I told you all the batty creationists were crawling out of the woodwork to crow over Xiaotingia‘s redefinition of Archaeopteryx‘s status as a victory for their ideology, when it really isn’t. Now another has joined the fray: Vox Day, creationist and right-wing lunatic. He makes a lot of crazy, ignorant claims in this short passage that I’ll answer one by one.
Oh, this is beautiful. Bill Nye (the Science Guy!) is being interviewed by a Fox News talking head who asks a surprisingly dumb question: Nye is talking about a volcano found on the moon, so he asks, “Does it go anywhere close to the climate change debate on earth?…we haven’t been up there burning fossil fuels.”. Bill Nye’s eyebrows shoot up, he pauses very briefly, and you can see him recalibrating his brain so he can answer as he would to a perky little 5 year old. It’s wonderfully amusing, and he does give a very good answer.
The Atheists, Skeptics, & Humanists Team on Folding@Home, the distributed computing effort to calculate protein folding properties, is looking for your spare CPU cycles. Join them! It’s a much more productive use of your computer than that silly SETI@Home project.
OK, I’m feeling guilty: I’m off at The Amaz!ng Meeting enjoying myself, and totally neglecting the blog readers who aren’t lucky enough to be here too. And since I’ve been getting lots of requests to put the full content of my talk online, I figured…yeah, sure, I can do that. So here you go, all of the slides and what I said about them, mostly, below the fold. Criticize and argue and do your usual.
That’s not hyperbole. I really mean it. How else could I react when I open up the latest issue of Bioessays, and see this: Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules. Just from the title alone, I’m immediately launched into my happy place: sitting on a rocky beach on the Pacific Northwest coast, enjoying the sea breeze while the my wife serves me a big platter of bacon, and the cannula in my hypothalamus slowly drips a potent cocktail of cocain and ecstasy direct into my pleasure centers…and there’s pie for dessert. It’s like the authors know me and sat down to concoct a title where every word would push my buttons.
The content is pretty good, too. It’s not perfect; the development part is a little thin, consisting mainly of basic comparative embryology of body plans, with nothing at all really about deployment of and interactions between significant developmental genes. But that’s OK. It’s in the nature of the Greatest Science Papers Ever Written that stuff will have to be revised and some will be shown wrong next month, and next year there will be more Greatest Science Papers Ever Written — it’s part of the dynamic. But I’ll let it be known, now that apparently the scientific community is aware of my obsessions and is pandering to them, that the next instantiation needs more developmental epistasis and some in situs.
This paper, though, is a nice summary of the emerging picture of cephalopod evolution, as determined by the disciplines of paleontology, comparative embryology, and molecular phylogenetics, and that summary is internally consistent and is generating a good rough outline of the story. And here is that story, as determined by a combination of fossils, molecular evidence, and comparative anatomy and embryology.
Cephalopods evolved from monoplacophoran-like ancestors in the Cambrian, about 530 million years ago. Monoplacophorans are simple, limpet-like molluscs; they crawl about on the bottom of the ocean under a cap-like shell, foraging snail-like on a muscular foot. The early cephalopods modified this body plan to rise up off the bottom and become more active: the flattened shell elongated to become a cone-like structure, housing chambers for bouyancy. Movement was no longer by creeping, but used muscular contractions through a siphon to propel the animal horizontally. Freed from its locomotor function, the foot expanded into manipulating tentacles.
These early cephalopods, which have shells common in the fossil record, would have spent their lives bobbing vertically in the water column, bouyed by their shells, and with their tentacles dangling downward to capture prey. They wouldn’t have been particularly mobile — that form of a cone hanging vertically in the water isn’t particularly well-streamlined for horizontal motion — so the next big innovation was a rotation of the body axis, swiveling the body axis 90° to turn a cone into a torpedo. There is evidence that many species did this independently.
We can still see vestiges of this rotation in cephalopod embryology. If you look at early embryos of cephalopods (at the bottom of the diagram below), you see the same pattern: they are roughly disc-shaped, with a shell gland on top and a ring of tentacle buds on the bottom. They subsequently extend and elongage along the embryonic dorsal-ventral axis, which becomes the anterior-posterior axis in the adult.
