Born in Chicago, George Gaylord Simpson, probably the most influential paleontological thinker of the mid-20th century, was trained at Yale in vertebrate paleontology. In 1927 he took up a curatorial position in fossil mammals at the American Museum of Natural History, where he spent most of his career before proceeding to Harvard and then to the University of Arizona. Together with his American Museum colleague, the ornithologist Ernst Mayr, and the Columbia University geneticist Theodosius Dobzhansky, Simpson was one of the three main architects of what became known as the “Evolutionary Synthesis.”
Following the rediscovery of the principles of Mendelian genetics in 1900, evolutionary science entered a period of ferment in which all possible avenues for explaining the emergence and establishment of evolutionary novelties were explored. Most of these explorations departed quite radically from purely Darwinian notions of natural selection— which were, indeed, somewhat out of fashion in the opening decades of the 20th century. During the 1920s, however, a series of contributions by such notables as R. A. Fisher, J. B. S. Haldane, and Sewall Wright placed the emerging science of population genetics on a solid mathematical footing, focusing attention upon processes influencing the survival and propagation of particular alleles in populations. These new perspectives attracted the attention of evolutionary biologists from other fields, and rapidly formed the foundation for a new view of the evolutionary process based on Darwinian principles. Simpson’s book Tempo and Mode in Evolution (1944), written in trying conditions while he was in the U.S. Army during WWII, is credited, along with Dobzhansky’s Genetics and the Origin of Species (1937) and Mayr’s Systematics and the Origin of Species (1942), with setting the foundations for the neo-Darwinian view of the evolutionary process that was to dominate in biology for the next half-century.
Particularly in its post-1950 “hardened” form, the Synthesis taught that most, even all, evolutionary phenomena could be traced back to gradual, generation-by-generation changes in population gene frequencies under the guidance of natural selection. Simpson’s particular contribution to the Synthesis lay in the eloquence and erudition of his attempt to reconcile the “gaps” in the fossil record with a linear notion of evolutionary process. In Simpson’s view, the larger gaps—which he regarded as real, rather than as artifacts of the record’s completeness—equated with episodes of fast evolution (“tachytely”) which, like the slower episodes (“bradytely”) that produced smaller gaps, resulted from the same basic set of “determinants”—population sizes and variabilities, generation lengths, mutation rates, selection pressures, and so on. It was differences in the combination of determinants that gave rise to differing evolutionary patterns. As an extension of this line of reasoning, Simpson saw species not as discrete entities but as arbitrarily delineated segments of evolving lineages.
In addition to his very considerable and influential theoretical output, Simpson was a phenomenally productive systematic paleontologist whose interests ranged widely. He is probably best remembered for his studies of horse phylogeny, but he was also a major influence in paleoanthropology, especially around the time that this field capitulated to the Synthesis in the 1950s and 1960s.
- Laporte, L. F. (2000). George Gaylord Simpson: Paleontologist and evolutionist. New York: Columbia University Press.
- Simpson, G. G. (1944). Tempo and mode in evolution. New York: Columbia University Press.