I am so going to hell for linking to this. If you love Jesus, don’t click on that.
(via Stupid Evil Bastard)
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
I am so going to hell for linking to this. If you love Jesus, don’t click on that.
(via Stupid Evil Bastard)
The speakers for the science panel at the Yearly Kos convention have been announced, and they are Chris Mooney, some goofy guy named PZ, and Wendy Northcutt. We’re going to Vegas! Since I only drink in moderation, never gamble, and the only showgirl I want to see is my wife (who will be coming along), I’m not going to have much to do other than castigate wobbly Democrats, and maybe argue with those annoying progressive Christians. There better be some firebreathing, let me tell you…the only magic show I want to see is a bunch of Democrats with spines, and skip the animal tamers—I want to see some fierce beasts breaking loose.
First, a tiny bit of quantitative morphological data you can find in just about any comparative anatomy text:
| mammal | number of vertebrae | ||||
|---|---|---|---|---|---|
| cervical | thoracic | lumbar | sacral | caudal | |
| horse | 7 | 18 | 6 | 5 | 15-21 |
| cow | 7 | 13 | 6 | 5 | 18-20 |
| sheep | 7 | 13 | 6-7 | 4 | 16-18 |
| pig | 7 | 14-15 | 6-7 | 4 | 20-23 |
| dog | 7 | 13 | 7 | 3 | 20-23 |
| human | 7 | 12 | 5 | 5 | 3-4 |
The number of thoracic vertebrae varies quite a bit, from 9 in a species
of whale to 25 in sloths. The numbers of lumbar, sacral, and more caudal vertebrae also show considerable variation. At the same time, there is a surprising amount of invariance in the number of cervical vertebrae in mammals — as every schoolkid knows, even giraffes have exactly the same number of vertebrae in their necks as we do. What makes this particularly striking is that other vertebrates have much more freedom in their number of cervical vertebrae; swans can have 22-25. I was idly wondering why mammals were so limited, and stumbled onto a couple of papers that addressed exactly that question (Galis & Metz, 2003; Galis, 1999). Galis’s explanation is that it is a developmental constraint that may have something to do with the incidence of cancer.
Development is an intricately choreographed process that treads a dangerous line. On one side is stability; but development is in many ways a destabilizing process, in which cells have to change their path and form new tissues, and stability is not compatible with it. On the other side is chaos, unregulated proliferation — cancer. During development, the organism has to foster proliferation and change to a greater degree than it can tolerate later, and that loosening of constraints represents a danger. Galis suggests that one reason we mammals may always have 7 cervical vertebrae is that the regulatory genes that specify the number of vertebrae are coupled to processes that otherwise regulate cell fates, and that modifications to those genes that would cause variation in vertebra number would also lead to unacceptable increases in the frequency of embryonal cancers.
This isn’t at all an improbable idea. Genes exhibit bewilderingly complex patterns of expression, and pleiotropy (the regulation of multiple phenotypic characters by a single gene) is the rule, not the exception. The Hox genes, the particular genes that control the identity of regions along the length of the animal, are known to switch on and off in proliferating mammalian cell lines in culture. Perhaps the Hox genes involved in defining cervical vertebrae are somehow also involved in controlling cell proliferation, making them dangerous targets for evolution to tinker with?
Galis provides several lines of evidence that this is the case. To see whether variation in cervical vertebra number leads to increased incidence of cancer, we need to look for instances of variation in mammalian vertebrae.
There isn’t much variation in cervical vertebra number, though. There is an exception: sometimes, the 7th cervical vertebra is found to undergo a partial homeotic transformation and forms a pair of ribs, which are normally found only on thoracic vertebrae. Humans develop cervical ribs with a frequency of about 0.2%; do they also develop cancers? The answer is yes, with a frequency 125 times greater than the general population.
Another place to look would be in phylogenetic variation — between groups rather than within a population. It turns out that there are two groups of mammals that do have a non-canonical number of cervical vertebrae: one manatee genus and two genera of sloths. No data is available on frequencies of embryonal cancers in either, and Galis reports that manatees at least seem to have a low incidence of cancer. One explanation is that both sloths and manatees have exceptionally slow metabolic rates, which in itself will reduce the frequency of cancer, since it will reduce the rate of oxidation damage; the idea is that this low cancer rate may have made these organisms more tolerant of variation in these genes.
An open question is how birds can have greater variability in the number of cervical vertebrae — they certainly don’t have low metabolic rates. One suggestion is that the coupling between these particular Hox genes and a predilection for cancer is unique to mammals. Another possibility is that birds possess other, unidentified mechanisms that reduce free radical production, reduces oxidative damage, and makes them relatively cancer-free. Galis cites several studies that show that birds do seem to be less severely afflicted with cancers than us mammals.
