The heartbreaking beauty of development

This is a spectacular video of the development of Clypeaster subdepressus, also called a sand dollar or sea biscuit. These are stunningly beautiful creatures (as are we all, of course), and it is so cool to see them changing here. The video starts with a little echinoderm porn — these animals are profligate with their gametes — and then we see early divisions, gastrulation, the formation of the pluteus larva, metamorphosis into Aristotle’s lantern (one of the more charming names for a developmental stage), and into an ungainly spiky juvenile.

This is why some of us are developmental biologists: it’s all about the exotic weirdness and delicate loveliness of transformation.


A Sea Biscuit’s Life from Bruno Vellutini on Vimeo.

Charismatic cephalofauna

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Christine Huffard sent me a note alerting me to the publication of her latest paper, and she thought I might be interested because I “seem to like cephalopods”. Hah. Well. I’ve noticed that Dr Huffard seems to have some small affection for the tentacled beasties herself.

The paper follows on an old tradition and an old problem. While people have no problem distinguishing human individuals, we have a tough time telling one individual animal from another. This perceptual difficulty complicates problems of studying variations in behavior or physiology, or monitoring numbers and behavior, in natural populations. One solution is tagging or marking the animals in some way, but that always has the risk of changing or harming the disturbed animals — non-invasive procedures are much preferred. This is an especially difficult problem with small animals, like zebrafish or small octopus; I’ve struggled myself with trying to track individual fish in experiments.

I came up with one solution, and Huffard et al. have come up with something similar: humans can be trained to recognize distinctive individual variations, and learn to identify single animals. In this paper, they describe a pattern of white pigmented regions that are consistent within single animals of the species Wunderpus photogenicus…and as you might guess, that is a great excuse to put together a collection of photographs of these aptly named animals.

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Entropy and evolution

Blogging on Peer-Reviewed Research

One of the oldest canards in the creationists’ book is the claim that evolution must be false because it violates the second law of thermodynamics, or the principle that, as they put it, everything must go from order to disorder. One of the more persistent perpetrators of this kind of sloppy thinking is Henry Morris, and few creationists today seem able to get beyond this error.

Remember this tendency from order to disorder applies to all real processes. Real processes include, of course, biological and geological processes, as well as chemical and physical processes. The interesting question is: “How does a real biological process, which goes from order to disorder, result in evolution. which goes from disorder to order?” Perhaps the evolutionist can ultimately find an answer to this question, but he at least should not ignore it, as most evolutionists do.

Especially is such a question vital, when we are thinking of evolution as a growth process on the grand scale from atom to Adam and from particle to people. This represents in absolutely gigantic increase in order and complexity, and is clearly out of place altogether in the context of the Second Law.

As most biologists get a fair amount of training in chemistry, I’m afraid he’s wrong on one bit of slander there: we do not ignore entropy, and are in fact better informed on it than most creationists, as is clearly shown by their continued use of this bad argument. I usually rebut this claim about the second law in a qualitative way, and by example — it’s obvious that the second law does not state that nothing can ever increase in order, but only that an decrease in one part must be accompanied by a greater increase in entropy in another. Two gametes, for instance, can fuse and begin a complicated process in development that represents a long-term local decrease in entropy, but at the same time that embryo is pumping heat out into its environment and increasing the entropy of the surrounding bit of the world.

It’s a very bad argument they are making, but let’s consider just the last sentence of the quote above.

This represents in absolutely gigantic increase in order and complexity, and is clearly out of place altogether in the context of the Second Law.

A “gigantic increase in order and complexity” … how interesting. How much of an increase? Can we get some numbers for that?

Daniel Styer has published an eminently useful article on “Entropy and Evolution” that does exactly that — he makes some quantitative estimates of how much entropy might be decreased by the process of evolution. I knew we kept physicists around for something; they are so useful for filling in the tricky details.

The article nicely summarizes the general problems with the creationist claim. They confuse the metaphor of ‘disorder’ for the actual phenomenon of entropy; they seem to have an absolutist notion that the second law prohibits all decreases in entropy; and they generally lack any quantitative notion of how entropy actually works. The cool part of this particular article, though, is that he makes an estimate of exactly how much entropy is decreased by the process of evolution.

First he estimates, very generously, how much entropy is decreased per individual. If we assume each individual is 1000 times “more improbable” than its ancestor one century ago, that is, that we are specified a thousand times more precisely than our great-grandparents (obviously a ludicrously high over-estimate, but he’s trying to give every advantage to the creationists here), then we can describe the reduction in the number of microstates in the modern organism as:

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Now I’m strolling into dangerous ground for us poor biologists, since this is a mathematical argument, but really, this is simple enough for me to understand. We know the statistical definition of entropy:

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In the formula above, kB is the Boltzmann constant. We can just plug in our estimated (grossly overestimated!) value for Ω, have fun with a little algebra, and presto, a measure of the change in entropy per individual per century emerges.

