Snake segmentation

Blogging on Peer-Reviewed Research

Life has two contradictory properties that any theory explaining its origin must encompass: similarities everywhere, and differences separating species. So far, the only theory that covers both beautifully and explains how one is the consequence of the other is evolution. Common descent unites all life on earth, while evolution itself is about constant change; similarities are rooted in our shared ancestry, while differences arise as lineages diverge.

Now here’s a new example of both phenomena: the development of segmentation in snakes. We humans have 33 vertebrae, zebrafish have 30-33, chickens have 55, mice have 65, and snakes have up to 300 — there’s about a ten-fold range right there. There are big obvious morphological and functional differences, too: snakes are sinuous slitherers notable for their flexibility, fish use their spines as springs for side-to-side motion, chickens fuse the skeleton into a bony box, and humans are upright bipeds with backaches. Yet underlying all that diversity is a common thread, that segmented vertebral column.

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(Click for larger image)
Vertebral formula and somitogenesis in the corn snake.
a, Alizarin staining of a corn snake showing 296 vertebrae, including 3
cervical, 219 thoracic, 4 cloacal (distinguishable by their forked
lymphapophyses) and 70 caudal. b, Time course of corn snake development
after egg laying (118-somite embryo on the far left) until the end of
somitogenesis (~315 somites).

The similarities are a result of common descent. The differences, it turns out, arise from subtle changes in developmental timing.

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Epigenetics

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Epigenetics is the study of heritable traits that are not dependent on the primary sequence of DNA. That’s a short, simple definition, and it’s also largely unsatisfactory. For one, the inclusion of the word “heritable” excludes some significant players — the differentiation of neurons requires major epigenetic shaping, but these cells have undergone a terminal division and will never divide again — but at the same time, the heritability of traits that aren’t defined by the primary sequence is probably the first thing that comes to mind in any discussion of epigenetics. Another problem is the vague, open-endedness of the definition: it basically includes everything. Gene regulation, physiological adaptation, disease responses…they all fall into the catch-all of epigenetics.

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Altenberg 2008 is over

Massimo Pigliucci has posted the notes, parts 1, 2, and 3, from the Altenberg meeting that was unfortunately over-hyped by the creationist crowd (no blame for that attaches to the organizers of this meeting). It sounds like it was a phenomenally interesting meeting that was full of interesting ideas, but from these notes, it was also clearly a rather speculative meeting — not one that was trying to consolidate a body of solid observations into a coherent explanation, but one that was instead trying to define promising directions for an expansion of evolutionary theory. That’s also the message of the concluding statement of the meeting.

A group of 16 evolutionary biologists and philosophers of science convened at the Konrad Lorenz Institute for Evolution and Cognition Research in Altenberg (Austria) on July 11-13 to discuss the current status of evolutionary theory, and in particular a series of exciting empirical and conceptual advances that have marked the field in recent times.

The new information includes findings from the continuing molecular biology revolution, as well as a large body of empirical knowledge on genetic variation in natural populations, phenotypic plasticity, phylogenetics, species-level stasis and punctuational evolution, and developmental biology, among others.

The new concepts include (but are not limited to): evolvability, developmental plasticity, phenotypic and genetic accommodation, punctuated evolution, phenotypic innovation, facilitated variation, epigenetic inheritance, and multi-level selection.

By incorporating these new results and insights into our understanding of evolution, we believe that the explanatory power of evolutionary theory is greatly expanded within biology and beyond. As is the nature of science, some of the new ideas will stand the test of time, while others will be significantly modified. Nonetheless, there is much justified excitement in evolutionary biology these days. This is a propitious time to engage the scientific community in a vast interdisciplinary effort to further our understanding of how life evolves.

That’s a little soft — there are no grand reformulations of the neo-Darwinian synthesis in there, nor is anyone proposing to overturn our understanding of evolution — but that’s what I expected. It’s saying that there are a lot of exciting ideas and new observations that increase our understanding of the power of evolution, and promise to lead research in interesting new directions.

Unfortunately, one reporter has produced an abominably muddled, utterly worthless and uninformed account of the Altenberg meeting that has been picked up by many crackpots to suggest that evolution is in trouble. This not only ignores a fundamental property of science — that it is always pushing off in new directions — but embarrassingly overinflates the importance of this one meeting. This was a gathering of established scientists with some new proposals. It was not a meeting of the central directorate of the Darwinist cabal to formulate new dogma.