The next division of the cephalopods occurred in the Silurian/Devonian, about 416 million years ago, and it involved those shells. Shells are great armor, and in the cephalopods were also an organ of bouyancy, but they also greatly limit mobility. At that early Devonian boundary, we see the split into the two groups of extant cephalopods. Some retained the armored shells; those are the nautiloids. Others reduced the shell, internalizing it or even getting rid of it altogether; those are the coleoids, the most successful modern group, which includes the squids, cuttlefish, and octopuses. Presumably, one of the driving forces behind the evolution of the coleoids was competition from that other group of big metazoans, the fish.
The nautiloids…well, the nautiloids weren’t so successful, evolutionarily speaking. Only one genus, Nautilus has survived to the modern day, and all the others followed the stem-group cephalopods into extinction.
The coleoids, on the other hand, have done relatively well. The number of species have fluctuated over time, but currently there are about 800 known species, which is respectable. The fish have clearly done better, with about 30,000 extant species, but that could change — there are signs that cephalopods have been thriving a little better recently in an era of global warming and acute overfishing, so we humans may have been giving mobile molluscs a bit of a tentacle up in the long evolutionary competition.
There was another major event in coleoid history. During the Permian, about 276 million years ago, there was a major radiation event, with many new species flourishing. In particular, there was another split: between the Decabrachia, the ten-armed familiar squid, and the Vampyropoda, a group that includes the eight-armed octopus, the cirroctopodes, and Vampyroteuthis infernalis. The Vampyropoda have had another locomotor shift, away from rapid jet-propelled movement to emphasizing their fins for movement, or in the case of the benthic octopus, increasing their flexibility to allow movement through complex environments like the rocky bottom.
Time for the big picture. Here’s the tree of cephalopod evolution, using dates derived from a combination of the available fossil evidence and primarily molecular clocks. The drawings illustrate the shell shape, or in the case of the coleoids, the shape of the internal shell, or gladius, if they have one.
The story and the multiple lines of evidence hang together beautifully to make a robust picture of cephalopod evolution. The authors do mention one exception: Nectocaris. Nectocaris is a Cambrian organism that looks a bit like a two-tentacled, finned squid, which doesn’t fit at all into this view of coleoids evolving relatively late. The authors looked at it carefully, and invest a substantial part of the review discussing this problematic species, and decided on the basis of the morphology of its gut and of the putative siphon that there is simply no way the little beast could be ancestral to any cephalopods: it’s a distantly related lophotrochozoan with some morphological convergence. It’s internal bits simply aren’t oriented in the same way as would fit the cephalopod body plan.
So that’s the state of cephalopod evolution today. I shall be looking forward to the Next Great Paper, and in particular, I want to see more about the molecular biology of tentacles — that’s where the insights about the transition from monoplacophoran to cephalopod will come from, I suspect.
Kröger B, Vinther J, Fuchs D (2011) Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules: Extant cephalopods are younger than previously realised and were under major selection to become agile, shell-less predators. Bioessays doi: 10.1002/bies.201100001.
When I was in London last month, Richard Dawkins and I had a public conversation sponsored by the British Humanist Association, and now you can watch it wherever.
And here’s the Q&A:
One of the advantages of being an old geezer is that I remember watching this kind of thing as it was happening, and having to keep the television on all day and all night to catch the latest news from the Moon. You young whippersnappers just get it handed to you at your convenience on this youtube thingie.
It was awesome then, and it’s awesome now.
(via Carl Zimmer)
I found a recent paper in Nature fascinating, but why is hard to describe — you need to understand a fair amount of general molecular biology and development to see what’s interesting about it. So those of you who already do may be a little bored with this explanation, because I’ve got to build it up slowly and hope I don’t lose everyone else along the way. Patience! If you’re a real smartie-pants, just jump ahead and read the original paper in Nature.
A little general background.
