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Wednesday, September 10, 2008

Darwin & Paleontology: 50 years of conflict

by Brian Switek

melittle.jpg Brian Switek is an ecology & evolution student at Rutgers University.

"... for in all the boundless realm of philosophy and science no thought has brought with it so much pain, or in the end has led to such a full measure of the joy which comes of intellectual effort and activity as that doctrine of Organic Evolution which will ever be associated, first and foremost, with the name of Charles Robert Darwin." - Edward Poulton, "Fifty Years of Darwinism" (1908)

Edward Poulton and T.C. Chamberlin may have been impressed by evolution by natural selection during the centenary celebration of Charles Darwin's (portrait on the lower right) birth in 1908, but paleontologist H.F. Osborn (see photo below) was not as enthusiastic. To commemorate Darwin's achievements, primarily On the Origin of Species, a series of lectures were organized by the American Association for the Advancement of Science which were later collected in the book Fifty Years of Darwinism. While Chamberlin and Poulton championed evolution by natural selection as triumphant over the "fad" of Neo-Lamarckism that had cropped up in the wake of Darwin's work, other contributors felt that Darwin's theory was not adequate to explain evolutionary change. Osborn was one of them, and although he had mostly abandoned the concept of acquired characteristics, he latched onto an adapted form of Lamarck's "complexifying force" in the form of orthogenesis. He laid out this program in his contribution on paleontology and Darwin's theory.

As I have written about before, paleontology was a problem for Darwin. The succession of life in the geologic strata was not inconsistent with his theory of evolution, yet the fossil record did not offer the minutely-graded transitional forms his theory predicted. (T.H. Huxley would later propose at least three major evolutionary transitions during the 1870's, but even he had to admit that many of the fossils represented only approximations of expected transitional forms. Huxley concluded that the actual line of descent for two of those transitions, terrestrial carnivorous mammals to whales and reptiles to birds, had yet to be found. The recent evolution of the one-toed horse from three-toed ancestors was a better example of a direct line.) Even beyond the conceptual issues, some of the most vociferous opponents of Darwin's theory were paleontologists and geologists. In 1860 Darwin wrote to the father of American paleontology, Joseph Leidy, complaining;

Most paleontologists (with some few good exceptions) entirely despise my work... all the older geologists with the one exception of Lyell, whom I look at as a host in himself, are even more vehement against the modification of species than are even the paleontologists.

In the face of such opposition, Darwin was primarily concerned with getting other workers to at least partially accept his conclusions. As he acknowledged in the letter to Leidy, he was sure that some of his ideas would turn out to be false, but he was sure that the core of evolution by natural selection was correct. In terms of paleontology, this would require a new type of program that specifically tried to identify transitions in the fossil record.


In Osborn's view, "Darwinian paleontology" did not really start until 1868 when German paleontologist W.H. Waagen described evolutionary changes in ammonites, or the coiled shells of ancient cephalopods. Yet Waagen's was not truly a Darwinian view. According to Peter Bowler's historical review The Eclipse of Darwinism, Waagen viewed ammonites as a group that underwent a kind of senescence in which they degenerated and became extinct because of mutations. (Although it would have been unknown to Waagen, the weird form Nipponites is a good example of the baffling arrangements seen in the later ammonites. These forms have long vexed paleontologists, as many seem to be caught in the act of uncoiling or becoming tangled up in their own shells.) Thus ammonites were not examples of evolution due to natural selection working on variation; there were internal forces involved that pushed groups to evolve and eventually to "over-evolve" particular traits to the point where they became extinct.

Even if the theories of change did not fit, Waagen at least tried to form an evolutionary sequence. Such quests were initially inspired by Darwin's work. Osborn compared Darwin to Moses, leading researchers to the Paleontological Promised Land but never entering it. Darwin had set paleontologists in search of lines of descent in the fossil record. As more strata were mined for fossils, particularly in America, the results clearly showed that evolution had occurred. Darwin was right, but Osborn only thought he was correct in the general sense.

Osborn thought that natural selection had an important role to play in nature; it acted as a discriminating force tied to extinction. As evidence of this Osborn suggested that a particular pattern of grinding teeth independently possessed by North American herbivores during the Oligocene and Miocene (spanning from about 34 million years ago to about 5 million years ago) all led them to extinction. Indeed, selection was primarily a destructive force. Beyond that, it appeared to account for little.

