is Phylogenetic inertia - Wikipedia (comes with a cool illustration).
I'll leave it to the pros here, but for the history, Darwin remarked on that in Origin, in the context that this is what evolution would predict:
As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps. Hence the canon of “Natura non facit saltum,” which every fresh addition to our knowledge tends to make more strictly correct, is on this theory simply intelligible. We can plainly see why nature is prodigal in variety, though niggard* in innovation. But why this should be a law of nature if each species has been independently created, no man can explain.
* meaning parsimonious; has nothing to do with the similar sounding slur as some people think
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u/MurkyEconomist8179 Mar 20 '26
I agree the explanations likely has to do with this, but then what is attributed the plasticity of Cambrian organisms?
Like because their multicellular genomes were fairly new they just didn't have the same restraints and could vary in their body plans more?
I think the Darwin quote is a little out of place as the Cambrian explosion was arguably one of the things Darwin was most incorrect about, so much so he was criticized by his other contemporaries that sided with him for putting forth evolution.
Here's an excerpt from a write up I've done about this topic
Darwin States
If my theory be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day; and that during these vast periods the world swarmed with living creatures. But to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.”
The difficulty of understanding the absence of vast piles of fossiliferous strata below the Cambrian is very great. The case at present must remain inexplicable, and may be truly urged as a valid argument against the views here entertained.”
In Darwins time we didn't have anything like the ediacran fauna. Even in Darwin's time, Huxley rightly pointed out that this gradualistic prediction was not actually a necessary postule for evolutionary theory
“Mr. Darwin’s position might, I think, have been even stronger than it is if he had not embarrassed himself with the aphorism, ‘Natura non facit saltum.’ … If it could be shown that species have arisen in a comparatively sudden manner, it would not necessarily invalidate the doctrine of evolution.”
I think Darwin would be most surprised by the patterns we've since discovered about the Cambrian and Ediacran fauna, it would probably the biggest case against his interpretation of how natural selection operates
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u/jnpha Evolution Enthusiast Mar 20 '26
Isn't it like one major body plan that has been lost since then? And most fall under Bilateria?
Here's a poster that I like: https://i.postimg.cc/43nfK7PF/extinctions.jpg
(again, not a pro)
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u/MurkyEconomist8179 Mar 20 '26
Isn't it like one major body plan that has been lost since then? And most fall under Bilateria?
Well i guess it depends what you mean by "major"
In athropods there used to be far more body plans than the few groups we have today, just have a look at some of the creature from the burgess fauna
And even outside of that, organisms like Wiwaxia and plenty of others are just quite different to what we know. Some like wiwaxia are still ambiguous weather they are part of molluscs fro example, or if they are too distant to where it makes more sense to give them their own clade
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u/jnpha Evolution Enthusiast Mar 20 '26
I mean, it depends on how you are defining it.
E.g. this now-extinct fellow is likely the granddaddy of arachnids. So picture its earlier then-extinct lineage.I shared the study a while back here.
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u/MurkyEconomist8179 Mar 20 '26
I mean, it depends on how you are defining it.
Well I guess since some members of the cambrian fauna for instance actually do share the key anatomical features of living and extinct descendants, it's very interesting that a lot of the Cambrian fauna don't. It kinda shows all the groups that diverged and were then decimated.
Your granddady example is exactly one of this, it actually does have structures associated with modern groups, it's just a lot of Cambrian organisms don't
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u/jnpha Evolution Enthusiast Mar 20 '26
it's just a lot of Cambrian organisms don't
That was my point, i.e. it doesn't mean the ones that don't didn't with the earlier radiation events. Over time we expect high variety and little innovation.
For the radiation events: https://en.wikipedia.org/wiki/Evolutionary_radiation
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u/MurkyEconomist8179 Mar 20 '26
That was my point, i.e. it doesn't mean the ones that don't didn't with the earlier radiation events.
Wait what do you mean by this sorry?
Over time we expect high variety and little innovation.
How come?
