Unit Four: Steven M. Stanley

Interview: Steven M. Stanley

Steven Stanley is a professor of paleontology in the Department of Earth and Planetary Sciences at Johns Hopkins University. He began his career studying living organisms and went on to apply his research to interpreting the fossil record. He has written one of the major books in contemporary evolutionary theory, Macroevolution: Pattern and Process (1979), and, more recently, the less technical Earth and Life through Time (1985).

Professor Stanley's research on fossilized shells has thrust him into the eye of one of evolutionary biology's stormiest debates in the past half century. A major issue of this debate is the tempo of evolution. Orthodox evolutionary theory emphasizes "gradualism," the idea that minute evolutionary changes accumulate over vast spans of time to produce evolution on a grander scale. This view of evolution implies that the fossil record should provide evidence of new species being linked to their ancestral species through a succession of intermediate fossil forms. In most cases, however, new species appear in the fossil record rather abruptly and then undergo little change for the rest of their existence. Steven Stanley interprets this feature of the fossil record as evidence that evolution occurs primarily in fits and starts rather than by a smooth continuum of minute changes.

Along with Stephen Jay Gould of Harvard and Niles Eldridge of the American Museum of Natural History, Dr. Stanley is one of the principal architects of a theory of evolution known as punctuated equilibrium. Its central point is that most anatomical change is compressed into bursts of evolution that punctuate longer periods of relative stasis. In the following interview, Dr. Stanley presents the case for a new evolutionary timetable.

Dr. Stanley, how did you become interested in paleontology?

I grew up in the country and liked nature. I always had an interest in both geology and biology. My sophomore year at Princeton I took a tremendously stimulating introductory biology course taught by Colin Pittendrigh, who was one of the great lecturers on campus. The text was Life, originally written by Simpson, Pittendrigh, and Tiffany, and we also read George Gaylord Simpson's little book, The Meaning of Evolution. This course was my first major exposure to Charles Darwin; I later read much more about 19th-century biology and geology.

That same year I took an invertebrate paleontology course taught by a very stimulating, brilliant man by the name of Alfred G. Fisher. Al Fisher knew a remarkable amount of biology and geology. And in a sense, through his influence and also through his course, fossils came to life for me. Evolution gave me an intellectual motivation for taking a strong interest in fossils, and I became fascinated by the idea of evolution as a field of study.

At what point did your interest in paleontology lead you to pursue a research career?

Within a year or two I discovered that I loved doing research. School had never been exciting for me because in those days, academic ability was defined as the ability to memorize everything in sight, and I wasn't very good at that. My senior thesis was the real thing: I studied an Ice Age coral reef that forms the rock of the upper Florida Keys, called the Key Largo Limestone. I studied the ecology of this ancient reef and the way in which it had turned into rock. And I did this in part by studying the modern reefs, developing an idea of what kind of reef it was and what kind of environment it represented, and learning about corals and mollusks that were present in the reef. They're all still living species.

It was very difficult because I was only 20 years old and had never done research and simply didn't know the techniques. I got a little bit of help from a lot of people: scientists who were in southern Florida at the time, graduate students, various people. I learned techniques and dug into the literature. I found that I was doing research and enjoying it: from developing hypotheses and trying to find solutions to problems, to carrying out the field research, to testing things and writing up the results. A whole new world had opened up to me.

I've always enjoyed the mixture of field and lab work. And publishing. My senior thesis was ultimately published. I think any young person has to experience a tremendous thrill seeing his or her name on an article for the first time and seeing the article in published form. There are many different dimensions of this enterprise we call science that I really enjoy.

As a scientist who gets at questions in evolution mainly by studying the fossil record, you look back over vast tracts of time. How does one comprehend time on this scale?

If you think about it, we deal with many orders of magnitude even in our everyday thinking. We can relate seconds to minutes and perhaps to hours, but we certainly can't relate seconds to decades. What paleontologists do is simply add a few more orders of magnitude, a few more decimal places. We often think in terms of millions of years, and, occasionally, we think in terms of billions of years. If we talk about the early Precambrian (4 billion years ago), we're really on a different scale than when we talk about the Ice Age. And even within the Ice Age, we're talking about a span of time from 1.8 million years ago to the final interval just a few tens of thousands of years ago. So there are a few discontinuities in conceptualizing time, and you never really do put the whole continuum together in your mind.

