Disparity, Diversity and the Cambrian Explosion

Posted 17 March 2008 by PvM

(Click picture to pop-up a larger version) Charles R. Marshall in EXPLAINING THE CAMBRIAN “EXPLOSION” OF ANIMALS, Annual Review of Earth and Planetary Sciences Vol. 34: 355-384, shows how the 'classical' view of the Cambrian as popularized by Gould cannot longer be maintained given the available data. Abstract: The Cambrian “explosion” is a unique episode in Earth history, when essentially all the animal phyla first appear in the fossil record. A variety of environmental, developmental (genetic), and ecological explanations for this complex and somewhat protracted event are reviewed, with a focus on how well each explains the observed increases in disparity and diversity, the time of onset of the radiation, its duration, and its uniqueness. The increase in disparity (the origin of the phyla) and diversity are best understood as being the result of the interplay of the combinatorial bilaterian developmental system and the increase in the number of needs the first bilaterians had to meet as complex ecological interactions developed. The time of onset is constrained by the evolution of the environment, whereas its duration appears to be controlled primarily by rates of developmental innovation. The uniqueness of the event is either due to ensuing developmental limitation, to ecological saturation, or simply to the exhaustion of ecologically viable morphologies that could be produced by the nascent bilaterian developmental system. A little background about the concepts of disparity and diversity may be in order

To assess the differences between organisms, the amount of diversity and disparity are estimated. Diversity is simply counts of taxa (sets of organisms related together by certain characteristics). In other words, it is the number of species, genera, families, and so forth in the traditional taxonomy of Linnaeus. Disparity, on the other hand, describes the extent of morphological differences between various taxa. This is essentially the organization of anatomical form that defines the larger sets of organisms.

Figure 2. Several hypotheses of the evolution of morphological diversity, or disparity. (a) Traditional model of gradually increasing disparity over time; (b) Model of maximum disparity early in evolutionary history and eventual stabilization; (c) Model of long-term gradually increasing disparity of cryptic deep Precambrian animals and followed by rapid stabilization; (d) Model of short-term gradually increasing disparity of cryptic late Precambrian animals followed by eventual stabilization. Image courtesy of M.A. Wills of the University of Bath, UK. Despite the incompleteness of the fossil record, it is agreed upon that Cambrian diversity is lower than that of today. But it has been shown that the disparity of organisms in the Burgess Shale was nearly equal to that of modern organisms. This has been done using a variety of independent methods, including empirical and theoretical approaches. This means that in only the first 10 percent of the history of multicellular life, around 80 percent of modern disparity was reached, demonstrating the rapidity of the evolution of form.

Another useful picture is from Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation, Paleobiology, 31(2), 2005, pp. 94–112 by Derek E. G. Briggs and Richard A. Fortey FIGURE 2. The fossil record and metazoan phylogeny. Relationships are illustrated as in Pennisi 2003. Dark lines represent the stratigraphic range of maj r metazoan taxa from their ﬁrst appearance in the fossil record, indicated by a square, to the present. Light lines represent ghost ranges implied by a previous occurrence of a related taxon. The data on Lobopodia, Brachiopoda, Mollusca, Hemichordata, and Echinodermata, as well as phyla that ﬁrst appear after the Cambrian, are from Benton 1993. Other ﬁrst appearances are discussed in the text. Since Wonderful Life (1989), when ﬁrst appearances were as indicated here by circles, discoveries in the Chengjiang fauna have pushed various Burgess Shale occurrences back 20 million years—Ctenophora, Lobopodia, Priapulida, Chaetognatha, Cephalochordata—and the vertebrates back from the Ordovician. The range of the tardigrades has extended from the Tertiary to the Cambrian, and the tunicates have acquired a reliable Cambrian record. Sponges have been discovered in the Vendian. And then we have the interpretation of the Cambrian by ID creationist Casey Luskin Figure 2. a) Representation of the tree of life and the fossil record, as predicted by Darwin. Fossils should show smooth transformations from one morphological form to another. b) An idealized schematic of the fossil record as predicted by punctuated equilibrium. Transitional forms are missing, as is predicted by "punctuated equilibrium", however breaks between morphology and various fossil species are not exceedingly large. c) A schematic representing the actual fossil record as it pertains to the major animal phyla. Morphologically diverse organisms appear all at once along the Cambrian / Pre-Cambrian boundary, without any fossil ancestors. Dubbed the "Cambrian Explosion," this "explosion" phenomenon is also recognized for the origin of fish, birds, mammals, and plants. Despite this evidence and the lack of fossils confirming Darwin's original theory of descent with modification (a), according to its proponents, Darwin's theory is not falsified. And since we are visiting with our creationist friends, let's address the following misunderstanding

