SYSTEMATICS AND THE ORIGIN OF SPECIES
On Ernst Mayr’s 100th Anniversary
Jody Hey, Walter M. Fitch, and Francisco J. Ayala Editors
THE NATIONAL ACADEMIES PRESS
THE NATIONAL ACADEMIES PRESS
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This volume is based on the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Systematics and the Origin of Species: On Ernst Mayr’s 100th Anniversary,” held December 16-18, 2004, at the Arnold and Mabel Beckman Center of the National Academies of Science and Engineering in Irvine, California. The articles appearing in these pages were contributed by speakers at the colloquium and have not been independently reviewed. Any opinions, findings, conclusions, or recommendations expressed in this volume are those of the authors and do not necessarily reflect the views of the National Academy of Sciences.
Library of Congress Cataloging-in-Publication Data
Systematics and the origin of species : on Ernst Mayr's 100th anniversary / Jody Hey, Walter M. Fitch, and Francisco J. Ayala, editors.
Includes bibliographical references.
ISBN 0-309-09536-0 (hardcover)—ISBN 0-309-54760-1 (pdf) 1. Species. 2. Biology—Classification. I. Mayr, Ernst, 1904-2005. II. Hey, Jody. III. Fitch, Walter M., 1929- IV. Ayala, Francisco José, 1934-
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Ernst Mayr, one of the 20th century’s greatest scientists and a principal author of the modern theory of evolution, passed away on February 3, 2005, at the age of 100. From December 16 to 18, 2004, before Mayr’s passing, a colloquium on “Systematics and the Origin of Species” sponsored by the National Academy of Sciences was held in his honor. The colloquium’s title was the same as that of Mayr’s 1942 book, generally considered one of the foundation books of the theory of evolution. The 17 papers that follow explore current knowledge about the main topics of Mayr’s book.
The modern theory of evolution embodies a complex array of biological knowledge centered around Darwin’s theory of evolution by natural selection couched in genetic terms. It is not one single theory with its corroborating evidence, but a multidisciplinary body of knowledge bearing on biological evolution: an amalgam of well established theories and working hypotheses together with the observations and experiments that support accepted hypotheses (and falsify rejected ones), which jointly seek to explain the evolutionary process and its outcomes. These hypotheses, observations, and experiments originate in disciplines such as genetics, developmental biology, neurobiology, zoology, botany, paleontology, and molecular biology.
Darwin’s theory of evolution (1859) argued that natural selection, the process accounting for the adaptation and diversity of organisms, emerges as a necessary conclusion from two premises: (i) the assumption that hereditary variations useful to organisms occur and (ii) the observation
that more individuals are produced than can possibly survive. A serious difficulty facing Darwin’s evolutionary theory was the lack of an adequate theory of inheritance that would account for the preservation through the generations of the variations on which natural selection was supposed to act. Theories then current of “blending inheritance” proposed that offspring struck an average between the characteristics of their parents. As Darwin became aware, blending inheritance could not account for the conservation of variations, because differences among variant offspring would be halved each generation, rapidly reducing the original variation to the average of the preexisting characteristics.
The missing link in Darwin’s argument was provided by Mendelian genetics. Mendel’s paper published in 1866 formulated the fundamental principles of a theory of heredity that accounts for biological inheritance through particulate factors (now called “genes”) inherited one from each parent that do not mix or blend but segregate in the formation of the sex cells, or gametes. Mendel’s discoveries, however, remained unknown to Darwin and, indeed, did not become generally known until 1900, when they were simultaneously rediscovered by several scientists.
The synthesis of Mendelian genetics with Darwin’s theory of natural selection was initially accomplished in the 1920s and 1930s through the theoretical work of several geneticists who used mathematical arguments to show, first, that continuous variation (in such characteristics as size, number of progeny, longevity, and the like) could be explained by Mendel’s laws and, second, that natural selection acting cumulatively on small variations could yield major evolutionary changes in form and function. Distinguished members of this group of theoretical geneticists were R. A. Fisher (1930) and J. B. S. Haldane (1932) in Great Britain and Sewall Wright (1931) in the United States. Their work provided a theoretical framework for the integration of genetics into Darwin’s theory of natural selection but had a limited impact on contemporary biologists because (i) it was formulated in a mathematical language that most biologists could not understand; (ii) it was almost exclusively theoretical, with little empirical corroboration; and (iii) it was limited in scope, largely omitting many issues, like speciation, that were of great importance to evolutionists.
The synthesis accomplished by the theoreticians was greatly expanded in the following decades by biologists coming from various disciplines who enlarged the initial theoretical synthesis with relevant concepts and theories and provided supporting empirical evidence. Several books are considered emblematic of this original expansion of the theory in addition to Mayr’s Systematics and the Origin of Species (1942), notably, Theodosius Dobzhansky’s Genetics and the Origin of Species, published in 1937, George Gaylord Simpson’s Tempo and Mode in Evolution (1944), and G. Ledyard Stebbins’ Variation and Evolution in Plants (1950). Three earlier
colloquia sponsored by the National Academy of Sciences were dedicated to current knowledge concerning the distinctive topics originally explored in these books (Ayala et al., 2000; Ayala and Fitch, 1997; Fitch and Ayala, 1995).
