The second theme is the extent to which strict adaptationism has to be compromised by considering the developmental and genetic restraints at work upon organisms, when you start considering the organism as a figure that pushes back against the force of natural selection. The best way to explain it is metaphorically. Under really strict Darwinism (Darwin is not a strict Darwinian), a population is like a billiard ball: you get a lot of variability, but the variability is random, in all directions. Natural selection is like a pool cue. Natural selection hits the ball, and the ball goes wherever selection pushes it. It's an externalist, functionalist, adaptationist theory. In the nineteenth century, Francis Galton, Darwin's cousin, developed an interesting metaphor: he said an organism is a polyhedron; it rests on one of the facets, one of the surfaces of a polyhedron. You may still need the pool cue of natural selection to hit it — it doesn't move unless there is a pushing force — but it's a polyhedron, meaning that an internal constitution shapes its form and the pathways of change are limited. There are certain pathways that are more probable, and there are certain ones that aren't accessible, even though they might be adaptively advantageous. It really behooves us to study the influence of these structural constraints upon Darwinian and functional adaptation; these are very different views.

The third theme is the extent to which a crucial argument in Darwinism — namely, that you can look at what's happening to pigeons on a generational scale and extrapolate that into the immensity of geological time — really doesn't work, that when you enter geological time there are a whole set of other processes and principles, like what happens in mass extinctions, that make the extrapolationist model not universal.

I'm attempting to marry those three themes — hierarchical selection, internal constraint, and the immensity of geological time — into a more adequate general view of evolutionary theory.

I should say that geological time is in there because it's so essential to strict Darwinian theory that you be able to use the strategy of bio-uniformitarian extrapolation; in other words, that you be able to see what happens in local populations, and then render the much larger-scale events that occur through millions of years to much larger effect by accumulation of these small changes through time. If, in the introduction of the perspective of millions of years, new causes enter that couldn't ever be understood by studying what happens to pigeons and populations for the moment, then you couldn't use the Darwinian research strategy. That's why Darwin himself was so afraid of mass extinction and tried to deny the phenomenon. The geological stage is really a critique of the uniformitarian, or extrapolationist, aspect of Darwinian thinking.

Richard Lewontin is my population-genetics colleague at Harvard, probably the most brilliant man I've ever had the pleasure of working with. We teach a "Basics of Evolution" course together. In 1978, there was a symposium on adaptation held by the Royal Society of London. It was a very pro-adaptationist symposium; that's the British hang-up, after all. I think John Maynard Smith was one of the organizers. Dick was invited to present a contrary view, because — particularly after the publication in 1975 of E.O. Wilson's Sociobiology, which is so strongly adaptationist — Dick had been quite vocal in his doubts about the adaptational parts.

Clearly, there's a lot of adaptation in nature. Nobody denies that the hand works really well, and the foot works well, and I don't know any way to build well-adapted structures except by natural selection. I don't have any quarrel with that, and I don't think any serious biologists do. But adaptationism is the hard-line view — which has been so characteristic of English natural history since Darwin — that effectively every structure in nature (there are exceptions of course) needs to be explained as the result of the operation of natural selection; that if we're not absolutely optimal bodies — because clearly we're not — we're at least maximized by natural selection.

Darwinian biologists will use it as the strategy of first choice. If you see a structure in a flower or in a mole, and you don't know what it's for, the first thing you assume is that it was built by natural selection for something, and your job is to figure out why it's there — the "why" being "What is it good for?" — because once you know what it's good for, then you know why natural selection made it. Although this is a technique that often works, it's inadequate in so many cases that it just doesn't suffice as a general strategy, the main problem being that many structures are built for other reasons that have nothing to do with natural selection. For example, they can arise as side consequences of other features that might have adaptive benefit. Having been built for other purposes, they may then prove useful; they can be coopted secondarily for utility. The bird's wing did not evolve for flight. If you want to know why it's there, seeing a bird fly isn't going to help you, because 5 percent of a wing doesn't fly. It must have been built originally for some other function.

Take the human brain. Most of what the human brain does is useful in a sense — that is, we make do with it — but the brain is also an enormously complex computer, and most of its modes of working don't have to be direct results of natural selection for its specific attainments. Natural selection didn't build our brains to write or to read, that's for sure, because we didn't do those things for so long.

Anyway, the Royal Society asked Dick to write a piece for the 1978 symposium. I had developed my own doubts about adaptationism, for a host of reasons. Part of it came from working on random models of phylogeny with Dave Raup and Tom Schopf and Dan Simberloff in the early seventies and coming to realize how much of an apparent pattern could be produced within random systems. Part of it came from writing my first book, Ontogeny and Phylogeny, in 1977, and coming in contact with the great German and French continental literature on structural, or nonadaptational, biology. That's the continental tradition as much as adaptationism is the English tradition. I also had been unhappy with the overuse of adaptation in sociobiological literature, so I had a whole variety of reasons to agree with Dick on those subjects.

Dick was going to be the one nonadaptationist speaker at the symposium. In fairness, he was going to give the last speech, and it was certainly given prominent coverage; the English are nothing if not fair.

Dick doesn't like to fly, and he had no particular desire to go there, and since we had pretty consonant views and I wanted to go to England anyway, we decided to write a joint paper. In fact I wrote virtually all of it. He was very busy, and I would be giving the paper anyway. The paper is a general critique of full- scale adaptationism, or panadaptationism. It's not an attempt to trash Darwinian natural selection, which obviously happens; it's an attempt to argue that adaptationism, or the notion that Darwinian selection is effectively responsible for everything in the form of organisms, just will not work.

One of the main reasons I'm proud of that paper is that I do believe in interdisciplinary perspectives and — as an essayist, particularly — the use of examples from other fields. The paper succeeded because I used a fairly arresting strategy of argument, by beginning with an architectural example.

The paper is called "The Spandrels of San Marco and the Panglossian Paradigm; A Critique of the Adaptationist Program." I began by talking about the spandrels under the domes of the cathedral of San Marco. I had been in Venice a few months before, and I had stood under the dome in San Marco, and I had worked out this argument for myself, and it was very enlightening to me. It helped me to see what's wrong with the adaptationist paradigm.

Here's the situation: You decide to build a church by mounting a circular dome on four rounded arches that meet at right angles. I'll accept that as an analog of adaptation; that's an engineering design that works. But once you do that, you have four tapering triangular spaces where any two arches meet at right angles. The spaces are called spandrels — or pendentives, but the more general architectural term is spandrels. They're spaces left over.

Previous Page 1 2 3 4 5 6 Next