Chapter 5 "WHY IS THERE SO MUCH GENETIC DIVERSITY"

Steve Jones [5.1.96]
Topic:

Stephen Jay Gould: I like Steve Jones' work. I've read most of his scientific papers. I work on pulmonate snails, and he's one of the best in this little field. I don't know him very well. He's a very good scientist. He's followed the path of a media person, but in my professional world — snail biology — his science is very good.

__________

STEVE JONES is a biologist; professor of genetics at the Galton Laboratory of University College London; author of The Language of the Genes: Biology, History, and the Evolutionary Future (1993); coeditor (with Robert Martin and David Pilbeam) of The Cambridge Encyclopedia of Human Evolution (1992).

Steve Jones' Edge Bio Page


[Steve Jones:] I've spent a long time working on snails. This seems an odd thing to do, but they remind us of one question that remains unanswered, and is effectively forgotten by many biologists: Why is there so much diversity? Without it there could be no genetics, no evolution, and — probably — no biology at all. It's a question that periodically comes to the surface and then sinks again; just like a political issue, it appears, disappears, and remains unresolved. We have the beginnings of an answer as to why, in some places, one snail species is so variable, but we have no real idea why in any species anywhere at any time no two individuals are identical. That's an essential question of evolution. All others flow from that.
I'm an evolutionary biologist. For most of us, from Darwin on, the organism you work on is the second question. The first is, What is going on in evolution? Inevitably, though, one ends up talking about the particular rather than the general. Snails are a microcosm of what Victorians used to think evolution was about, in that they're strikingly different from one another in appearance, and often from one species to another. Perhaps they're a good particular case to study for that reason alone — and, of course, they're easy to catch.

I don't think any scientists can explain, with any honesty, how they got into their own particular topic. My theory is that most people do it by accident, in spite of what they claim later. I got into evolution by chance. My tutor was a chap called Bryan Clarke, a very able scientist who worked on the genetics of snails. In those days before molecular genetics, snails were one of the few creatures whose diversity was easy to study, so it was less odd than it might seem today. I was assigned to his tutorial group on the basis of the first letter of my surname, and inevitably I started working on the same thing. Fortunately, it turned out to be fascinating (to me, if to no one else) and it's still my prime scientific interest — although I can't be accused of being a narrow specialist, as I have now moved into slugs.

Doing molluscs was probably the worst career move I ever made, because the people around me are now household names in biology — people like Ed Southern, who invented the Southern blot, a central technique in the new genetics. I could easily have gone into that field and perhaps have become slightly less obscure than I am today. But I didn't, and I don't really regret it.

Steve Gould has concentrated a lot on the differences in shape and form in the Bahamian snail he works on — a bigger and more difficult question. I work more on differences in color and pattern — and increasingly I'm asking the same questions about diversity at the DNA level. The shell-pattern variation is a classic of evolutionary biology. It was first looked at in the nineteenth century. We now have data on well over a million individuals who have been scored for their physical appearance — an awful lot of information about differences.

To summarize a lot of work, it seems clear that as you go across Europe, the snails' genes are different for reasons having to do with thermal relations in sunshine. Dark objects — and genetically dark snails — heat up more in the sun, and those genes are rarer in the south. The same is true on the smaller scale of a few miles. I did a lot of work on that on the border of Bosnia and Croatia. A few months ago on the news, I saw the town I was based in burning to the ground.

That leaves the more difficult question: given that in different places individuals with different genotypes are favored, why is every snail in a particular population not always the same?

We have, I think, sorted that out. What I did was to use the snails themselves as ecological monitors. I developed a paint that fades at a known rate when exposed to the sun. Put small spots of that on the shells of snails of different genotypes, return them to the wild, and come back after a month; the paint indicates the amount of solar energy soaked up. There are big differences between animals of different genetic constitution. They choose different times of day, or different parts of the habitat, to be active in, suggesting that an ecologically complex habitat might support more genetic diversity.

I developed another technique to test this, which has become known as "Jones' balls." These are snail-size spheres made of plastic, which you throw into a habitat. Then you pretend you're the sun, to put it childishly, and you take the world's cheapest satellite, which is a ladder, and scan the habitat on a track from sunrise to sunset. This measures the pattern in which snail- size objects are exposed to the sun or hidden by the vegetation at different times of day. It gives a kind of snail's eye view of the universe. I suppose it's a trivially simple idea, but it works. It measures habitat diversity as perceived by the snails. There's a good fit between genetic diversity, ecological diversity, and individual choice of microhabitat.

