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Species and Speciation in the Fossil Record
By Warren D. Allmon, Margaret M. Yacobucci The University of Chicago Press
Copyright © 2016 The University of Chicago
All rights reserved.
ISBN: 978-0-226-37758-2
CHAPTER 1
The "Species Concept" and the Beginnings of Paleobiology
David Sepkoski
In the late 1940s, the discipline of paleontology took major steps towards becoming more fully integrated into the community of evolutionary biology. A primary reason for this was the involvement of George Gaylord Simpson in the development of the Modern Synthesis. Along with works by Theodosius Dobzhansky (Genetics and the Origin of Species), Ernst Mayr (Systematics and the Origin of Species), Sewall Wright, and others, Simpson's contributions to the emerging Synthesis between genetics, population biology, and paleontology were extremely important in reorienting the priorities of evolutionary biologists and paleontologists. Simpson's Tempo and Mode in Evolution (1944) and the revised follow-up, Major Features of Evolution (1953), were read by generations of paleontologists, and in large part defined the scope of paleontological contributions to evolutionary theory between 1950 and the 1970s. (On the growth of evolutionary paleontology — or "paleobiology" — see Sepkoski 2009, 2012).
The resulting Synthesis defined evolutionary biology as a study of the movement (via inheritance and mutation) of genes within populations. One of the most important aspects of the synthetic approach was the development of a quantitative understanding of gene flow in populations, which allowed biologists to confirm that Darwin's qualitative assessment of the sufficiency of natural selection to produce evolution agreed with the modern understanding of genetics; some historians, including Provine (1971), view this as the major accomplishment of the Synthesis. Herein, however, were the seeds of a potentially intractable problem for paleontologists: fossils leave no record of genetic information, and it is notoriously hard to reconstruct assemblages of fossils that correspond to living populations. So how could paleontologists accommodate the idiosyncratic evidence of the fossil record to the increasingly precise resolution expected in the growing field of population genetics?
This chapter will address the 'species question' in paleontology, or the problem paleontologists faced during the 1940s and 1050s in accommodating fossil data to the populational understanding of species promoted by geneticists in the Modern Synthesis. One strategy adopted by paleontologists was to redefine the very notion of the "paleontological" species as a "historical" concept that did not grant species independent ontological existence. Debates carried out in the journals Evolution and Journal of Paleontology in the early 1950s, for example, show that considerable divergence of opinion existed over whether the term "species" meant the same thing in both paleontology and neontology. I will argue, however, that the solution to the paleontological species problem ultimately came not from arguments about definitions but from a new methodological approach: a select group of paleontologists, led by the American Museum of Natural History (AMNH) invertebrate specialist Norman Newell, began to develop statistical, quantitative methods for evaluating and interpreting fossil populations. This approach was important for three reasons: (1) it posed a solution to the problem of how to incorporate "populational thinking" into paleontology; (2) it introduced greater analytical rigor (of the type increasingly expected in mathematical population biology) into paleontology; and (3), it introduced new theoretical possibilities for interpreting the evolutionary significance of the fossil record, which contributed to the further growth of evolutionary paleobiology.
Paleontology in the Modern Synthesis
Historians have emphasized that the synthetic project was, in large part, an institutional project. The architects of the Modern Synthesis called for dialogue between a variety of evolutionary disciplines, including genetics, ecology, systematics, zoology, anatomy, and paleontology, which necessitated the orchestration of what Joe Cain has called "institutionalized cooperation." According to Cain, this allowed the architects to cross "disciplinary boundaries in pursuit of common problems ... to ensure inclusion and to elevate the status of fields and practices otherwise deemed marginal within biology" (Cain 1993, 2). This is part of the process of "unification" that Betty Smocovitis describes as involving several discrete steps, beginning with Dobzhansky's successful translation of mathematical population genetics into terms comprehensible to the average naturalist, and concluding with the establishment of the Society for the Study of Evolution and its journal, Evolution (Smocovitis 1996, 99–127).