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Centuries are awkward units, so Styer converts that to something more conventional: the entropy change per second is -3.02 x 10-30 J/K. There are, of course, a lot of individual organisms on the planet, so that number needs to be multiplied by the total number of evolving organism, which, again, we charitably overestimate at 1032, most of which are prokaryotes, of course. The final result is a number that tells us the total change in entropy of the planet caused by evolution each second:

-302 J/K

What does that number mean? We need a context. Styer also estimates the Earth’s total entropy throughput per second, that is, the total flux involved from absorption of the sun’s energy and re-radiation of heat out into space. It’s a slightly bigger number:

420 x 1012 J/K

To spell it out, there’s about a trillion times more entropy flux available than is required for evolution. The degree by which earth’s entropy is reduced by the action of evolutionary processes is miniscule relative to the amount that the entropy of the cosmic microwave background is increased.

This is very cool and very clear. I’m folding up my copy of Styer’s paper and tucking it into my copy of The Counter-Creationism Handbook, where it will come in handy.

Styer DF (2008) Entropy and evolution. Am J Phys 76(11):1031-1033.

Oxygen has eyelashes!

It’s cute: this exercise in molecular visualization has been all dolled up with anthropomorphized atoms to sneak it into kids’ attention spans.

I can’t be entirely dismissive, though. There’s some cool stuff lurking in the backgrounds of these scenes, it’s just unfortunate that the goofy cartoon stuff is always being placed front and center.

I am kind of hoping that the creationists, with all their talk of cars and buses and traffic lights in the cell, steal this video. I can almost imagine Michael Behe exclaiming that the sophisticated facial expressions of atoms are evidence of intent and design.

The Mason’s Apprentice

My latest Seed column slipped quietly onto the interwebs last week — it’s an overview of how the glues that hold multicellular organisms together first evolved in single celled creatures, represented today by the choanoflagellates.

Just as a teaser, the next print edition that should be coming out soon will continue the focus on enlightening organisms of remarkable simplicity with a description of the results of the Trichoplax genome. Get it! You will also be rewarded with a great issue focusing on science policy.

Epidexipteryx

The Mesozoic was inhabited by some strange-looking critters, and here’s another example: a Jurassic dinosaur called Epidexipteryx, which has spiky teeth, big claws, fluffy feathers all over its body, and four long decorative feathers coming off a stumpy tail. It resembles a particularly ugly bird with a nasty bite, but it couldn’t fly — none of the feathers covering its forelimbs are pennaceous, but are more like an insulating fur. Or, alternatively, its feathers were all about display, a possibility suggested by the odd long feathers of the tail. Here are the bones; as you can see, the integument is remarkably well preserved, with a scruffy ruff of short, non-shafted feathers over the body and limbs, and a surprising spray of just four very long feathers coming off the tail.

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(Click for larger image)
a, Main slab; b, c, skull in main slab (b) and counterslab (c); d, four elongate ribbon-like tail feathers; b’, c’, line drawings of b and c, respectively. Abbreviations: l1, l2 and l7, 1st, 2nd and 7th left teeth of upper jaw; l1′, r1′ and r5′, 1st left, 1st right and 5th right teeth of lower jaw; l2 and r2, 2nd left and right teeth of upper jaw.

And here’s what it would have looked like in life (only the colors are imaginary). It would have been about the size of a pigeon — I think a pack of these scurrying about New York’s Times Square would be both scenic and would quickly clean up the pigeon problem there.

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For all the details, read the write-up on Tetrapod Zoology.

Zhang F, Zhou Z, Xu X, Wang X, Sullivan C (2008) A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers. Nature 455:1105-1108.

Here’s something else Darwin didn’t have

Tracking of the HMS Beagle by a manned space station. I don’t know why; maybe those pre-Victorian Space Engineers had their steam-powered space-stations all tied up trying to find the source of the Nile or plotting invasion routes into Afghanistan, or something. This time around our 21st century panjandrums of outer space have their priorities a bit more in focus, and NASA has committed to using the ISS to watch the new Voyage of the Beagle. Read the Beagle Project for more details.

I’m just relieved that finally we’ve found something useful for these space nuts to do — providing supplemental assistance to a biological and historical project, instead of noodling around staring at space rocks, space debris, and space vapor.

A step closer to efficient solar power?

I have to confess that the title of this paper, The remarkable influence of M2δ to thienyl π conjugation in oligothiophenes incorporating MM quadruple bonds, is Greek to me, that the abstract was impenetrable, and the paper itself was thoroughly incomprehensible. I’m a biologist, not a chemist or materials engineer! Fortunately, there are a couple of summaries that simplify the explanation enough that I can understand the gist of it, and it’s cool stuff. Researchers have made a new material that promises to greatly increase the efficiency of solar cells. It works by collecting photons over a wider spectrum of wavelengths and by using both fluorescence and phosphorescence to create an electron flow, allowing it to both collect more energy per unit area and facilitating the production of current.