Let’s begin with an abstract map of a small piece of a strand of DNA. This is a region of fly DNA that encodes a gene called svb/ovo (I’ll explain what that is in a moment). In this map, the transcribed portions of the DNA are shown as gray shaded blocks; what that means is that an enzyme called polymerase will bind to the DNA at the start of those blocks and make a copy in the form of RNA, which will then enter the cytoplasm of the cell and be translated into a protein, which does some work in the activities of that cell. So svb/ovo is a small piece of DNA which, in the normal course of events, will make a protein.
Most of the DNA here is not transcribed. Much of it is junk — changing the sequence of those areas has no effect on the protein, and has no effect on the appearance or function of the organism. Some of it, though, is regulatory DNA, and its sequence does matter. The white boxes labeled DG2, DG3, Z, A, E, and 7 are regions called enhancers — they are not translated into protein, but their sequence affects the expression of svb/ovo. One way to think of them is that they are small parking spots for other proteins that will bind to the DNA sequences in each enhancer. These protein/DNA complexes will then fold around to make a little landing zone for the polymerase, to encourage transcription of the svb/ovo gene. This is why this is called regulatory DNA: it doesn’t actually make the svb/ovo protein itself, but it’s important in controlling when and where and how much of the svb/ovo protein will be made.
Now for some jargon; sorry, but you have to know what it is to follow along in the literature. Those little white boxes of regulatory DNA are often called cis factors, because they have to be located on the same strand of DNA as the protein-coding gene in order to work. In general, when we’re talking about cis factors, we’re talking about non-coding regulatory DNA. The complement of that is the actual coding sequence, the little gray boxes in the diagram, and those have the general name of trans factors.
There is a bit of a debate going on about the relative importance of cis and trans mutations in evolution. Proponents of the cis perspective like to point out that cis mutations can be wonderfully subtle and specific; you can make a change in an enhancer and only modify the expression of the gene in one tissue, or even a small part of one tissue, while changing a trans factor causes changes in every tissue that uses that gene product. Also, most of the cis proponents are evo-devo people, scientists who study the small variations in timing and magnitude of gene expression that lead to differences in form, so of course the kinds of changes that affect the stuff we study must be the most important.
Proponents of the trans view can point out that small changes in the coding regions of genes can also produce subtle shifts in what the genes do, and that mutations can also produce very large effects. Those cis changes appear to be little tweaks, while trans changes can run the gamut from non-existent/weak to strong, and so have great power. They also like to point out that most of the data in the literature documents trans changes between species, and that a lot of the evo-devo stuff is speculative.
It’s a somewhat silly debate, because we all know that both cis and trans effects are going to be found important in evolution, in different ways in different organisms, and that arguing about which is more important is kind of pointless — it will depend on which feature and which species you’re looking at. But the debate is also useful as a goad to urge people to look more at the subtleties and ask more questions about those enhancers, as in the paper I’m about to describe.
What is this svb/ovo gene?
This is a drawing of just the back end of a fly larva, and what you should be able to see is that they’re very hairy. Dorsally, there’s a collection of small hairs called trichomes, and ventrally there are some thicker, stouter hairs called denticles. If you destroy the svb/ovo coding region, these hairs don’t form — svb is an important gene for organizing and making hairs on the cuticle of the fly. It’s name should make sense: svb is short for shavenbaby. The gene is responsible for making hairs, but when you break it with a mutation you get embryos and larvae lacking those hairs, a shaven baby.
It also has the synonym of ovo, because it has another important function in the maturation of oocytes, something I’ll skip over entirely. All you need to know is that svb/ovo is actually a large complex gene with multiple functions, and all we care about right now is its function of inducing hair development.
Now let’s look at embryos of two different species of fruit flies, Drosophila melanogaster at the top, and Drosophila sechellia at the bottom. D. melanogaster is clearly hairier than D. sechellia, and you might be wondering if svb is the gene making a difference here, and if you’re following the debate, you might be wondering whether this is a change in the trans coding region or the cis regulatory region.
One way to figure this out is to sequence and compare maps of the svb region in multiple fly species and ask where the actual molecular differences are. This isn’t trivial: D. melanogaster and D. sechellia have been diverging for half a million years, and there have been lots of little changes all over the place, many of them expected to be neutral. What was done to narrow the search was to compare the sequences of five different Drosophila species with hairy embryos to the relatively naked D. sechellia, and ask which changes were unique to the less hairy form.
