>Maybe we could get the Venezuelan Bolivarians to send the Greens some
>statutes of General Simon Bolivar, suggesting it become the Green Giant
>Party, for, ecologic is not self-actuating theory.
>
>Charles
I read this book review (9.28.02 issue of The New Yorker, H Allen Orr) and thought it was interesting. Haven't had a chance to read the book, though. According to the review, Gould ended up reassessing his claims about "punctuated equilibrium"--basically gutting it. Gould conceded that change happens all the time. So, why the appearance of a pattern in the fossil record? "only if species split do the resulting differences last long enough to have a shot at showing up as fossils". From what the reviewer has argued, PE has been gutted as a theory of change and resurrected as a theory of why the fossil record appears as it does. An excerpt from TSET, <http://www.stephenjaygould.org/library/gould_structure.html>:
"The model of punctuated equilibria does not maintain that nothing occurs gradually at any level of evolution. It is a theory about speciation and its deployment in the fossil record. It claims that an important pattern, continuous at higher levelsthe 'classic' macroevolutionary trendis a consequence of punctuation in the evolution of species. It does not deny that allopatric speciation occurs gradually in ecological time (though it might notsee Carson, 1975), but only asserts that this scale is a geological microsecond."
From the book review: <quote> Gould ultimately conceded all this, though at an agonizing pace that spanned decades. (In this respect, if in no other, he was a gradualist.) In "The Structure of Evolutionary Theory," he admits that "we made mistakes," and that prominent among these were the claims about both constraint (idea one) and speciation (idea two). The theory part of punctuated equilibrium was thereby gutted. But what of the pattern part? If a punctuated pattern characterizes the fossil record - and Gould insists that it does, and spends a good deal of "The Structure" trying to show that new species often appear abruptly in the fossil record alongside ancestral ones - what explains it?
Gould concludes that the best guess was one made by the evolutionist Douglas Futuyma in the eighties. Futuyma's idea starts with the fact that different populations of a species often adapt to local features of their environments. Mice in the north, for instance, might evolve to be bigger than those in the south because big bodies lose proportionally less heat. But such local differences are usually too short-lived to show up in the fossil record. That's because populations are apt to come back into contact with each other, and when they do they begin to mate and so swap genes. As a result, any differences among the populations tend to get blended away: close encounters between northern and southern mice yield mid-sized mice. There's only one way to prevent this washing-out: populations must not only adapt but speciate. For separate species, by definition, can't swap genes and so won't lose their distinctive looks by genetic blending when they come into contact. This, then, !
might explain why, in the fossil record, change and speciation seem to go hand in hand. It's not that change is rare and happens only when species split; it's that change is common, but only if species split do the resulting differences last long enough to have a shot at showing up as fossils.
Futuyma's idea, though, had one embarrassing blemish: it's pure Darwinism. Gould and his allies thus found themselves in a peculiar position: the red-hot revolutionaries suddenly seemed staid traditionalists. This was not, of course, lost on evolutionists, many of whom were still steaming over the Darwinism-is-dead business. By the nineties, most evolutionary biologists had simply stopped paying attention to punctuated equilibrium. At best, the theory looked like a moving target, veering now at breakneck speed toward Darwinism. At worst, its chief advocate seemed muddled, a mixture of radical rhetoric and malleable ideas. Punctuated equilibrium was down, if not out.
In "The Structure of Evolutionary Theory," Gould fights mightily to defend his position against charges of collapse. He offers two main defenses. One is to deny reports of his retreat to orthodoxy by claiming that his model was fairly traditional all along. He was, he says, just trying to change paleontology by importing a "fully conventional" biological theory. But this won't do. Gould's model of speciation was far from conventional. And his ideas about strong developmental constraints were clearly and defiantly non-Darwinian. Gould's attempts to take the edge off his earlier views - as when he tells us that his claim that modern Darwinism was "dead" wasn't meant to imply that it was "wrong" - represent one of the least satisfying aspects of "The Structure."