One of the metaphors that Darwin used in The Origin of Species was that the fossil record was an imperfect book. Does the incompleteness of the fossil record still frustrate paleontologists?

The one place I would fault Darwin is in his very negative appraisal of the fossil record. He spent a great deal of time talking about what was wrong with the fossil record, and this idea was assimilated by paleontologists to the point where they themselves talked about the incompleteness of the record and its weakness with regard to the study of evolution. This idea has persisted until modern times and has hampered paleontologists in justifying their contributions to the study of evolution.

My own view is that the fossil record has been grossly underrated. Yes, it's full of gaps and there are parts of it that are abysmal, but there are also places where the fossil record shows remarkable continuity, where we can actually see sequential populations separated by rather narrow intervals of geological time. And those are the places one should go to test hypotheses and look for answers. For example, I've been looking at fauna in southern Florida that is 4 million years old and has something like 2,000 species of mollusks. They're beautifully preserved, they're still shiny, and some of them still have color patterns. There's no question about identifying species in many cases.

I'm sometimes frustrated when a biologist friend of mine, an expert on frogs, will talk about how you can't really tell frog species apart by their skeletons. Because of this, he will tend to question the value of the fossil record in a more general way. My attitude is to not study frogs in the fossil record, because they're very fragile and their fossil record is very poor.

Darwin seems to have had a decidedly gradualistic view of evolution: a view that minute changes, accumulated over vast tracts of time, could explain very large changes. You said in one of your books that Darwin's gradualism is understandable, given the scientific and philosophical atmosphere of his time. Would you comment on that?

One of the most remarkable facts about biologists of the early 19th century was that, by and large, they did not believe that species were variable. That is, they didn't acknowledge a range of sizes or shapes within a species. Darwin was a great empiricist, and he spent a great deal of time documenting variation within species in nature. His idea of natural selection was based on the existence of variability within species, but I think the social climate of the times constrained him from accepting the variation that he observed.

The easiest way around this for Darwin was to convince himself and others that evolution worked very slowly and required little variability. Had he presented a punctuational view of evolution, that evolution occurs in sudden spurts, he would have been unable to defend it against the prevailing creationist view of the time. He would have been saying that natural selection was operating in such a way that it couldn't be seen in the fossil record. And that would put him in a very difficult position to sell the idea of natural selection.

The gradualistic flavor of Darwinism persisted in the modern synthesis of evolution formulated in the 1930s and 1940s. The orthodox Darwinian view about the tempo of evolution is now being challenged by those who propose a punctuated model for evolution, and your research is at the center of this debate. What evidence is there from the fossil record for a jerky tempo to evolution?

Unfortunately, the fossil record very rarely documents rapid transitions. It's difficult to sample a given population at many intervals over a period of more than 50,000 years. What we can do is look at the other side of the question: stability. If we find that species last millions of years without changing appreciably, and, at the same time, we can see other evidence of evolutionary relationships between species, then we end up being forced to look to rapid punctuational transitions.

One of the factors we have to keep in mind is that no species is perfectly stable through time. To me, the key question is how much change is occurring in general in a clade [a segment of the tree of life]. If, in a given time interval, there are dramatically new forms appearing — new genera, for example — and at the same time the species are quite stable, then we must conclude that although there may be a little bit of evolution at the species level, the origin of new genera must come via rapid branching events. Maybe in some cases an entirely new genus forms via one event, or in some cases by way of three or four events.

Does this mean, then, that when large-scale change does occur, it must be relatively compressed, at least on the scale of time that geologists use?

Yes. On the geological scale of time, many large-scale transitions seem to be very rapid. We see this in the age of mammals: Modern mammals as diverse as bats and whales appeared within just a few million years of the beginning of the age of mammals. We see the same thing in a number of other groups. The echinoderms [starfish, sea urchins, sea lilies, and their relatives] produced a wide variety of classes way back in the Cambrian period.