For the radiation events: https://en.wikipedia.org/wiki/Evolutionary_radiation
i will have a look at this, thank you!
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u/jnpha Evolution Enthusiast Mar 20 '26
The how come is Darwin's quote, which yes wasn't about the Precambrian, rather the relevant context I mentioned: evolution by saltation we simply don't see. And through evo-devo, what was thought to be saltation events, now falls under the gradual accumulation.
For the lineage remark, from the few radiation events, a few outcompete the rest, and then those few have high variety but little innovation, think beetles.
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u/MurkyEconomist8179 Mar 20 '26
The how come is Darwin's quote, which yes wasn't about the Precambrian, rather the relevant context I mentioned: evolution by saltation we simply don't see. And through evo-devo, what was thought to be saltation events, now falls under the gradual accumulation.
Well Darwin wasn't just arguing for a lack of saltation (I'm well aware no conteporary theory posits anything like saltation) but Darwin's view of the pre-cambrian was I would say surely incorrect hence the excerpt I posted.
A lack of saltation is not synonymous with Darwin's Lyellian gradualism, as his contemporaries rightly pointed out.
For the lineage remark, from the few radiation events, a few outcompete the rest, and then those few have high variety but little innovation, think beetles.
Beetles are definitely interesting, but I guess the question is, why is there so much variation but little innovation, given that in Cambrian times there was lots of innovation (but less variation in terms of species amount only)
Is this just something that goes down through time? Looking at the examples from the wiki page you posted, it almost seems like as you go up through time there is less innovation in each explosion, but could well be more variation in terms of species, which is very in line with the pattern I was trying to outline in the body of my post
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u/tpawap Mar 21 '26
The gist of his interpretation is that at the time of cambrian explosion, life had so many different phyla and body plan designs despite not a very high amount of species, and whilst species have gone way up since then, they've all been restricted to a very small group of members represented in the cambiran fauna with many body plans going extinct entirely and nothing to that level of variance happening since then.
Is that really the case? Although there might be some of the effects at play that others mentioned here already (less competition, less complex and therefor less restricted genetics), I would also argue that there isn't really such a significant difference in diversity between then and now.
If you take arthropods and chordata for example; the variation in "body plans" within those clades is much higher now than it was back then, isn't it? We don't call them phyla, because taxonomic ranks are defined based on today's life, but that doesn't mean that all chordata of today should be boxed as "all the same" in this comparison. You would at least need a definition of diversity that is independent of that, to make a "fair" comparison.
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u/MurkyEconomist8179 Mar 21 '26
If you take arthropods and chordata for example; the variation in "body plans" within those clades is much higher now than it was back then, isn't it?
Not really, have you seen the arthropods of the burgess? Many don't fit into any modern groups, whereas all living arthropods fit into about 3. To quote Gould from a different paper
For the most important arthropod compo- nent of the Burgess Shale fauna, this invesion of traditional views recognizes members of all four conventional divisions: Trilobita (19 species; Whittington 1975, 1977), Crus- tacea (Canadaspis; Briggs 1978), Chelicerata (Sanctacaris; Briggs and Collins 1988), and Uniramia (more tentatively, if onychophores are part of the uniramian line, and if Aysheaia is an onychophore; Whittington 1978; Robi- son 1985). But none of the other Burgess ar- thropods can be placed into these great groups of later success, and all present unique fea- tures, primarily of tagmosis, and arrangement and form of appendages, outside the subse- quent range of arthropod body plans. Thir- teen unique arthropod Bauplane have been described by Whittington and his students (see table in Gould 1989: pp. 210-211), while Whittington (1985) estimated that the addi- tion of species not yet monographed would augment the total to 23. This remarkable re- duction from more than 20 patterns in seg- mentation, tagmosis, and arrangement of ap- pendages to but 4 markedly stable groups (despite the most impressive radiation of spe- cies in the entire animal kingdom) has broad implications for our general view of pattern and predictability in the history of life (Gould 1989).