As Doolittle indicates, from the base of the tree of life, it is not "tree-like." In the "bush" below (Figure 3), it is impossible to reconstruct such trees, as the observed distribution of characters create something which looks more like a tangled thicket or a bush. The three major "domains" of life--Bacteria, Archaea, and Eukarya have a distribution of characteristics which does not allow a tree to be constructed to describe their alleged ancestral relationships. This is due to a character distribution which is not what one would predict if they inherited their genes through common ancestry:
— Casey Luskin

and yet, science, contrary to ID creationist claims, has successfully recreated trees, with the addition of horizontal gene transfer Once again our friends at TalkOrigins help us out: The figure is from The net of life: Reconstructing the microbial phylogenetic network by Victor Kunin, Leon Goldovsky, Nikos Darzentas and Christos A. Ouzounis

It has previously been suggested that the phylogeny of microbial species might be better described as a network containing vertical and horizontal gene transfer (HGT) events. Yet, all phylogenetic reconstructions so far have presented microbial trees rather than networks. Here, we present a first attempt to reconstruct such an evolutionary network, which we term the "net of life." We use available tree reconstruction methods to infer vertical inheritance, and use an ancestral state inference algorithm to map HGT events on the tree. We also describe a weighting scheme used to estimate the number of genes exchanged between pairs of organisms. We demonstrate that vertical inheritance constitutes the bulk of gene transfer on the tree of life. We term the bulk of horizontal gene flow between tree nodes as "vines," and demonstrate that multiple but mostly tiny vines interconnect the tree. Our results strongly suggest that the HGT network is a scale-free graph, a finding with important implications for genome evolution. We propose that genes might propagate extremely rapidly across microbial species through the HGT network, using certain organisms as hubs.

Emphasis mine. For more on Dollittle.

23 Comments

Torbjörn Larsson, OM · 17 March 2008

Thanks for this, it will be interesting to read this more thoroughly. Meanwhile, following up on Doolittle's article layman fashion (i.e. whatever takes my fancy), there is this article on using a global constraint analysis on tRNA evolution (and accompanying and confirming papers on proteins) that manages to include viruses, establishes domains, lineage relationships, epochs and possibly root the studied tree:

In a process that reconstructs history from molecular sequence and structure and at the same time forces molecules belonging to lineages into groups, we tested alternative hypotheses of origin and established when major organismal lineages appeared in evolution. Remarkably, timelines showed that Archaea was the most ancient lineage on earth and that viruses originated early in the archaeal lineage. Our findings unroot the universal tree of life, and, for the first time, provide evidence for an evolutionary origin of viruses.

Our evolutionary timeline is also remarkable in that it identifies three epochs in the evolution of the organismal world that were analogous to those proposed earlier [16]: (1) an architectural diversification epoch in which tRNA molecules diversified their structural repertoires (light green areas in Figure 2), (2) a superkingdom specification epoch in which tRNA molecules sorted in emerging lineages that specified superkingdoms Archaea, Bacteria, and Eukarya (salmon areas), and (3) an organismal diversification epoch that started when all tRNA coalesced in each superkingdom (light yellow areas).

But as I understand it they observe that genomic diversity makes virus origins still compatible with just about any hypotheses out there, such as the ancient viral world. Nevertheless it seems to me to be a verification of the method discussed in the last part of Doolittle's paper as well as consistent with his description of domains/superkingdoms. And AFAIU it is descent pushed back, against Luskin's description of "failed" predictions.

harold · 17 March 2008

Interesting. I suppose I'll have to read on my own if I want to learn how "disparity" is quantified.