Where one ignorant kook dares to assert her inanity, you know the Discovery Institute will stampede after her. Both Paul Nelson and now Casey Luskin have cited her lunatic distortions favorably. Luskin’s account is egregiously incompetent, as we’ve come to expect — he even thinks Stuart Pivar was an attendee. Pivar is an eccentric New York art collector, heir to a septic tank fortune, who has no training in science and whose “theory” is a nonsensical bit of guesswork that is contradicted by observations anyone can make in a basic developmental biology lab. He was not at the meeting. No one in their right mind would even consider inviting him to such a serious event. Maybe if it was a birthday party and they needed someone to make balloon animals, he’d be a good man to have on hand.

Now we can move beyond the garbled hype of the creationists. Pigliucci lists several concepts up there that have promise for further research, and that may help us understand evolution better. That’s the productive result of the meeting, and the only part that counts. Those concepts are also going to be discussed by many other scientists at many other meetings — even I talked about some of them recently — but don’t let the liars on the creationist side confuse you into thinking that the fact that scientists are talking about new ideas is a sign that evolution is in crisis. Talking about new ideas is normal science.

Someone was awake at GECCO

Hooray, I have evidence that at least one person didn’t fall asleep at my GECCO keynote: here’s a summary. He even managed to pay attention right up to the dramatic conclusion, which I usually keep a secret, but now the beans have been spilled.

Wait…the biology-oriented keynote at last year’s GECCO was a panel with Lewis Wolpert, Steve Jones, and Richard Dawkins? I would have been a lot more nervous if I’d known I was following up that act.

Atlanta GECCO 2008

I’m on my way home, and am actually using a fast internet connection at the airport — I’d forgotten what it was like! I quickly uploaded a few essential files, and my mail software is downloading my email. Unfortunately, I’d need a really fast connection to handle all that — the number of messages pouring in might actually hit 5 digits. If you’re hoping for a reply to anything, you might well be out of luck here.

Atlanta has been very pleasant, with friendly people and good company. I’ll have to come back sometime. The meeting itself was challenging for a mere biologist, but I might have absorbed a few glimmerings. At least it’ll help me dig into the literature a little more.

As for my talk, and since I haven’t had time to put much science here lately, I’m making my GECCO 2008 talk available as a pdf. These presentations are always a little cryptic when handed out without my explanatory overview, but at least in this one I’ve included my presenter notes, which might help a little bit. The first half is an overview of some concepts in evo devo, which includes those little reminders of what I was supposed to say; the last half is a description of two experiments, and I’m afraid my notes are a little thin there — the data in the research always seems self-explanatory to me. Sorry about that, you should have registered for the conference!

Email download complete: it didn’t quite hit 5 digits, only 9865 messages in the last few days. Maybe if I included the spam that gmail filters out for me…

Ack! I couldn’t add this note from the airport because “Your computer was automatically blacklisted (blocked) by the network due to an abnormal amount of activity originating from your connection.” Curse you, Boingo! What good are you if I can’t even download email without you suspecting I’m up to no good?

Mad Scientist contest

There’s a new contest you can enter: Build a Lifeform. A real one.

Yes, it can be done now…or at least, we can insert new capabilities into existing organisms. Before you get too excited, though, most of this work involves directed tweaking of phage or E. coli, which is powerful stuff, but far removed from the dream of building Kelly LeBrock in my garage.

We’re going to need another decade or so before we can do that.

Ice, Mud and Blood

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I’m very fond of Chris Turney‘s book, Bones, Rocks, and Stars. It’s a slender, simple description of the many tools scientists use to figure out how old something is, and when arguing with young earth creationists, it’s become the first thing I recommend to them. It’s short and easy to read, and focuses on explaining how dating methods work.

Turney has a new book out: Ice, Mud and Blood: Lessons from Climates Past(amzn/b&n/abe/pwll). This is the one you’ll be able to hand to climate change denialists, and it’s a winner.

Its virtues are the same as his previous book, the careful documentation of exactly how we know what we know, and less dictation of the conclusions. This is useful, because as we all know, climate is a phenomenon that shows a lot of variability, exhibits patterns in its history, and also has large degrees of uncertainty, phenomena that denialists can seize upon to magnify that uncertainty into a basis for an unwarranted rejection of well-supported hypotheses. So while we can see distinct variations from simple linear uniformity of climate change like the Little Ice Age and the Medieval Warm Period, that doesn’t change the fact that greenhouse gases profoundly influence earth’s temperature, and it’s clear that CO2 has risen to levels the planet hasn’t seen in at least 650,000 years. The past tells us what we can expect in the future, and it’s grounds for serious concern. Yes, Turner comes down firmly on the side of an anthropogenic cause for the current trend of global warming, and he explains exactly why, step by step.