It's a fairly new idea to try to get a view of ecology through your organism's eyes, as it were. We've succeeded in doing the same kind of thing with fruit flies. The whole of genetics used to be Drosophila, and it's still a very important organism, because so much is known about it in the lab. Linda Partridge, a colleague of mine, and I, and Jerry Coyne from the University of Chicago were interested in trying to assess the ecological relationships of Drosophila using genetic means. We used a mutation that was temperature-sensitive: eye color depended on the temperature the fly experienced during development. We did the experiment in Maryland, where it was hot and steamy. We released millions of flies containing this mutation and collected their offspring, which had developed in the wild.

The flies themselves were acting as living thermometers. We could tell from their eye color what temperature they'd grown at. They occupied an extraordinarily wide range. Because flies growing at different temperatures emerge at different sizes, this explained an awful lot of the variation in shape and size which previously had been thought to be genetic. And, in turn, that helps explain another large but largely ignored question in evolution. For most creatures, it pays to be big. It makes you a better mate, better at dealing with enemies, better at coping with heat and cold. Why, then, is there any variation in size? Perhaps it's because most of the differences — in fruit flies, at least — are environmental, and not genetic at all.

I also have a vicarious interest in sex. After all, it's just a machine for generating diversity — differences between parents and offspring. Nobody really knows why sexual reproduction is there. About the only way to study it is to look at those few creatures who've given it up. Most slugs are hermaphrodites, but nearly all are relatively decorous about it; boy-girl meets girl-boy and nature takes its course. Some, though, have taken the easy way out. They fertilize themselves, effectively abandoning sex altogether. Their genes show that these species are essentially a mass of identical twins, with no diversity at all. We don't yet know why they do this: the only real pattern is that it pays to give up sex in the cold — in Norway, as compared with Spain, for example.

The pivotal influences on my work have been Bryan Clarke and Dick Lewontin. When I finished my Ph.D., I wrote to Lewontin in Chicago, asking him for advice as to where I could go for a postdoctoral fellowship. He wrote back almost by return mail saying, "Thank you for your application, which is accepted. We expect you at the beginning of next month." I hadn't even applied, but I went there like a shot and learned, more than anything else, how little I really knew.

He was then approaching the question I have been studying ever since, which is why genetic diversity exists. Dick is a man of tremendous brio and enthusiasm, who has the ability to fire people up with his ideas, however good, bad, or indifferent they might be. I have to say that I was greatly enthusiastic about one of his bad ideas — that it might be possible to take an isolated population of fruit flies living out in the California desert and use it as an artificial laboratory, change the flies' genes by flooding them with genetically different flies and see what happened to their evolution over a couple of years.

That was a great time. I traveled all over the deserts of California, into Mexico, looking for isolated populations. After three years and a lot of money and a deep tan, what we basically found was that these populations weren't isolated at all; there were flies flying in and out all the time. In its own narrow way, that was interesting for Drosophila genetics — though it certainly wasn't going to change the course of evolutionary biology.

Lewontin excited me about science more than anybody else has ever done. He did the same for lots of people. If you trace the family tree of evolutionary biologists in the world, a suspiciously large number of them lead straight back to him. He has been pivotal in the subject.

He's sometimes a pernicious influence, though, in the sense that Marx or St. Augustine were. They may both have been wrong, but life would have been a lot less interesting if they hadn't been around. At least they forced people to think about their ideas. Dick is an evolutionary gadfly, attacking whatever the dogma of the day might be. He's the embodiment of the idea that science is the art of the disprovable. He's destroyed lots of ideas, and that's a useful thing to do. He does it superbly, but science needs more than iconoclasts. It needs some people — hacks, like me — to build the icons up, even if their fate is to be knocked down by the Lewontins of this world. Still, I wish there were more people like him around.

I do know a lot about snail genetics. It's my narrow, limited, unintellectual kind of field. In many ways, though, it's a microcosm of evolutionary biology at its worst. Its literature is filled with the great vaguenesses of evolution — with words that, when you deconstruct them, are like shoveling fog; they don't mean much. "Coadaptation," "adaptive landscape," "punctuated equilibrium" — what I sometimes think of as theological population genetics. They're words that don't help at all when you're trying to decide what experiment to do next.

Words like these reflect the view that somehow one gene is there because it has adapted to the other genes that were there already. That the world somehow is a beautifully harmonious structure is an optimist's point of view: everything fits beautifully together, and if you see the whole edifice you don't have to worry about how it's constructed, it just stands up.