I would note that lateral transfer can occur between multicellular eukaryotes (viral vectors, or now, humans who work for Monstanto, for example, can affect it), but that it is far, far less of an issue in eukaryotic evolution than in bacterial evolution.

Casey Luskin's comments make no sense whatsoever, of course. Any nested hierarchy supports evolution. Whether an imbecile looks at it and thinks it looks like a "bush", a "tree", or a "portrait of Abraham Lincoln" makes no difference. If it's a nested hierarchy, that's what it is.

PvM · 17 March 2008

Interesting. I suppose I’ll have to read on my own if I want to learn how “disparity” is quantified.

Not a trivial undertaking. Often based upon a concept called 'morphospace' See: MDA: a MATLAB-based program for morphospace-disparity analysis Disparity References Let us know what you learn

Nigel D · 18 March 2008

Interesting article.

Luskin's words show that he has not even read TOOS (well, either that or he failed to understand it or is deliberately misrepresenting it). Darwin anticipated that transitional forms between species would be under-represented in the fossil record, and provided a quite lucid explanation of why this should be so.

In essence, differences between organisms lead to a decrease in direct competition; whereas similarities lead to increased competition between the organisms. Thus, natural selection drives divergence of species following a speciation event (Ack! I can't recall the term for a speciation event where two daughter species arise from a parent species!).

David Stanton · 18 March 2008

Nigel,

Is the term you are looking for cladogenesis as opposed to anagenesis, or am I not understanding your point?

Stacy S. · 18 March 2008

Founder effect?

Nigel D: (Ack! I can't recall the term for a speciation event where two daughter species arise from a parent species!).

Henry J · 18 March 2008

Founder effect (iirc) is a case of genetic drift caused by separation of a small group from the rest of the species. If that small group later grows in population, any genetic rarities that were carried by that group can spread a lot easier there than they could if the group were still interbreeding with their relatives.

Henry

teach · 18 March 2008

Could someone give me a quick rundown on how this disputes the "classical" view of the Cambrian explosion?

raven · 18 March 2008

The uniqueness of the event is either due to ensuing developmental limitation, to ecological saturation, or simply to the exhaustion of ecologically viable morphologies that could be produced by the nascent bilaterian developmental system.

My vote is for ecological saturation. A basic principle in ecology is that two species cannot occupy the exact same niche at the same time. Otherwise, the most fit will drive the other one to extinction. You see adaptive radiations only when new territory opens up or when mass extinction events clean off the table. That is why the cambrian explosion is unique IMO. What keeps the ecosphere and biosphere changing is that on longer term time scales the environment is in a constant state of change. We've even seen that in recent times, the retreat of the last ice age and the rise of intelligent tool users.

Stacy S. · 18 March 2008

Is the ensantina salamander an example of this? http://www.santarosa.edu/lifesciences2/ensatina2.htm "What is most interesting about this species of salamander, is that the two southern most subspecies, eschscholtzi and klauberi, meet in several locations. Near Mount Palomar, these two subspecies meet in a very narrow zone and hybridize infrequently. (Brown, 1974) To the south near Cuyamaca State Park, klauberi and eschscholtzi meet and apparently fail to interbreed under natural conditions even though they are narrowly sympatric. In fact, by analyzing electrophoretic separations of selected enzymes and studying DNA patterns, the two subspecies klauberi and eschscholtzi are different species by every definition."

Henry J: Founder effect (iirc) is a case of genetic drift caused by separation of a small group from the rest of the species. If that small group later grows in population, any genetic rarities that were carried by that group can spread a lot easier there than they could if the group were still interbreeding with their relatives. Henry

Mark Duigon · 18 March 2008

The Cambrian “explosion” is a unique episode in Earth history...

Or maybe not. There was an article in a recent issue of Science (I'll have to look for it when I get home) that described an Ediacaran "explosion" that differed in not leading to the modern phyla (perhaps, in a sense, a "failed" explosion).

s1mplex · 18 March 2008

Luskin:

As Doolittle indicates, from the base of the tree of life, it is not “tree-like...” [T]he observed distribution of characters create something which looks more like a tangled thicket or a bush.

