Evolution of Evolution — Text-only | Flash Special Report
Modern Paleobiology: Out of Darwin’s Shadow
Knowledge of early 21st cetury fossil record bears little resemblance to 1859
By Charles Marshall & David Sepkoski
Charles Darwin recognized that the fossil record—evolution's 'time machine'—provided the best available test of his hypothesis of descent with modification. “On the Origin of Species,” however, bears witness to his disappointment that the fossil record did not reveal the abundance of intermediate forms he expected to see. In fact, Darwin devoted an entire chapter in Origin, 'On the Imperfection of the Geological Record,' to explain why the absence of such forms might be expected as the inevitable result of an incomplete fossil record. Thus, while the central importance of the fossil record was recognized from the outset, its perceived failure in 1859 left a deep pall over its value to our understanding of evolution.
However the paleontology of Darwin's era bears little resemblance to modern paleobiology. First and foremost, our knowledge of the fossil record has increased enormously since Darwin's time, and is still accelerating. For example, in 1859 only three genera of non-avian dinosaurs were known; in 1990 there were 285 valid genera, and now there are over 527. Secondly, we have developed methods for quantifying the incompleteness of the fossil record that enable us to determine whether the fossil record will be of value for a given question. In primates, for example, it appears that only about 5 percent of all species that have ever existed have been found in the fossil record. At this preservation rate, we only expect about 9 percent of these fossil species to be directly ancestral to another fossil species. Indeed, for many groups, as Darwin observed, we do not expect to see direct evidence of descent with modification.
Paleobiology also has altered the character of modern evolutionary biology. Major contributions include punctuated equilibrium, the discovery that stasis is a major feature of evolution, and the development of hierarchical approaches to understanding the large-scale patterns of origination and extinction, including the invigorating debate over species-level selection—i.e., determining whether the process of natural selection shapes entire biological groups as well as smaller units such as individuals, genes and cells.
Less controversial, but equally vital, are paleobiology's ongoing contributions to our understanding of the assembly of ecosystems, and to the nature of biotic response[s] to short- and long-term climate change. Moreover, paleontology has been enriched by the integration of data and ideas from geology, evolutionary biology, ecology, molecular biology, evo-devo and genomics.
While Darwin's legacy may have been bittersweet, it had an undeniably salutary effect on the modern field of paleobiology. Many of the 20th century's greatest paleontologists, from George Gaylord Simpson to Stephen Jay Gould, took Darwin's gloomy assessment of the incompleteness of the fossil record as a challenge, rather than a discouragement. In many ways, the ongoing research program of paleobiology is a testament to the central importance Darwin placed on the fossil record for the study of evolution, even though the science of paleobiology has vastly outstripped anything Darwin could have imagined in 1859. Darwin's assessment of the value of the fossil record would most likely have been very different if he knew what we now know today.
Charles R. Marshal is a professor of organismic and evolutionary biology and a professor of Earth and planetary sciences at Harvard University. He is also curator in the Department of Invertebrate Paleontology at the Museum of Comparative Zoology. He uses techniques in paleontology, developmental biology, statistics and molecular and morphological phylogenetics to understand the nature and causes of evolutionary innovation and extinction over geological time scales. Marshall is widely published.
David Sepkoski is an assistant professor of history at the University of North Carolina Wilmington (UNCW). He received his Ph.D. from the Program in History of Science and Technology at the University of Minnesota in 2002. Before joining the department at UNCW in 2006, he taught in the history department at Oberlin College in Ohio. Sepkoski specializes in the history of evolutionary theory, particularly the interaction of paleontology and biology during the 20th century. He recently published “The Paleobiological Revolution: Essays on the Growth of Modern Paleontology” with Michael Ruse, a professor of philosophy and director of the Program in the History and Philosophy of Science at Florida State University.
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