In the gradualistic era, one specialist on fossil echinoderms suggested that somehow the fossil record must go way back into the Precambrian in order to produce all of these very distinct and different classes by very early in the Cambrian. Now most paleontologists are convinced that there cannot be a fossil record of the echinoderms going well back into the Precambrian. We know the fossil record too well. So here I would invoke the punctuational model. I don't see how else one can explain how entire species could become wholly transformed so rapidly in geological time.

So, the debate about the tempo of evolution, gradual versus punctuated, stems largely from different ways of looking at the fossil record?

Well, I think the basic issue here is whether we judge the fossil record to be a picture of the incompleteness of sedimentary deposits or take it at face value as a complete picture of evolution. Darwin concluded that it must be very incomplete. Some of us are now suggesting that the stability of species we see isn't a matter of bad record, but an important evolutionary phenomenon. Part of the problem has been that until recently, there was no reliable scale of absolute geological time, and we didn't know how long the stages lasted. With accurate absolute time scales, we are starting to see some remarkable things about the lengths of survival of individual species.

About how long, on the average, do species last?

This varies greatly from group to group. For mammals, I would say it's somewhere in the neighborhood of 2 million years for a species. Of course, there are some species of mammals that last for only a very short period of time. For many invertebrate groups, durations are more or less like those of mammals. And the average species of bivalve mollusks I study has probably lasted at least 15 million years. Many of the shorter durations result from real extinction, not from evolving into something else. At Calvert Cliffs, right here along the Chesapeake Bay, you can look at diatom floras that are about 13 or 14 million years old, and you'll find that half the species exist today, which suggests a mean duration longer than 20 million years.

If species spend such a small fraction of their existence actually undergoing significant change, does this explain the dearth of transitional forms in the fossil record?

I think that has to be the case. There are too many places where the fossil record is complete enough that we ought to see transitions occurring. Even in these cases, we see very few good examples of higher taxa evolving by gradual change. There may be a few examples here and there, but by and large, we just don't see the steps.

It seems that the debate between a gradual and a punctuated view of evolution is not just about tempo. It's also a debate about mechanism.

It is to a degree, but there has been much misunderstanding about this. Some geneticists have assumed that paleontologists adopting a punctuational position argue for a totally new genetic mechanism for evolution. That's not really true. The punctuational view is quite compatible with the view that natural selection operating on mutations over a period of generations is the prevailing mode of transitions. It's simply a matter of our compressing this into a shorter time and considering it as happening in small populations.

One of the basic notions of the modern synthesis has been that large, well-established species with subdivided populations offer the most effective conditions for evolution to occur. The punctuational scheme would argue that small populations evolve not just as parts of a whole complex that's evolving, but as individual units that are evolving and diverging rapidly. In fact, Ernst Mayr, a major proponent of the modern synthesis, laid the foundation for this whole viewpoint as early as 1954. He published a paper that suggested that the lack of continuity in the fossil record could well be a result of change taking place in small populations rapidly on the geological scale of time; yet the idea was never assimilated into the modern synthesis.

So transitional forms are so rarely observed in the fossil record because most speciation events involve very small splinter populations separated from a larger established population?

I think that's often the case. Evolution happens rapidly in small, localized populations, so we're not likely to see it in the fossil record. If you think about successful speciation events in terms of local diversity, there is a very revealing and simple notion. If you could sit and watch a particular group of animals or plants, say, in a family that includes 50 species, through 5 million years of time, each of those species would give rise to only one other species, on the average. Speciation is a very rare event. Because biologists often focus on the immediacy of things, they sometimes overlook how improbable and rare a speciation event actually is.

Even though speciation is rare, is the vastness of time an important factor here again?

That's right. A lot of things that seem improbable aren't improbable on the geological time scale. If you could transport yourself back in time 100 million years and look at a given group of organisms, you couldn't predict what changes would occur, but you could predict that something strange would happen. But you'd never be able to predict these changes in their particulars. A lot of it depends on the chance factors: what population happens to become isolated, where it becomes isolated, what genetic changes happen to crop up, and in what ecological context. There are all sorts of things that are unpredictable.