I see little difference, as both the Tree and Bush were most likely Intelligently Designed Acts of Creation by God. :|

PvM · 18 March 2008

You are correct, there were at least one more "explosions" in the ediacaran called Avalon.

Mark Duigon:
The Cambrian “explosion” is a unique episode in Earth history...
Or maybe not. There was an article in a recent issue of Science (I'll have to look for it when I get home) that described an Ediacaran "explosion" that differed in not leading to the modern phyla (perhaps, in a sense, a "failed" explosion).

Tyrannosaurus · 18 March 2008

[/sarcasm on/]
No, No, No is all designed and POOF!!!
[/sarcasm off/]

Henry J · 18 March 2008

[/sarcasm on/] No, No, No is all designed and POOF!!! [/sarcasm off/]

But that's only if'n ya have poof of purchase...

Torbjörn Larsson, OM · 18 March 2008

both the Tree and Bush

Hmm. Wouldn't cladistics be sufficiently mathematical inspired to call those ordered hierarchical structures tree and lattice? Fine, just stay with the biological imagery, as if this is ... oh.

Nigel D · 19 March 2008

David Stanton: Nigel, Is the term you are looking for cladogenesis as opposed to anagenesis, or am I not understanding your point?

Yes, cladogenesis (as opposed to anagenesis). Thanks, David.

Nigel D · 19 March 2008

teach: Could someone give me a quick rundown on how this disputes the "classical" view of the Cambrian explosion?

OK, Teach, I'll have a go (but please recall that my area of expertise is biochemistry, not palaeontology), so this is all based on my own understanding: Luskin's argument boils down to this: "Darwinian mechanisms are always gradual. Even punctuated equilibrium requires short steps with stable periods between them. And even though creationists everywhere have claimed that the fossil record is full of holes, it is a perfect record of the Cambrian explosion. In the Cambrian explosion, all of the major phyla appeared at exactly the same time, which cannot occur by Darwinian mechanisms, because I have defined these as demanding gradual change". In fact, the fossil record at the Precambrian / Cambrian boundary is difficult to interpret, because there were so few (or, initially, no) organisms with hard body parts. It is known that soft body parts rarely fossilise. However, the "Cambrian explosion" took place over about 30 - 40 million years, and shows a clear progression in the fossil record from low diversity and relatively simple fossils (in fact, the earliest fossil are trace fossils, i.e. things like worm casts and burrows and so on, so the organisms that made them must be inferred) to increased diversity and increased complicatedness. The "classical" view (say, 20 - 30 years ago) was that it was hard to explainwith the known fossils, and that the fossil record must be a poor representation of what occurred, because it showed this apparent sudden appearance of identifiable ancestors of the modern animal phyla. Creationists misconstrued this to indicate the sudden appearance of all the modern phyla, without understanding that, in the Cambrian, the modern phyla would be no more different from one another than species are today. Subsequent developments, both in our understanding of evolution and in the discovery of new fossils, have brought us the knowledge that the term "explosion" is extremely misleading. There is, as always, more to it than this, but I have no more time just now. Hope it helps.

teach · 19 March 2008

Nigel

Thanks for your response; now let me see if I can clarify my understanding a bit more. First, I'm ignoring most of what Luskin says, as I resolved his arguments a long time ago. But as I understand Gould's "classical" interpretation, Burgess represents a rapid radiation (to get away from the explosion idea) followed by winnowing and then stasis. Is his classical view challenged by the presence of simpler fossils in older strata? Is his view challenged by the gradual change of fossils after the radiation? (I'm referring to the info in the abstract with these questions).

I ask so much because I am around teenagers all day and don’t always get to ask in depth questions. I also ask because I get so tired of cdesign proponentists yelling about no one questioning the dogma of evolution. Gould did that in the 70s with punctuated equilibrium and now others do it to his interpretations as well.

Nigel D · 19 March 2008

teach: Nigel Thanks for your response; now let me see if I can clarify my understanding a bit more. First, I'm ignoring most of what Luskin says, as I resolved his arguments a long time ago. But as I understand Gould's "classical" interpretation, Burgess represents a rapid radiation (to get away from the explosion idea) followed by winnowing and then stasis.

