Evolution of Evolution — Text-only | Flash Special Report
Interview with Charles Marshall & David Sepkoski
BOBBIE MIXON: Charles Darwin expected little from the fossil record to help verify his theory of natural selection. In fact, “On the Origin of Species” argues that the imperfections of geology held the most obvious and serious objections to the theory. But since 1859, when Origin was published, new science and new knowledge has unearthed a wealth of new answers--and new questions. Two experts on the forefront of the advances in geological sciences are Harvard University biology and geology professor, Charles Marshall. Welcome, Dr. Marshall.
DR. CHARLES MARSHALL: Thank you.
BOBBIE MIXON: And David Sepkoski, assistant professor of history, specializing in evolutionary theory, at the University of North Carolina Wilmington. Thank you for joining us.
DR. DAVID SEPKOSKI Thanks for having me here.
BOBBIE MIXON: Dr. Sepkoski, let's start with you. Darwin questioned the geological record's ability to address questions of evolution. Do we have more questions today or more answers from the geologic record?
DR. DAVID SEPKOSKI Well, I think that what we have now is an entirely different way of looking at the fossil record than existed, or was even possible, in Darwin's day. Darwin was unable to ask many evolutionary questions about the fossil record because, frankly, the record was very poorly known and he lacked the tools to do so. In fact, in the early 1840s, one of Darwin's contemporaries, an English geologist named John Phillips, attempted the first really serious accounting of the fossil record and from this he was able to establish patterns in the history of life. But, even Phillips' attempt was handicapped by the tremendous gaps in knowledge of the record that existed at the time. But, intriguingly, the basic patterns that were first detected by John Phillips, which he used to demarcate, to establish the major eras in history of life have been confirmed by studies using massive fossil databases and computer analysis over the last several decades.
BOBBE MIXON: Now, Dr. Marshall, you write that the fossil record contains only about 5 percent of all the species to ever exist. Despite our improved knowledge, was Darwin right about the fossil record's inability to answer the big questions about evolution?
DR. CHARLES MARSHALL: In Darwin's day, he was certainly right because our knowledge of the fossil record was essentially zero of the over 500 species of dinosaur that we now know. In his time he knew just three--just three. That five percent number really just refers to our estimates of the completeness of the primate fossil record. There are many groups that are much more complete than that. For example, brachiopods, a marine group, has in the order of 90 percent completeness and most [inaudible] in those groups have percentages in the order of 50 percent complete made in a number of different ways. So, in fact the fossil record now is quite complete, especially for a wide range of theoretical questions of the sort that David just alluded to.
BOBBIE MIXON: Sticking with you for a moment, Dr. Marshall, I want to turn to this question of punctuated equilibrium, the idea that there are long periods of biological stability for a given organism interrupted by periods of rapid change. When Stephen Jay Gould and Niles Eldredge proposed the theory, what question were they trying to answer and did they answer it?
DR. CHARLES MARSHALL: I think they were trying to do two things. The first one is trying to understand just what are the patterns of morphological change in--on geologic timescales but they were also trying to establish that the fossil record has primary contributions to make to our understanding of evolution and isn't just sort of a secondary verification tool.
BOBBIE MIXON: Turning back to you, Dr. Sepkoski, Darwin's argument was that evolution was a slow, steady, gradual process. Does punctuated equilibrium challenge Darwin's understanding of evolution?
DR. DAVID SEPKOSKI: Yeah, that's an interesting question. I think that it's important to understand that punctuated equilibrium is an interpretation of the pattern of the fossil record, not a new theory of evolution and both Gould and Eldredge have stressed repeatedly that they are not attempting to overturn Darwinism. Punctuated equilibrium does not question Darwin's mechanism of natural selection at the level of individual organisms. It does not argue, as people have sometimes misinterpreted, that new species are produced by sudden massive mutations. What it does do is question Darwin's assumption that evolution is always slow, steady and gradual.
BOBBIE MIXON: I'm interested in getting both your thoughts on what's the biggest remaining geological question related to evolution. Dr. Marshall?
DR. CHARLES MARSHALL: To me, the most essentially interesting question is the extent to which abiotic, that is the sort of geologic climatic factors, have shaped the course of evolution as opposed to biotic factors…co-evolutionary factors. We do now know that the modern biosphere has been shaped by the great mass extinctions, particularly the End Permian mass extinction and also, to some extent, the End Cretaceous mass extinction. So we do know that the biota that we see now has been shaped by events in the distant past, things we could not have known without the geologic and fossil records. And so now comes the question, in detail, how is evolution--how is the biosphere co-evolved with the geosphere?
BOBBIE MIXON: Dr. Sepkoski?
DR. DAVID SEPKOSKI: Yeah. I think I'm going to partially agree with Charles on that question. I think one of the most exciting contributions of paleobiology over the last 50 years has been the confirmation that major extinction events, where anywhere from 30 to 90 percent of existing groups of organisms have been wiped out over a short period of time do, in fact, happen. Now, Darwin had rejected that idea because he associated it with biblical catastrophism but these events do take place, we now know, and with some regularity in the history of life and, as Charles alluded, they may often have abiotic, non-biological, causes, things like extraterrestrial bolide or, you know, comets, asteroid impacts, and that means they've had a major impact on evolution in ways that I think we're only just now beginning to fully understand.
BOBBIE MIXON: Dr. Charles Marshall, Harvard University, and Dr. David Sepkoski, University of North Carolina Wilmington. Thanks for joining us.
DR. CHARLES MARSHALL: Thank you very much. You're most welcome.
DR. DAVID SEPKOSKI Thanks so much for having me.