Doubting Didymus

I am currently reading ‘Dawkins vs. Gould’, a little book by Kim Sterelny, and one of the first things he brings up is the whole ‘animal phyla all having their beginnings represented at the same time in the fossil record (Cambrian)’ thing. Basically, Gould sees this as representing a different ‘kind’ of evolution, an ability to ‘invent’ fundamentally different body plans of the kind that denotes a phyla. In this sense, that ‘ability’ of evolution is no longer around today. Dawkins sees it as a matter of semantics, ‘phyla’ being a term invented by humans for the sake of convenience, and says that, way back at the time they were diverging, these new phyla that were appearing were just species level events, speciations that just happened to be successful enough to survive for millennia and found whole hordes of new species, which we then came along and slapped the title ‘phylum’ on.

Up until recently, this was my view also. However, now that I am reading more of Gould I have come to realize that the semantics of the taxon are irrelevant to Goulds main point, which is not that the ‘phyla’ all come into being at the same time, but that the body plans that are represented today all came into play at about the same time. This can not be dismissed as semantic, so what is going on?

For the sake of speculation, is it possible that the reason no new fundamental body plans come into existence is similar to the reason no new abiogenesis events happen? I.e. no new body plans are being thrown about because there is no niche for anything to fill. Consider: to create a new body plan, evolution would need to start a new multicellular lineage from a single cell, or very simple multicellular organism, otherwise the ‘new’ body plan would not be recognised as being ‘fundamentally’ different, but merely an alteration of an existing one. (for example, a whale is not considered a different body plan from other mammals, even though it is highly modified). So perhaps starting a new multicellular lineage is impossible, simply because all of the ecological niches that could possibly be filled by the new lineage are already filled by the existing batch of organisms? Also, even if there were unexplored niches, existing multicellular animals are in a much better position to get there first.

This would place the ‘why are there no new body plans’ question on a similar footing to the ‘if abiogenesis is possible, why did it only happen once’ question. The same answer applies to both: once it had happened the first time, there was no more room for it to happen again. This would also explain why the phyla appear to occur simultaneously: as soon as multicellular bodies were on the agenda, multiple lineages jumped on the concept, which both guaranteed that newcomers would be blocked out of the new niches and guaranteed that those that did make it would appear to have arrived simultaneously.

I picture the collective possible animal niches like a vacant, empty room, and the emerging multicellular lineages as people or machines with large balloons of different colors, inflating them as fast as they can through the doorway. What you are going to see is a relatively small number of the starting balloons will get through the door first, and rapidly take up all the room inside until they are pressed against one another. At this point we might ask, why are no new colors of balloon emerging? The answer is that the room is already fairly full, and the doorway is blocked. Those colors of balloon that got there first are now free to slowly take up all the rest of the space using modifications of their existing body plans.

Peez

I generally agree with your analysis. New body plans are unlikely to evolve now because of the competition that would be faced from organisms already extant. Also, as you implied, evolving a new body plan from a complex multicellular organism may be much more difficult than evolving a new body plan from a unicellular or simple multicellular organism. This is because the complex multicellular organism relies on a number of complex systems to survive, and those systems may be difficult to radically change without compromising their function. Simpler life forms may be much more flexible.

The comparison to abiogenesis has merit, but I would add that even if there were no living organisms today new abiogenesis events would not be comparable to our original abiogenesis. Even before a new organism can compete with extant life, it must form in the first place. Current conditions on this planet are quite different than they were about 4 billion years ago, so there is no reason to expect the same reactions to be taking place. That being said, I cannot claim that new life could not form under current conditions. Of course, even if conditions were right the activity of ubiquitous life here would almost certainly make development of novel life impossible. On top of that, even if novel life somehow got formed, it would then have to compete with life that has been evolving for billions of years.

Getting back to the evolution of body plans, I still tend to side with Dawkins. The fundamental processes that were responsible for the evolution of Phyla were almost certainly the same as those responsible for the evolution of new varieties within a species. The "Cambrian explosion" reflects the evolution of a genetic system that allowed multicellular organisms (with cells that "know" where they are in the body and what they are "supposed" to do). Suddenly (in geological terms) it was possible for truly multicellular animals to evolve (as opposed to clusters of essentially unicellular organisms). Mutation, genetic drift, and natural selection. The situation certainly was different, in terms of the genetic make-up of the extant organisms and the available niches, but the fundamental mechanisms was likely the same.

Peez

pz

I would also add that the modular nature of developmental programs also makes it difficult to compete. Existing phyla have these lovely, well-refined molecular pathways to assemble, for example, a limb; adding a limb to a limbless body plan is going to be difficult, because it may have to be built from scratch and will therefore not be as effective as one from a phylum that has honed theirs over half a billion years.

Even chordates, an initially limbless phylum and one in which even now limbs are not considered a canonical part of their body plan, had to struggle to get where they are now. They had to a) coopt an existing developmental module for anterior-posterior patterning to generate proximo-distal information, b) move into a niche in which there were no arthropod competitors, and c) take 50-100 million years to refine a crude paddle into a true limb.