Gould's second defense is far more important. Early in the debate, he began to reassess just what was revolutionary about punctuated equilibrium. He came to think that the truly outre aspect of the theory was something called species selection. In Darwin's account, natural selection acts at the level of organisms, not at the level of species. Some organisms are better adapted to their environments than others, and so have more progeny. Imagine, for instance, two kinds of moths belonging to the same species. One is white and easily seen by bird predators; the other is brown and often mistaken by birds for a dead leaf. The result is that the brown moths typically have more offspring: brown moths have a higher "fitness" than white ones. The percentage of brown moths will, consequently, increase with each generation. This is normal "organismal selection" - it yields organisms that neatly fit their environments.
But why, Gould asked, can't we extend the selectionist logic down or up the biological ladder, and talk about competition among genes, or cells, or populations, or species? The downward extension has, in fact, been achieved: William Hamilton, Richard Dawkins, and others argued that selection typically acts at the level of genes. Gould, though, was more interested in the up direction - in the idea that selection can act at the level of species. According to this theory, some species split faster or go extinct more slowly than others, and therefore become more common through time. Imagine, for example, two plant species. One, a dandelion, disperses its seeds on the wind - seeds sail away to new locations. The other, a cocklebur, disperses its seeds by hooking them onto the fur of passing animals. Now, plants that disperse by wind might be more likely to speciate than those which disperse by animals: wind-dispersed seeds might be more likely to travel long distances and so foun!
d a ne w population on some distant shore. Such a population - living in blissful isolation from the rest of the species - has a good chance of someday evolving into a new species. If wind dispersers speciate faster than animal dispersers, the percentage of wind-dispersed species will increase over long stretches of time. Thus if we start out with one dandelion species and one cocklebur species, the fast-speciating dandelion will give rise to more "daughter" species, which, in turn, will give rise to more "granddaughter" species, and so on. After millions of years, the earth might well be enveloped by more dandelion-like than cocklebur-like species. This is species selection. The important point is that this process has nothing to do with Darwinian competition at the level of organisms. Wind-dispersed species aren't getting more common because a wind-dispersed organism outcompetes an animal-dispersed organism. They're getting more common because the wind-dispersed species form new !
specie s faster than the animal-dispersed ones. Note also that the critical quantity here - speciation rate - is a property of whole species, not of individual organisms. You can't point to one petunia in your garden and ask if it has a higher speciation rate than another. But you can point to wind-dispersed species and ask if they have a higher speciation rate than animal-dispersed ones. Species selection thus involves a higher level of fitness than biologists are used to thinking about - one that reflects the birth and death of species, not of organisms. Gould rounds out his picture of natural selection by arguing that it acts simultaneously at many levels - on genes, on organisms, on species, and perhaps even on higher levels. What he calls "hierarchical selection" is the claim not that species selection is right and organismal selection is wrong but that both act at once.
Gould has high hopes for hierarchical selection, and it takes center stage in "The Structure of Evolutionary Theory." Hierarchical selection, he says, means that we must "reconceptualize all of evolution, and revise both our worldview and our language, accordingly." Indeed, he goes on, "we cannot fairly portray these expanded views as pure sweetness and light for orthodox Darwinism. Much that has been enormously comfortable must be sacrificed." Though the rhetoric is extreme - and it's hard to see what species selection has to do with punctuated equilibrium anyway - Gould's argument for higher-level selection is important. Twenty-five years ago, biologists typically believed that selection was more or less constrained to act on organisms or genes. This is no longer the case, and Gould deserves much of the credit for the change. His defense of species selection - at least as a formal possibility - is in places brilliant. This is Gould at his best. It is also, not coincidental!
ly, Go uld on one of the themes nearest his heart: the sheer complexity of the biological world, its stubborn refusal to submit to a simple analysis or a single equation.
But hierarchical selection is not without problems. The chief difficulty, which Gould concedes, is that it's extremely hard to distinguish species selection from other evolutionary forces when you're looking at evidence from the natural world. An increase in the percentage of dandelion-like species might be due to species selection, but it might also be due to a number of other processes, including old-fashioned organismal selection. The consequence, which Gould also acknowledges, is that there are no undisputed cases of species selection in the real world. This does not mean that the idea is wrong; I, for one, suspect that species selection acts in nature. But it does mean that Gould's species selection has a very different status than Darwin's organismal selection. Organismal selection made sudden sense of a vast number of puzzling phenomena (why, for instance, some moths look like dead leaves). Species selection remains an explanation in search of something to explain.
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