That's a pernicious idea. It's an anti-intellectual, working-out-God's-plan, know-nothing kind of idea. In what must have been a moment of extreme tedium, I once read a book by a South African general, Jan Smuts, called Holism. Smuts was a strange, interesting guy, who dabbled in philosophy. Everything you saw in the world was all part of a great scheme, and there was no point in trying to work out what individual parts of the scheme were for, because it made sense only when you saw it as a whole. He was a rather weak philosopher. But his idea pervades a lot of biological thinking. Evolution is a magical thing, with an intrinsic beauty of its own, which you can't hope to break down into the individual genes that make it happen. In other words, there's a limit to reductionism.

Well, maybe there is; but the beauty of reductionism is that it gives you something to do next. Once you start saying that something's unexplainable, then there's no point in trying to explain it. Steve Gould and Dick Lewontin made a famous and very funny attack on reductionism; but in some ways it shows the weakness of what I guess we can call the Argument from Smuts. It was the talk on the spandrels of San Marcos, at the Royal Society. It made an important point about hyperadaptationist views — that everything is the way it is for a reason that can be explained in simple biological terms. The extreme reductionist might write learned books about the Spandrel School — about the deep artistic reasons why the painter made his paintings in this particular shape, and what he was trying to say by not making them square. But the shape of the paintings is there for a reason that had nothing to do with painting. Gould and Lewontin made great play with the parallels between the Spandrel School and the many evolutionists who say that every character in every animal is there for an adaptive reason and if you look hard enough you'll find it.

There's some truth in their argument, but to accept it as the only truth is basically to give up and walk away, to stop being an ornithologist and turn into a bird-watcher. You become somebody who observes rather than analyzes. What they're saying to lots of biologists is, "Abandon hope, go home, and become a liberal-arts graduate!" I may be overcriticizing the Lewontin and Gould view; both of them like to poke people with their sharp pitchforks. The spandrels were a particularly successful poke. But what happened as a result of the famous spandrel paper? The answer is, not much.

Contrast that with the views of someone who is definitely not on the side of the Angels of San Marcos, Richard Dawkins. His views are — to simplify them — simplistic. You can deconstruct everything down to a series of units, the genes — although Dawkins himself would admit that it's naive to say that organisms are just vehicles for carrying DNA around and that everything they do is in the interests of their "selfish genes." But his metaphor has turned out to be extraordinarily productive and useful, because it gives you all kinds of ideas about how to test it. Again, reductionism provides the scientist with raw material, which is a lot more than spandrelism does. That's the beauty of the selfish-gene idea. You can grab it and test it. You can look at the idea and look at the genes. It may well be that the idea will turn out to be wrong. But it sparked a lot of very interesting work. The idea nowadays is that the most fundamental rules of biology — Mendel's laws themselves, even — are a reflection of a truce in a battle between selfish genes. That's a remarkably interesting thought, which leads to some testable predictions.

If I learned anything from my work on snails, it's that reality is getting your feet wet. The only way to approach the truth in snails, or in anything else, is to go out and do the work. Leave pontificating to the Pope. That may sound trivial, but it's important. Science is data-led, not theory-led. I never feel usefully employed in science except when I'm gathering data. Unfortunately, the system conspires to stop you; and instead I give interviews like this or write brittle little pieces in The Daily Telegraph. Old age, idleness, administration, grant- starvation, all those terrible things don't help either.

The questions I ask myself today are the questions I was asking thirty years ago. The only thing that's changed in the last thirty years in genetics is that humans have become the new fruit flies — the organisms that are technically accessible to asking questions about genes. How different are two people? Why are two human groups different from each other? What's the history of human diversity? That's what I shifted to, but now I'm increasingly a voyeur of science rather than a doer of science.

Because I do a lot of writing and broadcasting, I'm better known as a geneticist by the general public than I am by other geneticists. Although I write a lot about it, I've never done any serious work of my own in human genetics, so I'm a spectator of the subject rather than a participant. I'm grateful to my colleagues for being slightly less cynical about that than they have the right to be. I think they see that there's a role for the reporter in science. However, I can console myself with the thought that I'm one of the top six snail geneticists in the world, out of a field of perhaps half a dozen.


Back to Contents

Excerpted from The Third Culture: Beyond the Scientific Revolution by John Brockman (Simon & Schuster, 1995) . Copyright © 1995 by John Brockman. All rights reserved.