A hotspot lit up in the comparison: there is one region, about 500 base pairs long, in the enhancer labeled “E” in the diagram at the top of the page, which contained 13 substitutions and one deletion unique to D. sechellia, in 7 clusters. This is very suggestive, but not definitive; these are consistent differences, but we don’t know yet whether these molecular differences cause the differences in hairiness. For that, we need an experiment.
The experiment.
This is the cool part. The investigators built constructs containing the E enhancer coupled to the svb gene and a reporter tag, and inserted those into fly embryos and asked how they affected expression; so they could effectively put the D. sechellia enhancer into D. melanogaster, and the D. melanogaster enhancer into D. sechellia, and ask if they were sufficient to drive the species-specific pattern of svb expression. The answer is yes, mostly: they weren’t perfect copies of each other, suggesting that there are other elements that contribute to the pattern, but the D. sechellia enhancer produced reduced expression in whatever fly carried it, while the D. melanogaster enhancer produced greater expression.
But wait, there’s more! The species differences were caused by differences in 7 clusters within the E enhancer. The authors built constructs in which the mutations in each of the 7 clusters was uniquely and independently inserted, so they could test each mutational change one by one. The answer here was that each of the seven mutations that led to the D. sechellia pattern had a similar effect, reducing very slightly the level of svb expression. Furthermore, they had a synergistic effect: the reduction in hairs when all 7 mutations were present was not simply the sum of the individual effects of each mutation alone.
What does it all mean?
One conclusion of this work is that here is one more clear example of a significant morphological difference between species that was generated by molecular modification of cis regulatory elements. Hooray, one more data point in the cis/trans debate!
Another interesting observation is that this is a phenotype that was built up gradually, by a set of small changes to an enhancer element. D. sechellia gradually lost its trichome hairs by the accumulation of single-nucleotide changes in regulatory DNA, each of which contributed to the phenotype — a very Darwinian pattern of change.
By modifying the regulatory elements, evolution can generate distinct, focused variations. Knocking out the entirety of the svb gene is disastrous, not only removing hairs but also seriously affecting fertility. The little tweaks provided by changes to the enhancer region mean that morphology can be fine-tuned by chance and selection, without compromising essential functions like reproduction. In the case of these two species of flies, D. sechellia can have a functional reproductive system, the full machinery to make functional hairs, but at the same time can turn off dorsal trichomes while retaining ventral denticles.
It all fits with the idea that fundamental aspects of basic morphology are going to be defined, not by the raw materials used to build them, but by the regulation of timing and quantity of those gene products — that the rules of development are defined by the regulatory activity of genes, not entirely by the coding sequences themselves.
Frankel N, Erezyilmaz DF, McGregor AP, Wang S, Payre F, Stern DL (2011) Morphological evolution caused by many subtle-effect substitutions in regulatory DNA. Nature 474(7353):598-603.
Phil Senter has published the most deviously underhanded, sneaky, subtle undermining of the creationist position I’ve ever seen, and I applaud him for it. What he did was to take them seriously, something I could never do, and treat their various publications that ape the form of the scientific literature as if they actually were real science papers, and apply their methods consistently to an analysis of taxonomy. So on the one hand, it’s bizarre and disturbing to see the like of Ken Ham, Jerry Bergman, and Henry Morris get actual scientific citations, but on the other hand, seeing their claims refuted using their own touted methods is peculiarly satisfying.
Senter has published a paper in the Journal of Evolutionary Biology that takes their claims at face value and analyzes dinosaur morphology using their own methods. ‘Baraminologists’ have published a set of taxonomic tools that use as input a matrix of morphological characters for an array of animals, and then spits out numbers that tell whether they were similar enough to be related. You can guess what the motivation for that is: they want to claim that Noah didn’t have to carry representatives of every dinosaur species on the Ark, but only representatives of each ‘kind’, which then diversified rapidly after the big boat landed to generate all the different species found in the fossil record.