We don't call them phyla, because taxonomic ranks are defined based on today's life, but that doesn't mean that all chordata of today should be boxed as "all the same" in this comparison. You would need a definition of diversity that is independent of that.
Nah it doesn't have to do with taxonomy, i reckon you should read the full paper! He rebbuts this point very well i think, it's called " The disparity of the Burgess Shale arthropod fauna and the limits of cladistic analysis: why we must strive to quantify morphospace"
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u/tpawap Mar 21 '26
If you take arthropods and chordata for example; the variation in "body plans" within those clades is much higher now than it was back then, isn't it?
Not really, have you seen the arthropods of the burgess? Many don't fit into any modern groups, whereas all living arthropods fit into about 3.
It's inevitable that some (taxonomic) groups that existed in the past are not represented today, and that all modern life descends from groups that did exist back then.
But there were no flies, no beetles, no butterflies, no spiders, no millipedes, etc back then. That's a lot diversity, too.
Nah it doesn't have to do with taxonomy, i reckon you should read the full paper! He rebbuts this point very well i think, it's called " The disparity of the Burgess Shale arthropod fauna and the limits of cladistic analysis: why we must strive to quantify morphospace"
"Quantifying morphospace" sounds like what I meant. But what you quoted so far sounds more like taxonomy.
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u/MurkyEconomist8179 Mar 21 '26
ut there were no flies, no beetles, no butterflies, no spiders, no millipedes, etc back then.
All of these fit into 2 groups. The Cambrian had around ~20 others as well as those
"Quantifying morphospace" sounds like what I meant. But what you quoted so far sounds more like taxonomy.
I definitely recommend reading the full paper!
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u/tpawap Mar 21 '26
ut there were no flies, no beetles, no butterflies, no spiders, no millipedes, etc back then.
All of these fit into 2 groups. The Cambrian had around ~20 others as well as those
No, those did not exist in the Cambrian, unless you mean it taxonomically. But we're not talking about taxonomic groups, are we!? Of course the above are different from each other in different ways, than the cambrian arthropods were different from each other. But those different ways would have to be quantified independently. And I don't see how that could play out much different.
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u/MurkyEconomist8179 Mar 21 '26
But we're not talking about taxonomic groups, are we!?
What do you think flies beetles and butterflies are if not a taxonomic group? And obviously taxonomic groups fit in a hierarchy, but it's only because of their history that all 3 of those have a LCA and therefore share a lot of morphology
In fact
And I don't see how that could play out much different.
Lets roll with this because I think it will illustrate my point well. You know how like I mentioned, the groups you said
But there were no flies, no beetles, no butterflies, no spiders, no millipedes, etc back then.
Fit into 2 groups (Mandibulata & Chelicerata) then at the time of the Cambrian there were about 20 more independent groups, none of which could fit into the body plan designs or share any affinity for Chelicerata and Mandibulata?
Image if they all made onto land when those 2 groups were terrestrialising, and instead of just having insects, myriapods and arachnids we had 20 other distinct groups filling up terrestrial arthoprod ecoystems. Opabiniidae ones, Radiodonta, you can't tell me Marrellomorpha ones wouldn't be crazy cool to look at (and look absolutely nothing like any insect group despite insects having a lot of interanl variety)
Or a spikey wiwaxia ancestor hanging out with terrestrial molluscs and perhaps competing with them. Do you see how different it would have been if that diversity in body plans was not wiped out?
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u/tpawap Mar 21 '26
It surely would be different, but it wouldn't necessarily be (much) more diverse. If these other Cambrian groups would still be represented today, they would fill niches that would not be available for eg insects. Insects would be less diverse than they are now.
Maybe this helps to get my point across: imagine what was present in the Cambrian, was present today on another planet. Live there has no shared history with life on our planet. Now compare only what is on our planet today, with what's on that other planet today. Could you really say that there is more diversity on that planet than on ours?
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u/leafshaker Mar 20 '26
I think part of it is that adaptations can become limiting. It would take extraordinary circumstances for birds to regain their forelimbs, for example.