OK, I think we have to be very careful interpreting the term "disparity". I do not fully understand how it is used in evolutionary biology, so I try not to use it, but stick to terms that I can explain. However, since it seems to be a core part of this paper, I will do my best. Gould's model is that disparity was large in the Cambrian, but decreased rapidly and then remained approximately constant. That is not to say that evolutionary change was not happening. IIUC, disparity is not a measure of evolutionary change, but a measure of how different different organisms are from one another.

Disparity, on the other hand, describes the extent of morphological differences between various taxa
— PvM quoting the cited article

Thus, while modern vertebrates are diverse (because there are many families, genera and species), the disparity amongst vertebrates is relatively modest, because all modern vertebrates have a similar body plan (even if that body plan has adapted to perform many different functions).

Is his classical view challenged by the presence of simpler fossils in older strata?
— Teach

Not necessarily. Simpler fossils indicate a more primitive (and thus less derived) character among the organisms, but only in those organisms that fossilised. First, the fossilised organisms could show a large disparity while showing a relatively low diversity, if their morphology diverged rapidly since their last common ancestors. Second, there may be many organisms that lived in the early Cambrian that are simply not represented at all in the fossil record. In this case, Gould's hypothesis may one day be challenged if we ever have reason to believe that our knowledge of the early Cambrian nears completion, and if that knowledge indicates a lower disparity amongst the organisms that lived then than Gould hypothesises.

Is his view challenged by the gradual change of fossils after the radiation? (I'm referring to the info in the abstract with these questions).

It seems to me that the info in the abstract is indicating an initial low value for both disparity and diversity, followed by a relatively rapid increase in both, followed by a very much slower rate of change subsequently. This does challenge Gould's 1989 model that contained an initial high disparity, which decreased rapidly and then remained largely the same for a significant time. However, to know for sure whether this challenge is successful, I would need to both read and understand the paper in its entirety, and this may require that I get stuck into the evolutionary biological literature to an extent for which I do not currently have the time.

I ask so much because I am around teenagers all day and don’t always get to ask in depth questions.

Hmm, tricky.

I also ask because I get so tired of cdesign proponentists yelling about no one questioning the dogma of evolution. Gould did that in the 70s with punctuated equilibrium and now others do it to his interpretations as well.

Well, yes and no. There are still debates about the details, but the core mechanisms have been in place for a significant time. Natural selection was widely accepted as an agent of biological change in the latter half of the 19th century. This is partly because Darwin's ideas were argued very persuasively and supported wuith a wealth of observations (throughout TOOS, Darwin keeps apologising that the limited space available prohibits him from including all of the detailed data). It is further supported by subsequent discoveries that bore out Darwin's theory. For example, the first Archaeopteryx was discovered not long after the publication of TOOS, and it is still a classic example of a transitional form (some of its features are reptilian, while some are characteristic of birds). The rediscovery of Mendelian inheritance mechanisms around the start of the 20th century provided a process whereby heritable changes are passed on without dilution (the predominant theory before Mendel involved "blending" of parental characteristics), allowing the propagation of inherited changes through a population. I think that there has always been a frontier of research around evolutionary biology where details are unknown, where different hypotheses compete, and where new discoveries are making a contribution. As this frontier moves and our knowledge increases, more details become firmed up, more hypotheses are confirmed or refuted and more components become incorporated into MET (modern evolutionary theory). In addition to Eldredge and Gould's punctuated equilibrium hypothesis, population genetics (for example) has made significant contributions, including the recognition of the importance of genetic drift. Thus, the details at this frontier are continually being challenged and explored. Once the data support one hypothesis over its competitors, research moves on and the newly-firmed-up hypothesis becomes regarded as true until the data indicate otherwise. I hope this was not too rambling, and has been of some value for you.

teach · 19 March 2008

Nigel

That all makes perfect sense. You did a great job of explaining and I thank you.

Nigel D · 20 March 2008

teach: Nigel That all makes perfect sense. You did a great job of explaining and I thank you.

Teach, you're welcome.

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