There are cases in the fossil record of evolutionary trends — an increase in size, for instance — affecting many species belonging to the same genus or family. How can the punctuated view of evolution account for evolutionary trends?

There are a couple of ways a punctuational view can explain evolutionary trends on a large scale without invoking gradual transformation. There may be a tendency for speciation events to move in some particular direction because certain changes may be more favorable than others. Or perhaps the environment tends to push speciation events in one direction consistently. For example, one might have an environmental gradient toward drier or warmer conditions, where the original species is at one end of the gradient and the adaptive opportunities are primarily toward moving along that gradient.

A second possibility is that trends develop simply by chance. You happen to get a speciation event in this direction, and one in that direction, but then you get five more in the first direction, for no good reason. It's like genetic drift on a macroevolutionary scale. The problem is that, like genetic drift, it's very difficult to separate chance events from those having an identifiable cause.

Can you give an example from the fossil record of an evolutionary trend produced by species selection, by a pattern of branching events rather than cumulative change in continuing branches?

It's difficult to reconstruct a huge phylogeny for a long period of time. Usually, you're dealing with good segments that represent just a few million years. But there are examples where we see particular taxa declining while other related taxa are expanding. The horse is the most famous example, the classic story of one genus turning into another. Now it's becoming apparent that there's an overlap of these genera and that there were many species belonging to each one. It's a very bushy sort of pattern that is, I think, much more in line with the punctuational model; there isn't just a simple, gradual transition from one horse to another. This is now becoming fairly well known.

You used the word chance earlier. Evolutionary theory has always acknowledged the role of chance in the origin of variations. In the so-called modern synthesis, the role of chance has even been acknowledged for change in terms of genetic drift, and so on. Is part of the current debate a matter of the relative role that chance has played in the history of life?

In a sense, a part of it is. The raw material of species selection has a strong element of chance. When I use the word chance to refer to this kind of thing, I'm using it in the sense of unpredictability. Where a particular speciation event happens to develop, what the environment is, what the genetic changes are, what the small subpopulation is — these are things that are highly unpredictable. This is not to say that the actual transition from one species to another is a matter of chance. When you talk about chance in this way, some people assume you're talking about macromutations or genetic drift and throwing out natural selection. You're not. You're just saying that you can't predict what direction selection will go in or when it will occur.

If, in fact, the more common tempo of evolution is in fits and starts rather than gradual change, what are the implications for human evolution? Is there any possibility that speciation events could occur again from Homo sapiens?

The first members of our modern group, Cro-Magnons, appeared about 40,000 years ago, and their skeletons look virtually identical to those of modern Europeans. That's 40,000 years with very little change. We also know that Neanderthals were around for many tens of thousands of years without really changing very much. The punctuational prediction would be that we are not going to change a great deal, except by our own devices.

I think further speciation from Homo sapiens is highly improbable in the kind of world we have today because our transportation and communication systems are all-pervasive. There is no way that a small population is going to become isolated and flourish to the extent of becoming a new species. I think one of the last things that our activities would allow to happen would be the formation of a new species similar to us. Our gene flow is just too great. But there are two possibilities — and this is pure fantasy — from which a new hominid species might emerge. One would be a small isolated space colony. The other is if we had a nuclear war. Then we might end up with a few small splinter populations that could conceivably undergo rapid change in isolation. But I don't think that in the normal course of things on Earth, we're going to have a new species rising up.

This debate about the tempo of evolution spills over into the grandest events observed in the fossil record: episodes of mass extinction and episodes of adaptive radiation, where many new life forms seem to spring into existence over relatively short spans of geological time. Beginning with mass extinctions: Are they real, or are they artifacts of an imperfect fossil record?

Well, I think it's a question of definition. If you define mass extinctions as the disappearance of large percentages of higher taxa over brief geological intervals, they're real. We can see times when, for example, 30% to 50% of families of marine life have died out in less than 10 million years, which is a pretty drastic turnover. But if you define mass extinction as an instant wipeout, the kind that might be envisioned by the landing of an extraterrestrial object, such as a comet or meteor, that's another story. In the past, we've never defined extinction as being geologically instantaneous, and I don't think we should now.