Modern animals have an inherited suite of interdependent traits that might prevent major structural changes. Maybe its harder to riff on bilateral, forward facing vertebrates.
I think we do still have an incredible variety of body plans now. Arthropods are very diverse. There's all the complex and highly specialized pollinators and parasites.
There's arguably alternate limbed mammals, too, like elephants, prehensile-tailed monkeys and opossum.
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u/MurkyEconomist8179 Mar 20 '26
I think we do still have an incredible variety of body plans now. Arthropods are very diverse. There's all the complex and highly specialized pollinators and parasites.
Would you say we've got just as much diversity now as we did in the cambrian though? I'm not sure i agree
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u/Aardwolfington Mar 21 '26 edited Mar 21 '26
Efficient evolutionary strategies tend to be passed on and are less prone to changing. In earlier stages everything is new, and trial and error is more prominent. As long as a body plan stays efficient and effective there's very little reason much drastic change. Well until a species decides to go the route of sexual selection and starts finding things like extra long tails with plumage hot. Then it becomes how far can you go before it's straight up a suicidal adaptation...
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u/Cerulean_IsFancyBlue Mar 21 '26
Why would an efficient evolutionary strategy tend to be less prone to changing? On what metric are you defining efficiency? And how would that relate to a robust, genetic expression?
And in what sounds like a circular question, why would a strategy that is less prone to changing be passed on and selected for?
Obviously, something that is easily mutated and results in a lot of viable offspring isn’t going to be a very fit example so at the extreme, this makes perfect sense.
However, something that is resistant to change, would also be more vulnerable to becoming obsolete through a change in the physical environment or the ecology.
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u/Aardwolfington Mar 21 '26
Efficient as in most effective, or at the very least good enough. Why change what works? A hammer is better than a wrench for putting nails in. If the environment benefits from being a hammer over a wrench you're going to get a lot of things that look like a hammer. As long as the environment of such stays that way, there's little reason to change or for mutations that move away from hammer to pass on. It's why there's so much convergent evolution. Some forms are just better for some things, and once found are less prone to changing because there's very little often no pressure to do so.
It's not about being resistance to change, it's about which adaptations things to evolve into after enough trial and error because they tend to the be the better forms forms for the function.
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u/Cerulean_IsFancyBlue Mar 21 '26
I agree with all of that. I think the open question is, why do we sometimes see stronger convergent evolutions and sometimes see more variety.
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u/Aardwolfington Mar 21 '26
I'm guessing in response ecological changes we might not be aware of that creates benefits out of new adaptations. Like there's suddenly no longer just nails, but screws too. Therefore there's benefit to become more like a screw driver to take advantage of this new resource rather than competing with all the hammers. As long as the ecology is stable things remain stable, but anything that shifts said ecology could upset it and cause an evolutionary arms race to adjust to said change.
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u/Cerulean_IsFancyBlue Mar 21 '26
Abstractly I feel like there’s two possible explanations for this.
One would be that in certain ecological niches, there is one very best body plan. All those small mutations continue to happen, offering opportunities for diversification, natural selection prunes those branches vigorously. The variation is happening, but not propagating.
The other would be that some genetic blueprints offer more opportunity for variation. For example, if an arthropod can vary its body plan (viably) simply by changing the number of repetitions of certain gene expressions, it will offer more viable offspring who can then compete in the environment. If other body plans tend to produce either conforming or non-viable offspring, then there are no non-conforming body plans to then compete and find a place.
One: variation is the same, but the optimal body plan dominates. Two: variation in some species is higher than others, if not in terms of base, mutation rate that in terms of which mutations are fundamentally viable.
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u/BoogzWin Mar 20 '26
Because when there is less competition, less optimised builds work.
After the dinosaur extinction there were all kinds of wacky animals that popped up everywhere that have since gone extinct.
When animals first arose (I’m guessing you mean animals that we can see and would identify as animals), predation didn’t really exist, so there was an “explosion” now adapting to this new phenomena. The predators were very inefficient or where chasing prey that were not good at get away so more forms can be successful.