ORIGINS OF
DARWIN’S EVOLUTION
SOLVING the SPECIES PUZZLE Through TIME and PLACE
J. DAVID ARCHIBALD
Preface
For many people Charles Darwin epitomizes evolution even though he did not discover it. Before Darwin’s birth in 1809 the topic proved of great interest to scientists such as Jean-Baptiste Lamarck and Darwin’s grandfather, physician Erasmus Darwin. Although lost in the shuffle of today’s popular press, Charles Darwin rightfully receives credit (along with Alfred Russel Wallace) as the discoverer of the driver of evolution—natural selection. Unlike anyone before him, in 1859 Darwin presented in his magnum opus On the Origin of Species evidence establishing that evolution had occurred. In the ensuing years, natural selection waned in its acceptance until its final proof in the first third of the twentieth century, but never again would evolution be doubted by the scientific community. In popularized accounts of what Darwin accomplished, we see a mix of experiments on pigeon breeding, selective breeding of livestock, Lyellian uniformitarian principles used to explain gradual biological change over vast stretches of time, and the ancestry of humans. Although none of these parts to the story proves to be wrong, Darwin’s real basis for evolution too often hides in plain sight or at least becomes secondary in its telling. The most important source of information Darwin first used to prove evolution was historical biogeography, the subset of biogeography dealing with the study of the geographical distribution of organisms in time and place. Historical biogeography neither existed as a name nor as a science for much of the nineteenth century. Although not thought of as a great biogeographer like his friends and colleagues Joseph Hooker and Alfred Wallace because he did not publish major works on the topic, Darwin nonetheless
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must be placed in their ranks. Because of the immense scientific and cultural impact of Darwin’s theory of descent with modification by means of natural selection, the path to his discovery became secondary in many versions of the story. In the process of discovering his evidence for evolution, Darwin also helped to develop the science of historical biogeography. As Michael Ghiselin (1969) noted, “The fact that the general features of distribution can hardly be interpreted otherwise than as the result of evolutionary process forms one of the strongest arguments in The Origin of Species” (38). Darwin wrote as much in the first paragraph of On the Origin of Species in 1859 and later in his autobiography written in 1876 (but not published until after his death in 1887). Importantly, this historical biogeography, rather than simply being interpreted in the light of evolution, if viewed the other way around serves as the major proof of evolution. The three main parts of this proof included faunal succession (especially on continents such as Australia and South America), the geographical distribution of living plants and animals on the various continents and in the seas, and the rather peculiar kinds of plants and animals found on oceanic islands far from the nearest mainland. These in combination served as Darwin’s first major proof that evolution occurs. One might fairly ask how these three approaches prove evolution to be true. As I show in this book, the three parts separately do not rise to the level of a proof, but in combination they provide powerful evidence, so important for Darwin that he devoted three full chapters in On the Origin of Species to these three topics alone. What might have been the other major evidentiary chapter in On the Origin of Species preceded these three chapters and came with the curious title “On the Imperfection of the Geological Record.” Darwin’s schooling in geology began in the older ideas of catastrophist geology and continued in a self-taught manner in the newer ideas of uniformitarian geology espoused by Charles Lyell. The latter views shaped and directed much of his work and thinking while on board the HMS Beagle. He in no manner abandoned his deep love of geology, but by the time he came to write this chapter his confidence in the rock and fossil record as a major proof of evolution had waned greatly, as witnessed by the chapter title if nothing else. He harbored serious doubts about the help that this topic could lend to his evolutionary theorizing. This became worse with subsequent editions, especially as establishing the age of Earth proved an insoluble problem during Darwin’s lifetime. He felt opposed by many in the paleontological and
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geological establishments, even listing nine of their number at the close of this chapter. One can fairly ask, if Darwin found the geological record imperfect, what about faunal succession or “the law of succession of types” he proposed in 1839? The two are clearly related topics both dealing with fossils, but for the former he meant our general inability to trace slowly changing fossil species through the extent of the rock record. Albeit with almost two more centuries of collecting, we still realize the incompleteness of the rock record. By faunal succession he meant the overall pattern one sees through time on any given continent. Mammals proved a favorite of his for such discussions, with the faunal histories of South America and Australia playing the greatest role. Today the general public knows of the very peculiar marsupialdominated mammalian fauna of Australia. Those of South America are lesser known, but as we shall see through Darwin’s eyes, the continent provided an equally unusual mammalian fauna in its past as well as today. Although he had previous work to fall back on, his fossil discoveries helped begin our modern understanding of mammals on this continent. He became interested in using the whole ancient history of a continent compared to its modern biota rather than trying to trace changes step by step for each species through the fossil record. The fossil record may be quite incomplete in its details, but the overall pattern remained intact, a fact Darwin knew well and used to his advantage. In his autobiography (1887) Darwin enumerated the three facts from his voyage on the Beagle that “could only be explained on the supposition that species gradually become modified” through what he termed “the transmutation of species.” First, discoveries of great fossil animals covered with armor like that on living armadillos deeply impressed him. Second, closely related animals replaced one another moving southward in South America. Third, plants and animals on the Galápagos archipelago show great affinities to counterparts in South America, these species differ slightly from island to island of the group, and none of the islands appear to be very geologically old. Again, these three components of historical biogeography proved not to simply be explained by evolution; they in fact, in tandem, served as his earliest strong support of evolution. For any given region, according to Darwin, be it a continent or an archipelago, its biota might be seen as the continuity of its fossil past into its