In terms of gradual changes seen in the fossil record (i.e. the gradual lengthening of the neck of the giraffe), Osborn stated that there was no evidence of natural selection. What's more, he defied the implications of Dollo's Law (namely that what has already evolved dictates what can evolve, therefore preventing exact evolutionary reversal; there's no going backwards), and stated that the conditions of ancestors do not dictate future evolution. Speaking specifically in terms of the proportions of the skull and foot, Osborn asserted that with each generation the creatures were free to evolve in any particular direction. Indeed, he may have made concessions to Darwin in the introduction, but it was not long before the truth came out;

In all the research since 1869 on the transformations observed in closely successive phyletic series no evidence whatever, to my knowledge, has been brought forward by any paleontologist, either of the vertebrated or invertebrated animals, that the fit originates by selection from the fortuitous.


For Osborn the core of evolution had little to do with chance. The driving force of transmutation was some unknown internal growth force. What's more, whatever this internal driving force may have been it was too slow to be observed in anything but the fossil record. Time made all the difference, and if variations meant little for evolution then there had to be something else pushing creatures forward;

The law of gradual appearance or origin of many new characters in definite or determinate directions from the very beginning I regard as the grandest contribution which paleontology has made to evolution.

Yet Osborn still had to recognize the role of adaptation. Just how had the important traits that allowed creatures to occupy a particular niche or have a certain mode of life arise? Osborn proposed that some internal directing force caused the emergence of new structures that were already adaptive when they arose. He called these "rectigradations";

... the law of gradual change in certain determinate, definite, and at least in some cases adaptive directions, through very long periods of time, and the absence of chance or non-direction in the origin of a large number of adaptive and other new characters, is the common working principle both in vertebrate and invertebrate paleontology.

Perhaps most shocking of all, Osborn asserted that Darwin was on the path to coming to a similar view of evolution;

Here we find ourselves expanding a principle which was clearly foreshadowed by Darwin, and which, had he pursued it to its logical sequence, would have brought him to orthogenesis, namely, that variations may not be without direction, but that law may lie among the hidden recesses of the nature of the organism; in other words, Darwin himself frequently professed ignorance of the laws of variation as well as the belief that such laws might be discovered.


Perhaps this was a bit of wishful thinking on Osborn's part (if he was right it would make him an important heir to Darwin's evolutionary legacy), and I do not think that Darwin would have come around to Osborn's view. The reason for this has to do with the way each researcher viewed nature, which is somewhat reminiscent of the debates between the French naturalists Geoffroy St. Hilaire (an evolutionist) and Georges Cuvier (an anti-evolutionist, painting on the left). (See The Cuvier-Geoffroy Debate for a historical analysis.) Although their arguments have commonly been construed to center on evolution, the basis of their debate hinged on different ways of understanding nature. Geoffroy had a focus on the emergence of form, primarily through development, while Cuvier was concerned with adaptation. (This may seem strange as we often associate the term "adaptation" with evolution, but before Darwin adaptation was often discussed in terms of how the Designer had perfectly adapted organisms to their habitats. Darwin turned this on its head by throwing out the designer and showing that natural selection could act as a directing force.) Adaptation caused by natural selection on fortuitous variations was key for Darwin while Osborn's view has more to do with potentially unknowable internal forces. It was the difference between evolution being driven from within (Osborn) or without (Darwin).

Osborn closed his contribution by quoting Aristotle, presuming that there is some unknown (or unknowable) law that gave purpose to structures. Teeth are for eating and eyes are for seeing, after all; how could chance variations ever produce such adaptive structures? Other paleontologists might have shied away from such views, but Osborn was one of the most prominent voices for paleontology during the first quarter of the 20th century. What he said has often been taken as an indicator for the state of paleontology during the time when he lived (which may not be true; we need more historical scholarship).

Whether paleontologists agreed with Osborn or not, pioneering lab work in genetics made field work and paleontology seem anachronistic. While workers in labs were slowly reviving natural selection, a few prominent paleontologists preferred orthogenic trends. This likely contributed to the divide between the "field sciences" and "lab sciences" in biology, and by the time G.G. Simpson published his major synthetic work Tempo and Mode in Evolution in the 1940's, he had to mention the tendency of geneticists and paleontologists to dismiss each others work.

Just as it had been in other areas of science, non-Darwinian mechanisms of evolution were eventually booted out of paleontology. Simpson's work marked a sea change in which natural selection and the fossil record were married together. Indeed, natural selection was successfully revived as the primary mechanism of evolution in all areas of biology. Yet I am still curious about the time between the publication of On the Origin and Tempo and Mode. Why did orthogenesis hang on for so long? Proponents of the concept in paleontology certainly thought they had a viable scientific alternative, yet I am sure that social factors (particularly in Osborn's case) made it an attractive hypothesis. Osborn might have loudly proclaimed his notions from the AMNH, but what did other paleontologists think? I don't have any answers (Osborn is a lightening rod for historical study), but I think the area is ripe with research opportunities. We really do need a history of paleontology from 1870-1940.

Original here

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