The problem for them is that Senter found that it works far too well. Using creationist techniques, all of the Dinosauria reduce to…eight kinds. That makes the boat haulage problem relatively even easier.
Here is the summary diagram, illustrating the derived creationist tree of common descent. Oops.
At first, the results of the taxon correlation analyses appear to imply good news for the creationist world view, on several fronts. First, seven major dinosaurian groups (birdlike coelurosaurs, Tazoudasaurus + Eusauropoda, Stegosauria, Ankylosauridae, Neoceratopsia, Hadrosauridae and basal Hadrosauriformes) are separated from the rest of Dinosauria by morphological gaps (Fig. 15). Creationist inferences that variety within Eusauropoda (Morris, 1999) and Ceratopsidae (Ham, 2009) represent diversification within separately created kinds are congruent with these results. Second, each morphologically continuous group found by taxon correlation includes at least some herbivores. This is congruent with the creationist assertion that all carnivorous animals are descendants of originally herbivorous ancestors (Unfred, 1990; Gish, 1992; Ham, 1998, 2006, 2009; Larsen, 2001; McIntosh & Hodge, 2006). Third, although creationists have answered the problem of room on Noah’s ark for multiple pairs of gigantic dinosaurs by asserting that only about 50 ‘created kinds’ of dinosaurs existed (Ham, 1998, 2001, 2006, 2009; Morris, 1999), the problem is solved even better by the results of this study, in which only eight dinosaur ‘kinds’ are found.
Awww. I guess I’m going to have to become a creationist, now that the evidence shows that dinosaurs are related by common descent…oh, hey, wait. Isn’t that what evolution says? And isn’t that easier to accommodate within the idea that they did this over millions of years, rather than the freakishly unrealistic hyper-speciation within a few thousand years that the creationists insist on?
However, a second look reveals that these results are at odds with the creationist view. Whether there were eight dinosaur ‘kinds’ or 50, the diversity within each ‘kind’ is enormous. Acceptance that such diversity arose by natural means in only a few thousand years therefore stretches the imagination. The largest dinosaurian baramin recovered by this study includes Euparkeria, basal ornithodirans (Silesaurus and Marasuchus), basal saurischians, basal ornithischians, basal sauropodomorphs, basal thyreophorans, nodosaurid ankylosaurs, pachycephalosaurs, basal ceratopsians, basal ornithopods and all but the most birdlike theropods in an unbroken spectrum of morphological continuity. The creationist viewpoint allows for diversification within baramins, but the diversity within this morphologically continuous group is extreme. Also, the inclusion of the Middle Triassic non-dinosaurs Euparkeria and Marasuchus within the group is at odds with the creationist claim that fossil representatives of the predinosaurian, ancestral stock from which dinosaurs arose have never been found (DeYoung, 2000; Ham, 2006; Bergman, 2009).
So, effectively, these results, made using the creationists own tools, demonstrate a genetic relationship between a diverse group of animals that evolution predicted, and confronts young earth creationists with the problem of a kind of frantically prolific speciation that is unimaginably rapid. If species are that fluid and can change that rapidly, their own claims of fixity of species are patently wrong.
The final word:
The results of this study indicate that transitional fossils linking at least four major dinosaurian groups to the rest of Dinosauria are yet to be found. Possibly, some creationist authors will hail this finding as evidence of special creation for those four groups. However, such enthusiasm should be tempered by the finding here that the rest of Dinosauria–including basal members of all major lineages–are joined in a continuous morphological spectrum. This confirms the genetic relatedness of a very broad taxonomic collection of animals, as evolutionary theory predicts, ironically by means of a measure endorsed and used by creation science.
This is so wonderfully, evilly devious. Superficially, it seems to support creationist methods—but what it actually is is a grand reductio ad absurdam. Laugh wickedly at it now, but laugh even harder when you see creationists citing this paper in the future, as you know they will.
Senter P (2011) Using creation science to demonstrate evolution 2: morphological continuity within Dinosauria. J Evol Biol. doi: 10.1111/j.1420-9101.2011.02349.x.