And what about subsequent adaptive radiations?

Many adaptive radiations, including that of the modern mammals, were triggered by mass extinctions. Mass extinctions, in effect, reset the evolutionary system, so you end up starting over again in a lot of ways. They've served to remove potential competitors, open up environments to new forms of life, and limit predators: things that are very important in giving certain groups a chance to diversify.

The most engaging episode, at least in the eyes of the general public, is the mass extinction that occurred at the end of the Cretaceous period (65 million years ago) and claimed the dinosaurs, among other victims. There's the provocative idea of a huge asteroid colliding with Earth and throwing up a cloud of dust that changed the world climate. What is your opinion of extraterrestrial explanations for mass extinctions?

There is some tangible evidence for the late Cretaceous collision in the form of the iridium anomaly. Iridium is a very rare metal on Earth, and its presence in sedimentary strata of the right age in many locations around the world is intriguing. But in my view, the late Cretaceous mass extinction did not result entirely from an extraterrestrial event; this may have been the final blow for some groups, but it's not the whole story.

The fossil record suggests that the dinosaurs and a number of groups of marine organisms were already declining substantially. There is a great deal of evidence of general cooling as you go from the Cretaceous to the Paleocene. The timing isn't particularly well worked out, but the study of terrestrial plants shows very great cooling and the spread of cold-adapted marine life toward lower latitudes. So there may have been a general pattern of cooling upon which you impose this shock of the extraterrestrial object. Some things that died out might have died out anyway.

As for earlier mass extinctions, there were three major ones in the Paleozoic, and two of them coincided with the onset of major glacial intervals. There is no good evidence of meaningful iridium anomaly for any of these earlier mass extinctions. I would argue strongly that climatic change, particularly cooling trends, are the most important factors in these events.

We see patterns of extinction in tropical flora and fauna that seem to support this hypothesis. If you cool down the whole Earth, things that live at high latitudes tend to migrate toward the tropics, because the zones to which they are adapted migrate. The tropical zone can become less fully tropical and disappear altogether. Again and again, we see groups like the tropical reef communities dying out. Every mass extinction seems to hit that community very hard. In fact, those three Paleozoic mass extinctions had a heavier impact on the marine realm than did the Cretaceous extinction that did in the dinosaurs. So I think there is much evidence that we don't have to look beyond the Earth for explanations for mass extinctions.

Under the pressure of the many lively debates going on today in evolutionary biology, is the Darwinian foundation of evolutionary theory beginning to crack at all?

What's remarkable about evolutionary theory since Darwin is that it's really withstood the test of time. Evolutionary theory is not getting older; it's getting better. Evolutionary biologists and paleontologists have molded it and modified it, but it has actually grown stronger through all this testing and through the enriching of the theory with the development of genetics, molecular biology, modern ecology, and a much better knowledge of the fossil record. So even though we inevitably argue over details and suggest modifications, what is truly remarkable is that something so general can hold up in its essence.

If you could go back in time and sit down with Charles Darwin, what would you like to ask him, and what do you think would be important to tell him about what's happened to his theory?

What I would ask him, the question that everyone would want to ask him, is why he delayed so long in publishing The Origin of Species. It would be interesting just to talk to him about his trepidations about publishing. He might have worried about attacks from the clergy, attacks from other scientists. But the biggest question people have is when he really began to believe in the idea of natural selection. Was it on the voyage of the Beagle or later?

From my own personal standpoint, I'd like to tell him about the punctuational scheme and some of the things we've learned about the fossil record. I would like to address a number of major problems that he faced, such as the origin of higher taxa. He was very concerned about how all these forms of life could appear so suddenly. The punctuational view gets us around these problems, and we also have a much better view of what happened in their origin and early diversification. I also think telling him about genetics would be interesting and important. And I'd suggest that he find that reprint of Mendel's that was sitting unopened on his desk! [laughs]

©2005 Pearson Education, Inc., publishing as Benjamin Cummings