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living present, that such current distributions change in space over the region in question, and that species spread from nearby areas. Although this generalizes Darwin’s examination of South America, his intent was clear. South America presented a specific case of a more general pattern for evolutionary change, and fossils play a role in understanding the processes. Darwin offered what we would call a biogeographic history of South America to support his contention that such history offers support for evolution, but he did not argue that the fossil record per se could demonstrate a process of evolution through intermediary fossil forms. Darwin’s use of the fossil record as an independent proof of evolution proved much more circumspect and at times even apologetic, likely in large measure because of prevailing views of prominent geologists of the time, who for the most part were clearly anti-evolutionary in their outlook. I do not claim novelty for any of the specifics I present here; rather, I attempt to emphasize the importance of Darwin’s use of historical biogeography as his earliest major proof of evolution. This book seeks to bring these matters to the fore in understanding Darwin’s contributions to evolutionary theory.
“This is a fresh and stimulating reevaluation of the nature of Darwin’s argumentation behind his theory of evolution through natural selection. Particularly important is the focus on the evidence Darwin himself thought most important: the geographical distribution of organisms around the globe. This is a book that should be read both by Darwin scholars and by today’s practicing evolutionists.” —Michael Ruse, au thor of Defining Darwin: Essays on the History and Philosophy of Evolutionary Biology
Photo by Gloria E. Bader
“In this thoughtful and carefully researched book, Archibald makes it abundantly clear that it was biogeography, not geology or the fossil record, that provided Darwin and his supporters with the earliest compelling evidence for evolution. Origins of Darwin’s Evolution fills a significant gap in the literature on Darwin’s research methods and the birth of the modern theory of evolution.” —Michael Ghiselin, au thor of The Triumph of the Darwinian Method
Origins of darwin’s evolution
—Kevin Padian, Museum of Paleontology, Univer sit y of California , Berkeley
Solving the Species Puzzle Through Time and Place
J. David Archibald is professor emeritus of biology at San Diego State University and curator of mammals in the SDSU Vertebrate Collections. His books include Dinosaur Extinction and the End of an Era: What the Fossils Say (1996) and Aristotle’s Ladder, Darwin’s Tree: The Evolution of Visual Metaphors for Biological Order (2014), both from Columbia University Press.
“Charles Darwin begins The Origin of Species by saying that while on HMS Beagle he was struck by two classes of facts: the strange distributions of plants and animals on Earth, and the progression of forms in the fossil record from the oldest rocks to the youngest. These, and not variations in populations, first led him to doubt theories of special creation and the fixity of species. In this book, J. David Archibald shows how the facts of paleontology and biogeography led Darwin to suspect that organisms changed through time, and eventually to develop the central theory of all of biology. A very nice read that will open the perspectives of a great number of readers.”
Archibald
Praise for Origins of Darwin’s Evolution
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Origins of
Darwin’s Evolution
Solving the Species Puzzle Through Time and Place
J. David Archibald
Historical biogeography—the study of the history of species through both time and place—first convinced Charles Darwin of evolution. This field was so important to Darwin’s initial theories and line of thinking that he said as much in the very first paragraph of On the Origin of Species (1859) and later in his autobiography. His methods included collecting mammalian fossils in South America clearly related to living forms, tracing the geographical distributions of living species across South America, and sampling peculiar fauna from the geologically young Galápagos Archipelago that showed evident affinities to South American forms. Over the years, Darwin collected other evidence in support of evolution, but his historical biogeographical arguments remained paramount, so much so that he devotes three full chapters to this topic in On the Origin of Species. Discussions of Darwin’s landmark book too often give scant attention to this wealth of evidence, and we still do not fully appreciate its significance in Darwin’s thinking. In Origins of Darwin’s Evolution, J. David Archibald explores this lapse, showing how Darwin first came to the conclusion that, instead of various centers of creation, species had evolved in different regions throughout the world. He also shows that Darwin’s other early passion—geology—proved a more elusive corroboration of evolution. On the Origin of Species has only one chapter dedicated to the rock and fossil record, as it then appeared too incomplete for Darwin’s evidentiary standards. Carefully retracing Darwin’s gathering of evidence and the evolution of his thinking, Origins of Darwin’s Evolution achieves a new understanding of how Darwin crafted his transformative theory.