Where Turner departs from the formula of his last book, though, is that this one, while still fairly short, is much denser and more technical. Anyone can read it — I managed, despite knowing next to nothing about climatology — but it’s not the kind of thing you’ll be able to do in an evening or two of light reading. There are a few places where the level of detail slowed me down to a steady slog rather than a fast flit, but face it, any book that tries to untangle ocean currents, monsoons, El Nino, and past current reversals is going to occasionally demand the same level of studious, focused attention you’d need to clean up a snarled fishing reel. This is a book that is describing some extraordinarily complicated stuff.

If you want just one book, not too thick or too technical, that will give you the intellectual tools to at least understand what the climate change experts are talking about, this is the one. I recommend bringing it to the beach and reading it there — you’ll appreciate the rising tide and the ocean beaches a little more, and perhaps regard them with a little more respectful dread.

Evolving proteins in snakes

Blogging on Peer-Reviewed Research

We’ve heard the arguments about the relative importance of mutations in cis regulatory regions vs. coding sequences in evolution before — it’s the idea that major transitions in evolution were accomplished more by changes in the timing and pattern of gene expression than by significant changes in the genes themselves. We developmental biologists tend to side with the cis-sies, because timing and pattern are what we’re most interested in. But I have to admit that there are plenty of accounts of functional adaptation in populations that are well-founded in molecular evidence, and the cis regulatory element story is weaker in the practical sense that counts most in science (In large part, I think that’s an artifact of the tools — we have better techniques for examining expressed sequences, while regulatory elements are hidden away in unexpressed regions of the genome. Give it time, the cis proponents will catch up!)

This morning, I was sent a nice paper that describes a pattern of functional change in an important molecule — there is absolutely no development in it. It’s a classic example of an evolutionary arms race, though, so it’s good that I mention this important and dominant side of the discipline of evolutionary biology — I know I leave the impression that all the cool stuff is in evo-devo, but there’s even more exciting biology outside the scope of my tunnel vision. Also, this paper describes a situation and animals with which I am very familiar, and wondered about years ago.

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Ventastega

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The paleontologists are going too far. This is getting ridiculous. They keep digging up these collections of bones that illuminate tetrapod origins, and they keep making finer and finer distinctions. On one earlier side we have a bunch of tetrapod-like fish — Tiktaalik and Panderichthys, for instance — and on the later side we have fish-like tetrapods, such as Acanthostega and Ichthyostega. Now they’re talking about shades of fishiness or tetrapodiness within those groups! You’d almost think they were documenting a pattern of gradual evolutionary change.

The latest addition is a description of Ventastega curonica, a creature that falls within the domain of the fish-like tetrapods, but is a bit fishier than other forms, so it actually bridges the gap between something like Tiktaalik and Acanthostega. We look forward to the imminent discovery of yet more fossils that bridge the gap between Ventastega and Tiktaalik, and between Ventastega and Acanthostega, and all the intermediates between them.

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Amphioxus and the evolution of the chordate genome

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This is an amphioxus, a cephalochordate or lancelet. It’s been stained to increase contrast; in life, they are pale, almost transparent.

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It looks rather fish-like, or rather, much like a larval fish, with it’s repeated blocks of muscle arranged along a stream-lined form, and a notochord, or elastic rod that forms a central axis for efficient lateral motion of the tail…and it has a true tail that extends beyond the anus. Look closely at the front end, though: this is no vertebrate.

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It’s not much of a head. The notochord extends all the way to the front of the animal (in us vertebrates, it only reaches up as far as the base of the hindbrain); there’s no obvious brain, only the continuation of the spinal cord; there isn’t even a face, just an open hole fringed with tentacles. This animal collects small microorganisms in coastal waters, gulping them down and passing them back to the gill slits, which aren’t actually part of gills, but are components of a branchial net that allows water to filter through while trapping food particles. It’s a good living — they lounge about in large numbers on tropical beaches, sucking down liquids and any passing food, much like American tourists.

These animals have fascinated biologists for well over a century. They seem so primitive, with a mixture of features that are clearly similar to those of modern vertebrates, yet at the same time lacking significant elements. Could they be relics of the ancestral chordate condition? A new paper is out that discusses in detail the structure of the amphioxus genome, which reveals unifying elements that tell us much about the last common ancestor of all chordates.

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