Plato’s Beard

Here is a simple truth that you won’t hear from all the high priests and popularisers of science (in this case, biology):

“If you dissect the past, you can see that luck is a big part of everything in the grand scheme of evolution,” says lead author Stephen Brusatte, a researcher at the American Museum of Natural History.

The researchers whose work is being commented upon above, examined the historical record of dinosaurs and crurotarsans:

[E]arly dinosaurs were contemporaries of crurotarsans, croc ancestors, during the late Triassic period about 230 to 200 million years ago. This reptilian group ranged from quick predators to two-legged vegetarians to leisurely grazers. Then, as the Triassic turned into the Jurassic, the creatures roaming the planet changed drastically. Most crurotarsans disappeared from the fossil record. But many dinosaurs survived—and flourished, diversifying into meat-eating giants, armored warriors and winged aviators.

If dinosaurs were more fit for the environment, they should have had a higher rate of evolution and more diverse body types. Instead the researchers found that the two groups evolved at similar rates and that the crurotarsans had a wider range of body types, suggesting that they had actually adapted to more lifestyles and ecological niches.

The authors argue that because dinosaurs and crurotarsans were living parallel lives together for so long, it is unlikely the dinosaurs necessarily ruled. If you could travel back to the Triassic, Brusatte says, you would have guessed that the crocodilians would have won out. “There’s no way you could argue that dinosaurs were superior to them,” he says. Instead, he thinks an extinction event at the beginning of the Jurassic some 205 million years ago—like runaway global warming or an asteroid crash—may have just been bad luck for the crurotarsans.

The orthodoxy has objections:

“I think that the conclusions of the authors aren’t warranted,” says Kevin Padian, a dinosaur paleontologist at the University of California, Berkeley. “Good luck isn’t an evolutionary force…. Extinctions aren’t random.”

I am no evolutionary biologist or paleontologist, but this sort of response seems to miss the point, logically speaking. Properly understood, there is no implication that there is true randomness in evolution — or rather survival (after all, all events have causes). Rather, it seems to me, the parsimonious claim is that species flourish or perish not entirely due to their own adaptations in the constant presence of environmental pressures (and while we are at it, I thought multi-level selection was taboo and we are only to talk of individuals, not species?), but also often due to large environmental events that alter their fate. One could of course present these events as selective pressure and the pre-existing advantages of the organism as “adaptations” but I think such a tautology would rob the theory of much of its value.

[Link: Was the Dinosaurs' Long Reign on Earth a Fluke?: Scientific American ]

Plato’s Beard » Biology

Plato’s Beard » Biology

Some recent majordomos like Jerry Coyne (University of Chicago) have used language akin to George W. Bush’s “aiding and comforting the enemy” to shut down or criticise any criticism of the orthodox position (The New Synthesis) in Evolutionary Biology. To them, everything from Jerry Fodor’s controversial essay, “Why pigs don’t have wings” to the “Altenberg 16” and their meditations on an “extended evolutionary synthesis”, do nothing more than arm the creationists and ID theorists. While it is true that this line of thought is easily misused by the creationist gang (search for the Altenberg 16 in Google and the first two links are one titled “The Evolution Industry” and another that of an ID proponent), that is no more a reason to avoid it than the sort of criticisms that Bush wants to silence. But the more nuanced and less religious approach to understanding evolution is not new, as Lewontin shows in the previously mentioned review of books on and by Stephen Gould:

Free Expression: The Triumph of Stephen Jay Gould, By Richard C. Lewontin

Gould’s interest in form and function led him to revive interest in what was for some time a neglected aspect of evolution, the change in relative size of different body parts. It was well known in the nineteenth and early twentieth centuries that during the course of evolution different parts of similar organisms changed their relative size. So, for land vertebrates, as bodies grow larger in evolution, limbs get relatively thicker, but cranial size grows larger more slowly than the rest of the body so that heads get relatively smaller.

A major change in this relation of head size to body size occurred, however, in the immediate ancestors of humans. In the human line, cranial size has grown larger more rapidly than body size so we have, compared to apes, heads too big for our bodies. On the other hand our teeth have become much smaller. In recent years this regularity of differential growth, or allometry, of body parts is sometimes neglected because of a fashion for adaptive explanations of evolutionary change. A famous example is the Irish elk, a very large deer with grotesquely enlarged antlers. It was common to explain these huge appendages as the result of natural selection favoring males with the largest antlers, who would then win out in head-on struggles with other males in the competition for females. It was further imagined that the Irish elk as a species went extinct because it was too much of a strain to carry all that weight around on their heads.

Steve Gould neatly deflated this just-so story by showing that the antlers of the very large Irish elk were exactly the size that one would expect from the general differential growth rate of body and antlers in deer. As for the Irish elk’s extinction, their more modestly antlered ancestral species also became extinct, so it is not clear why we should pick out their extravagant headgear as the cause. The important point is that it is easy to make up adaptive stories out of one’s imagination for any feature of any organism, but that there are concrete realities of growth and physiology that need to be taken account of before lapsing into unchecked fictions. A chapter in The Richness of Life, coauthored with David Pilbeam, develops this theme for human evolution.

The phenomenon of allometry is only one of a number of possible factors that need to be considered in explaining particular evolutionary changes. It is a vulgar error to reduce the explanation of all evolutionary changes to the action of natural selection. It is not only in everyday language, however, that evolution is made synonymous with the effect of the higher reproductive fitness of some forms. Within the profession of evolutionary biology during the last thirty years there has developed an overwhelming fashion for adaptive explanations of every feature of organisms. This has been encouraged by the development of theories of kin selection and group selection in which it can be shown formally that a trait may spread in a population even though it is a disadvantage to its carrier, provided that close relatives or the population as a whole may have an increased reproductive rate. This form of reasoning, easy to do in theory but extremely difficult to demonstrate convincingly in nature, has swept through evolutionary biology.

One essay, “The Spandrels of San Marco,” reprinted in The Richness of Life, has itself become the subject of a very considerable literature. It argues that there are multiple possible explanations for evolutionary change besides direct natural selection for a trait.[4] Steve Gould was enamored of early Italian church architecture and familiar with spandrels—the triangular spaces between a series of arches and the straight cornices running above them. He suggested the spandrel as a metaphor for anatomical features of organisms that were not themselves adaptive, but were the architectural consequences of building another feature, just as the spandrels filling in the space surrounding a church dome are a necessary outcome of placing a circular object on a square base. As the church spandrels may then incidentally become the locus for decorations such as portraits of the four evangelists, so anatomical spandrels may be co-opted for uses that were not selected for in the first place.

Gould’s favorite example is the human chin, whose presence is an incidental consequence of the differential growth rate of two bones in the lower jaw. The dentary bone which carries the teeth elongates more slowly than the jawbone itself, so the chin juts out. In our ape-like ancestors the jawbone grows more slowly so no chin develops. Of course one can always try to invent a story about why having a chin confers more reproductive potential, but that is a parlor game, not science.

More on the Fodor affair and related matters later!

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