A New Synthesis
Paleobiogeography: Using Fossils to Study Global Change, Plate Tectonics, and Evolution. Bruce S. Lieberman. xvii + 208 pp. Kluwer Academic/Plenum Publishers, 2000. $72.50.
To nonspecialists, it might seem obvious that paleontology includes the study of how geological and geographical factors have shaped the course of evolution. So how can a book titled Paleobiogeography be published with endorsements from eminent paleontologists Niles Eldredge and Warren D. Allmon proclaiming it to be (in Allmon's words) "an important attempt to synthesize the fields of biogeography and paleobiology, which have been strangely isolated from each other for too long, to the detriment of both"? The puzzle is more perplexing since, as Lieberman shows, the issues he studies were taken seriously by the founders of evolutionary biology, including Charles Darwin, Alfred Russel Wallace and Thomas Henry Huxley. Even in the early decades of the 20th century it was still taken for granted that paleontology and biogeography went hand in hand.
So why did this early interaction cease in the mid-20th century? Contrary to the received wisdom, there seems to have been an element of disciplinary fragmentation in the Darwinian synthesis that came to dominate biology in the 1940s. Paleontologists certainly remained interested in the effects of geological changes (for example, as agents causing mass extinctions) and in large-scale geographical factors. They adopted the theory of continental drift to explain the dispersal of species as the ancient continent of Pangaea fragmented. But they did not have techniques that would allow a more fine-grained analysis of geographical factors.
Meanwhile, field naturalists such as Ernst Mayr used the greater precision that can be obtained from a study of the locations of living species to address issues such as speciation through geographical isolation. As a leading Darwinian, Mayr stressed the dispersal of organisms through migration and the adaptation of isolated populations to their environments. His approach was challenged in the 1970s by the advocates of vicariance biogeography, which rejected dispersal and attributed speciation to the breaking up of the ancestor's once-continuous geographical range. The more extreme vicariance biogeographers were openly anti-Darwinian, and the school became linked with the more radical pioneers of the cladistic technique of classification (see David Hull's Science as a Process [University of Chicago Press, 1988] for details of these controversies). No one noticed that the new approach to taxonomy might provide the tools needed for a more detailed account of paleobiogeography. Lieberman mentions the tensions between dispersal and vicariance biogeography, but if he had told this story in as much detail as he does that of Darwin and Wallace, he would have helped his readers understand why there is such a need for the synthesis he is attempting.
Lieberman's book is not a popular account of how the fossil record can be used to study mass extinctions or the movements of continents. It is a much more technical attempt to show that modern techniques of the cladistic analysis of characters can provide new lines of evidence that will allow us to reconstruct ancestral distributions and the subsequent evolutionary changes brought about by geological and geographical factors. He uses studies of North American trilobites to show how the creation of an ancient supercontinent could allow for "geodispersal"?the dispersal of many species simultaneously over a wider territory (as opposed to the chance migrations favored by Darwinism). This approach also challenges the refusal of vicariance biogeographers to admit the possibility of dispersal events, always a blank spot in their thinking.
Lieberman's book thus promises to initiate a new synthesis in the life sciences. But it also warns us that connections that appear obvious in retrospect may not be made because the internal dynamics of the scientific community encourage fragmentation rather than synthesis. There is surely a positive role for the sociology of science if it can help us to understand why such episodes occur.?Peter J. Bowler, Anthropological Studies, The Queen's University of Belfast, Ireland.