It’s an interesting idea, but the evidence so far is a collection of correlations. I’d be interested in seeing some direct analyses of the role of patterning genes on carcinogenesis. Still, it’s the first answer I’ve seen to explain why such a peculiar restriction in morphology should be nearly universal within a whole class of animals, when other classes allow so much more diversity.
Galis, F and JAJ Metz (2003) Anti-cancer selection as a source of developmental and evolutionary constraints. BioEssays 25:1035-1039.
Galis, F (1999) Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J Exp Zool (Mol Dev Evol) 285:19-26.
…for us readers. I can’t even imagine how the organizers are coping. The latest category posted is Most Deserving of Wider Recognition, and there are 300 weblogs in there!
A few months ago, after learning that Bill Gates was giving money to the Cascadia branch of the Discovery Institute (which studies transportation issues in the Pacific Northwest), I wondered if the DI was as incompetent and delusional about transportation as evolution. Here’s one answer—not surprisingly, they may again be tools of interests opposed to real advances. I am not by any means an informed expert on these issues, but I do know the Seattle area desperately needs better mass transit—I have seen rush hour on I5, and do not know how people can stand it—yet what the DI offers is a distracting welter of speculative and untried ideas that seem calculated more to muddy the waters and preserve the profitably wasteful status quo than anything else.
A typical right-wing think-tank, in other words.
Since I mentioned yesterday that penis size mattered, upon stumbling on this article about the horrific effects of a trematode infestation, I thought everyone might enjoy a grim and vivid picture of what trematodes can do to a poor, innocent mollusc.
This is a photo of a trematode, or fluke. Trematodes are parasitic flatworms with very complex life cycles; this particular one is a cercaria, or tailed larva. They swim about and infest various hosts at various stages, proliferating and spreading through tissues, before moving on to infect the next host in their cycle.
Leiter Reports has become a group blog. I hope the new guys have been thoroughly instructed in how to maintain the standards—I don’t want to see that uncompromising irascibility diluted!
Another new group blog has formed, Minvolved, focusing on Minnesota politics.
Alas, my daughter and I are big fans of Vonnegut’s writing, but he’s showing signs of losing it. He sounds terribly unhealthy on the radio, and his performance on the Daily Show a while back was depressing. This morning, Vonnegut was on NPR, and said scientists were defending evolution because of “tribalism”, and that “my body and your body are miracles of design”, and that “natural selection couldn’t possibly have produced such machines.” Please, please remind me to stop blogging when my mind deteriorates that far, OK?
To call the body a “miracle of design” is begging the question, while denying the possibility of evolution is the argument from incredulity. Neither is at all persuasive. I would like to know if Vonnegut thinks all those scientists who insist that the Earth is roughly spherical are also arguing for tribal dogma, or whether he suspects that they might actually be relying on this little thing called evidence…and why he thinks biologists fall in the former category and not the latter.
Out of respect for his past writing career, though, I will refrain from cutting him up with a razor here. There were some signs in his interview that he hasn’t drunk deep of the ID kool-ade, but overall it was a sadly muddled exercise in sloppy reasoning, spoken without the sharpness and clarity I’ve expected from Vonnegut.
Carnival of the Godless #32 is available for reading. Once you’ve read through all that, there’s also a somewhat interesting theistic point of view to consider. The author quotes Pascal:
Let the skeptics first learn what religion is before attacking it. If religion boasted that it offered a clear vision of God, and if it asserted that there was ample evidence of his existence, then the skeptic could simply argue that the evidence is not conclusive. But religion says the opposite. It recognizes that people are in darkness, remote from God, that God is hidden from their understanding. Yet it proclaims that God has given signs for those who truly seek him with their hearts. Thus the skeptics could only successfully attack Christianity if they themselves had sincerely sought God, and failed to find any signs.
He takes the claim that God is remote and difficult to know in a curious direction…as an indictment of the so-called ‘spiritual’ leaders who offer simplistic recipes derived from their religious absolutism.
For the fact is, most of those who set themselves up as religious or spiritual authorities in that country [the US], especially in the Christian religion, are just quacks. Fundamentalists are to real spiritual leaders as creationists are to real scientists—in fact, that’s why fundamentalists and creationists overlap so profoundly. They’re a big happy coterie of quackery.
I agree (no one is surprised, I’m sure). But I don’t think the author goes far enough. If gods are murky and nearly unknowable, with no clear evidence for them, why believe in them at all? We shouldn’t trust the charlatans who define Christian behavior so sharply, but why then should we trust any assertion about the nature of any gods, including the claim that there is no “clear vision” of them? Throw out the whole business of god-belief, I say.
As for that concluding bit that says that skeptics who claim to have sought gods and failed to find them had not sought them “sincerely”, well, that’s simply the old No True Scotsman fallacy. Why is it that everything I’ve read of Pascal’s theology suggests that it was painfully simple-minded (I could bring up Pascal’s Wager, the worst argument for gods ever, but I’ll spare you)? The CotG is much more satisfying.