Dobzhansky's Genetics and the Origin of Species (1937) provided the wake-up call to biologists, but it was Julian Huxley and Mayr who took the lead in advancing the institutional agenda of the Synthesis. Shortly after the publication of Dobzhansky's monograph in 1937, Huxley began organizing a movement in Britain to redefine the field of systematics in light of advances in genetics. This activity culminated in the book The New Systematics (1940), which Huxley edited for the British Systematics Association, which intended "to integrate the various studies of divergence and isolation and relate them to taxonomic groups and evolutionary mechanisms" (Cain 1993, 4–5). Meanwhile, Huxley pursued a similar reform project in America, and was able to generate interest among a number of important biologists, including Alfred Emerson, Dobzhansky, and Mayr, sufficient to launch a working group called the Society for the Study of Speciation in 1940 (Cain 1993, 7; Smocovitis 1994, 1–2). This group was short-lived, as the intervention of the war and the arrival of Dobzhansky to the faculty at Columbia shifted the center of the Synthesis to New York, where the AMNH also played a prominent role. There the Columbia biologist L. C. Dunn oversaw publication of the Columbia Biological Series of monographs, whose titles included Dobzhansky's Genetics and the Origin of Species, Mayr's Systematics and the Origin of Species (1942), Simpson's Tempo and Mode in Evolution (1944), Bernhard Rensch's Evolution above the Species Level (1959), and other seminal works of evolutionary biology. This series was an enormously effective tool for promoting the agenda of the Synthesis, and was centered around Dobzhansky's influential interpretation of population genetics. It also provided a vehicle for members of disciplines outside genetics to promote their own theoretical legitimacy: as Smocovitis argues, these monographs "were written by individuals who, engaging in dialogue with Dobzhansky, in turn legitimated as they grounded their disciplines with Dobzhansky's evolutionary genetics" (Smocovitis 1996, 134).
Simpson and other paleontologists firmly believed that paleontologists could not become part of the community of evolutionary biology if their work was only presented to, and read by, other paleontologists. For this reason, paleontologists who were committed to the project of the Modern Synthesis, like Simpson, Norman Newell, Carl Dunbar, Benjamin Burma, and others, actively sought to present their ideas in the journal Evolution, the newly established organ of the Society for the Study of Evolution, rather than in traditional paleontological outlets like Journal of Paleontology. During the early 1950s, this led to the somewhat awkward circumstance in which paleontologists actively carried out parallel public debates over the species problem in two separate forums — Evolution and Journal of Paleontology — that only partially overlapped. However, because I am most interested in understanding how paleontologists self-consciously attempted to integrate themselves into the broader evolutionary community, I will focus my discussion on a series of arguments that were presented in Evolution between 1947 and 1951 and their aftermath.
From one perspective, it was a legitimate triumph for paleontologists that their discipline was recognized so prominently in the institutionalization of the synthetic theory. Without question, this was largely due to Simpson's efforts, which were undeniably heroic. It would be a mistake, however, to conclude that paleontology was, whether in 1940, 1944, 1946, or afterwards, a fully equal and respected partner in the community of neo-Darwinian evolutionary biology. In fact, considerable pressure was exerted by biologists to ensure friendly paleontologists' "cooperation" in adhering to the synthetic party line. In 1944, the Princeton biologist Glenn Jepsen stressed to Mayr that "paleontology presents good evidence, as you know, that evolution proceeds by microgenetic rather than macrogenetic alterations and that this evidence is in harmony with experimental genetics," and he appealed to Mayr's colleague, the invertebrate paleontologist Kenneth Caster, "I hope you will be willing to make a statement on this subject" (quoted in Cain 1993, 12). The concern here was that many paleontologists had in the past been "seduced" by macromutations and saltations as explanatory mechanisms for major evolutionary change. Indeed, Mayr recalled many years later that "most paleontologists were either saltationists or orthogenesists, while those we believe to have been neo-Darwinists failed to write general papers or books" (Mayr 1980, 28). As a response, Cain concludes, biologists "effectively controlled the identity of biology through the synthesis period," and "'synthesis,' from this perspective, meant the expansion of laboratory work together with the subsumption of descriptive studies by field and museum workers who were 'brought into line'" (Cain 1993, 17–18).
The trend in paleontological statements about evolution, then (as evidenced for example in published annual presidential addresses of the Paleontological Society from that period), reflects paleontology's move closer to the mainstream of biological evolutionary theory (and away from saltationism and orthogenesis), but also indicates the extent to which the "disciplining" efforts of synthetic biologists were successful in engineering agreement with neo-Darwinian principles. Paleontologists would certainly benefit from greater participation in the evolutionary biology community — more secure institutional positions, greater respect for their data, better access to mainstream publications and conferences, and a larger stake in theoretical discussions all followed over the next few decades. But there was a cost as well: as Patricia Princehouse argues, "in large part the Modern Synthesis served to sideline major research traditions in paleontology" (Princehouse 2003, 21). One of those traditions involved approaching macroevolutionary analysis of the fossil record with confidence that paleontology had unique access to patterns and processes of evolution undetectable by genetics or systematics.