This is promising news, and also illustrates why we need to fund basic research — these are the kinds of discoveries that can’t be simply planned and forced into existence, but require the liberty of the research enterprise to explore new ideas freely.

Don’t get too excited just yet, though. The research has uncovered useful properties of a combination of molecules that have only been tested in minute quantities. It remains to be seen if it can be scaled up efficiently, if it can be made cost-effective, and whether it can be simply made to work at a practical level. It’s still an exciting idea — they’re talking about nearly 100% efficiency.

Sometimes ink is just ink

At first, I was a bit disappointed in this result, but then I realized it’s actually rather interesting in a negative sense. Investigators tested the effects of squid ink on other squid; the entirely reasonable idea being that it could contain an alarm pheromone that would have the function of alerting neighboring squid in the school of trouble. It works — adding ink to a tank of Caribbean reef squid sends them scurrying away.

However, when they removed the pigments from the ink and added that, the squid couldn’t care less. That says there is no chemical signal, only a visual signal.

That makes sense, I suppose — oceans are big and would dilute any chemical signal fairly rapidly, so pheromones would only work well over a fairly short range (although some fish certainly do have extremely sensitive olfactory senses, so it could be done). Still, Aplysia eject some potent chemical signals with their secretions, which work when directly squirted into the face of a predator, so there was a chance the cephalopods might have evolved something similar.

Will the availability of C-sections give humans bigger brains?

Blogging on Peer-Reviewed Research

While Steve Jones might think human evolution has stopped, I have to say that that is impossible. If human technology removes a selective constraint, that doesn’t stop evolution — it just opens up a new degree of freedom and allows change to carry us in a novel direction.

One interesting potential example is the availability of relatively safe Cesarean sections. Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults. While our brains have gotten larger over evolutionary time, they have not gotten proportionally larger in utero, because large-headed babies increase the difficulty of labor and cause increased mortality in childbirth. If childbirth could bypass the pelvic bottleneck, that would allow for fetal heads to grow larger without increasing the risk of killing mother and/or child.

And childbirth is a risky proposition for women; 529,000 die every year from this natural process (although only about 1% of those deaths occur in places where women have access to good, modern medical facilities — hooray for modern medicine). About 8% of those deaths occur from obstructed labor, where the fetus is unable to proceed through the birth canal for various reasons, and these are the kinds of birth problems that can be circumvented by C-sections. In practice, teaching health care workers how to carry out emergency C-sections has been tested in regions in Africa, where it has actually worked well at reducing maternal mortality.

This is the subject of an article by Joseph Walsh in the American Biology Teacher, which suggests that C-sections will have an effect on human evolution.

“Nothing in biology makes sense except in the light of evolution.” This was the title of an essay by geneticist Theodosius Dobzhansky writing in 1973. Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from natural selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelves will get smaller because there is nothing to prevent it.

The evidence so far is entirely circumstantial, but Walsh makes an interesting case. There are several correlations that imply an effect, but I can’t help but think there are alternative explanations that may swamp out any heritable, evolutionary effect. The kinds of evidence he describes are:

  • A known trend for increasing birth weight in the US, by about 40 g over 18 years in one study. It’s there, all right, but these studies don’t demonstrate a genetic component to increased size — it could be a consequence of better nutrition and medical care.

  • An increasing frequency of C-sections. Again, this isn’t necessarily genetically based at all, but could be a consequence of fads in medicine, or social factors, such as an increase in the likelihood of medical malpractice suits making doctors more cautious.

  • Walsh describes a couple of studies that seem to show that cephalopelvic disproportion (small pelvis or large babies or both together) does have a genetic component. So at least it is likely that there are heritable variations in these parameters that could influence the likelihood of obstructed labor.

  • There is statistical variation in neo-natal mortality that varies with birth weight in a suggestive way. Low birth weight clearly puts infants at risk, and there is an optimum weight around 3600 grams for newborns that minimizes mortality. Death rates also rise with increasing birth weight above the optimum. There is some data that suggest that availablity of modern medical care and C-sections reduces infant mortality at larger birth weights.

That increasing availability of C-sections might lead to an evolutionary shift towards increasing cranial capacity at birth is a reasonable hypothesis, but I’m not convinced that it has been convincingly demonstrated yet. There are too many variables that effect brain size at birth to make a clean analysis possible; in addition, many of the measures are indirect. Often, we use birth weight as a proxy for cranial capacity, and that means the numbers and correlations are sloppier than they should be. Many of the measurements made are of factors that are readily influenced by the environment, which makes it difficult to imply that these are the product of genetics.

So the idea is weakly supported, but tantalizing. Even as a purely theoretical exercise, though, what it does say is that it is obvious that human culture cannot end human evolution…all it can do is shape the direction in which it can occur.

Walsh J (2008) Evolution & the Cesarean Section Rate. The American Biology Teacher 70(7):401-404.