MrDarwin

I think they're both right. I think the Cambrian Explosion was a unique, one-time event that occurred precisely because multicellular animals were just beginning to evolve, and were not competing with anything remotely similar that was already in existence. But I definitely agree with Dawkins that our labeling of "phyla" is strictly semantics; we are simply recognizing the lineages that have survived and diverged from a series of species that were in existence at one time.

All kinds of "new body plans" (anybody care to define the term?) have appeared since the origins of the phyla. My favorite example is the parasitic crustaceans (forget the name) that are parasites in crabs and other bigger crustaceans--they are amoeba-like endoparasites that lack body parts of any kind except reproductive organs, are about as different from arthropods as you can get. But they are still defined as arthropods because their nauplius larvae still indicate their crustacean ancestry, and because their closest relatives, barnacles--themselves rather odd crustaceans--are clear intermediates between the parasites and more typical free-living crustaceans.

And this is what an organism would have to overcome to be considered a new phylum: it would have to have no connection to any living phylum, which would be impossible because it would have to be derived from a living phylum, and would end up being shoehorned into that phylum no matter how much it had diverged.

Ojuice5001

MrDarwin:

They're called Sacculina.

ps418

Another example of this is the carnivorous sponges (Cladorhizidae) discovered in the Mediterranean. The 'body plan' and mode of life of these sponges is radically different than other sponges, but they are still considered sponges because they have spicules. I first heard about these in essay written by Glenn Morton quoting Vacelet and Boury-Esnault (Carnivorous Sponges, Nature 373, pp. 333-335). They wrote:


Other stuff about the carnivorous sponges:


<a href="http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/ps_sponges.html" target="_blank">Attack of the Killer Sponges??</a>

<a href="http://www.clemson.edu/pal/111/diversity/Porifera/Porifera_08.htm" target="_blank">Asbestopluma, seen in a cave in the Mediterranean</a>

MrDarwin

Another example is the tongue worms. They were formerly classified in their own phylum, Pentastomida, but recent research has shown them to be derived from within the Arthropoda:

<a href="http://mbe.library.arizona.edu/data/1989/0606/8abel.pdf" target="_blank">Molecular Evidence for Inclusion of the Phylum Pentastomida in the Crustacea</a> (pdf file)

The pentastomids were presumably given their own phylum because their "body plan" was different from that of any other phylum. Their reclassification as arthropods demonstrates that it takes more than a novel body plan to get your phylum!

Edited to add that, the more I think about it, the more I can think of problematic groups that may or may not be their own phyla, e.g., tardigrades and velvet worms. Especially with the influence cladistics has had on classification in the last couple of decades, categories like "phylum" are being re-defined such that they must include all descendants of some common ancestor, no matter how much they may have diverged from each other, or from that common ancestor. Another example is birds now being classified as dinosaurs.

[ September 29, 2002: Message edited by: MrDarwin ]</p>

Doubting Didymus

Aha! I didn't think of birds. Surely birds can be considered a fundamentally new body plan, even though it is clearly derived from a sauropod.

This also supports my suggestion that the lack of new body plans are due to an absence of empty niches. For what to we see whenever an existing lineage manages to find and colonise a new niche? we see a brand new bodyplan (in the case of mr darwins ameobic crustacian parasites), or a very highly modified bodyplan, such as birds.

In both cases, a new bodyplan is the result of 'discovering' a new ecological niche, such as crustacians finding a specific new kind of parasitism, or for large animals to conquer the sky.

These examples are limited (e.g. they are not as drastic as the arthropod/tetrapod divide) but, if my OP is accurate, that is to be expected as a completely new bodyplan of that kind would have to evolve from a unicellular starting point, and is thus outcompeted by the existing multicellular lines.

The Lone Ranger

Quote:

Surely birds can be considered a fundamentally new body plan, even though it is clearly derived from a sauropod.

Just to be nitpicky, birds evolved from theropods, not sauropods. Indeed, the fossils of the earliest birds are essentially identical to those of small theropods like Compsagnathus.

I agree with what seems to be the consensus. There was a period of adaptive radiation at the time of the "Cambrian Explosion," when all sorts of vacant niches were available for colonization. In this sense, the Cambrian Explosion was no different from the explosive radiation of mammals that occurred after the non-avian dinosaurs became extinct.

There's no evidence that there was any special or unprecidented kind of evolution going on at the time, however. I think that Dawkins is perfectly correct to point out that if you could go back in time and watch the evolution of these creatures, there would be nothing at all remarkable about it -- just organisms evolving to fill vacant niches.

Cheers,

Michael

pz

Nope, sorry, not at all. Birds are just a minor variation of the standard chordate body plan. It's got a notochord, pharyngeal arches, somites, and a post-anal tail. Those are the features that count, not whether one of the pairs of limbs (a secondary feature anyway) have become specialized in some particular way.

A body plan is a common suite of diagnostic features that represent a fundamental level of organization within a phylum. Another way to think of it is that it is a particular way of patterning the spatial compartmentalization of early developmental processes, especially of the selector (or Hox) genes and their associated signalling proteins.