The Species Question
As early as 1940, Huxley situated the species problem at the heart of the emerging evolutionary Synthesis. In his introduction to The New Systematics, Huxley defined the project of the "new systematics" as "detecting evolution at work," and in particular answering the question of "how discontinuity in groups is introduced into the biological continuum," or in other words answering whether the divisions imposed by taxonomy correspond in some natural sense to real populations of organisms (Huxley 1940, 2). Huxley's answer was fairly unequivocal: in most cases "species can be readily delimited, and appear as natural entities, not merely convenient fictions of the human intellect," and he concluded that "species are in some sense valid natural groups" (Huxley 1940, 11, 16). The same year, Mayr offered his now-famous definition of the "biological species concept": species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (Mayr 1940). This definition was reprinted two years later in Systematics and the Origin of Species (Mayr 1942), where it had wide influence in the emerging Synthesis.
While Mayr's discussion of the species concept sidestepped questions about the ontological reality of biological species, it is important to note that this definition effectively excluded paleontologists from the discussion. Paleontologists have no way of grouping fossils into interbreeding populations or of assessing reproductive isolation, and so are limited to estimating degrees of morphological similarity. Unlike Huxley, who had rather dismissively claimed that paleontology "can only give us information concerning the course of evolution, and not concerning its mechanism," Mayr did acknowledge the challenge of addressing what he calls "allochronic" (temporal) species in an imperfect fossil record. Ultimately, however, the best he could offer is that paleontologists and biologists mean different things when they discuss species: as he put it, "the 'species' of the paleontologist is not necessarily always the same as the 'species' of the student of living faunae," since paleontologists have no choice but to impose arbitrary divisions based on breaks in stratigraphic sequences (Mayr 1942, 154).
One of the first responses to this problem from the paleontologists came from Norman Newell, who published a paper titled "Infraspecific Categories in Invertebrate Paleontology" in the third issue of the journal Evolution. Newell, in 1947, was recently arrived as curator of invertebrate paleontology at the American Museum of Natural History, working under G. G. Simpson. It can fairly be said that nobody did more to promote the agenda of evolutionary paleontology in the 1950s and '60s than Newell, and his influence, measured directly through his work and indirectly through his mentoring of students and younger paleontologists, was profound. Newell's hand touched nearly every major aspect of paleobiology during his career, and he can be said to have been directly responsible for, in no particular order: the investigation of broad patterns in the fossil record, the development of quantitative approaches to fossil databases, the study of the evolutionary significance of mass extinctions, and the creation of the subdiscipline of paleoecology. Throughout his career, Newell also tirelessly promoted the institutional agenda of paleobiology, and he trained many of the leaders of the movement's next generations.
Newell's 1947 paper was clearly a call to action for both paleontologists and biologists. The opening line proclaimed that "evolution as a modern philosophy requires the synthesis of paleontology, genetics, and neontology," and Newell proceeded to diagnose just what he thought paleontology could and should contribute to that Synthesis (Newell 1947, 163). It is noteworthy that Newell favored the term "neontology" here and in later publications; he regarded paleontology and biology as sister disciplines, and in part his championing this notion influenced important members of both communities. Newell's target in this essay was the gap between the paleontologic and neontologic understandings of "species," the closure of which, he argued, was a crucial step in facilitating greater synthesis. The problem involved the common paleontological practice of basing taxa on single type specimens, which, in Newell's mind, failed consider "the variability of organisms in taxonomy." In other words, whereas biologists understood "species" as populations of organisms exhibiting graded variability, paleontologists tended to assign specimens that differed only slightly from one another to separate taxa.
The solution, according to Newell, was for paleontologists to adopt the biological concept of "subspecies" and to develop greater sophistication in establishing methods for discerning the true relationships between related organisms. His definition of subspecies as "entire populations, or races, which have become differentiated through some degree of isolation" was drawn from Mayr's 1942 Systematics and the Origin of Species, and explicitly conceived of these populations as geographic and populational units (Newell 1947, 164). Of course, paleontologists are unable to establish genetic relationships between fossil organisms, but Newell argued that paleontology would benefit simply from paying closer attention to the populational and biogeographical vocabulary of biology. For instance, he urged paleontologists to abandon the imprecise concept "variety," which does not necessarily connote a distinct population, in favor of the biological "subspecies," which usually does.
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Excerpted from Species and Speciation in the Fossil Record by Warren D. Allmon, Margaret M. Yacobucci. Copyright © 2016 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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