One key development of the theory of evolution is the replacement of “population thinking” by “typological thinking.” Darwin had postulated that hereditary variations occur in organisms that are useful to the organisms themselves. Natural selection could only occur if such variations were pervasive. In genetics, populational thinking gave rise to a new branch of genetics that, as Dobzhansky (1937) noted, “has as its province the processes taking place in groups of individuals—in populations—and therefore is called the genetics of populations…. The rules governing the genetic structure of a population are distinct from those governing the genetics of individuals.”
Mayr’s Systematics and the Origin of Species (1942) represents a selfconscious effort to explicate the significance of population variation in the understanding of evolutionary processes and the origin of new species. “It is true that the change from the static species concept of Linnaeus to the dynamic species concept of the modern systematist has not entirely escaped the attention of progressive students of genetics and evolution. However, the whole significance of the polytypic species, of the phenomena of geographic variation, of the differences between geographic and other forms of isolation are by no means as widely appreciated … as they deserve” (1942).
Mayr would later write: “Systematics, contrary to widespread misconceptions, … was not at all in a backward and static condition during the first third of the 20th century…. Population thinking was widely adopted, and, as a consequence, variation within and between populations was actively studied, which led to the development of the biological species concept, to the widespread adoption of polytypic species taxa, and to the study of species in space and time as adapted systems…. [T]he experimental geneticists, with few exceptions, were quite unaware that a populational species concept had been widely adopted by naturalists” (1980a). “The biological species concept emphasizes the species as a community of populations, reproductive isolation, … and the ecological interactions of sympatric populations that do not belong to the same species” (1980a). Species as taxonomic entities and, most of all, as populations and units of evolution have remained Mayr’s supreme subject of intellectual engagement. In his most recent book, Mayr writes: “The species is the principal unit of evolution. A sound understanding of the biological nature of species is fundamental to writing about evolution and indeed about almost any aspect of the philosophy of biology…. I define biological species as ‘groups of interbreeding natural populations that are repro-
ductively (genetically) isolated from other such groups.’ The emphasis of this definition is … on genetic relationship…. This new interpretation of species of organisms emphasizes that biological species are something very different from the natural kinds of inanimate nature” (2004).
Ernst Mayr was born on July 5, 1904, in Kempton, Bavaria, Germany. On July 1, 1926, he became an assistant at the University Museum in Berlin “but left for New Guinea and the Salomon Islands in February 1928. I did not return until the end of April 1930” (Mayr, 1980b). He came to the United States in 1931 to be curator for birds at the American Museum of Natural History in New York. In 1953, he became Alexander Agassiz Professor of Zoology at Harvard University, where from 1961 to 1970 he was director of the Museum of Comparative Zoology and retired from the faculty in 1974.
Mayr’s scholarly publications span >80 years, starting with his first two scientific papers, published in 1923, and reaching to 2004. Walter J. Bock, who has written a fairly comprehensive overview of Mayr’s career, divides his contributions into three major periods. “The first period (1923 until 1953 when he left the American Museum of Natural History) was devoted mainly to avian systematics and the theory of systematics. This work formed the foundation for the second period (beginning in 1942 but becoming more dominant in the latter part of the 1940s and lasting until his formal retirement from Harvard University in 1974), which was devoted largely to evolutionary theory. His systematic and evolutionary contributions, in turn, provided the basis for the last period (beginning in the early 1970s), devoted chiefly to the history and philosophy of biology” (Bock, 1994).
The bibliography listed by Bock includes 176 publications (1923–1994), all but a baker’s dozen single-authored by Mayr. Some of Mayr’s most important books, in addition to Systematics and the Origin of Species, are Animal Species and Evolution (1963), Principles of Systematic Zoology (1969), Populations, Species, and Evolution (1970), Evolution and the Diversity of Life (1970), The Growth of Biological Thought (1982), and Toward a New Philosophy of Biology (1988). Remarkably, Mayr has continued publishing essays, articles, and books to the present. Mayr’s most recent book, What Makes Biology Unique? Considerations on the Autonomy of a Scientific Discipline (2004), was published in August 2004, one month after his 100th birthday. In 2001, at age 97, Mayr had published two other important books: Ernst Mayr and Jared Diamond’s The Birds of Northern Melanesia: Speciation, Ecology and Biogeography (2001) and Ernst Mayr’s What Evolution Is (2001). Mayr’s other recent books are This Is Biology: The Science of the Living World (1997) and One Long Argument: Charles Darwin and the Genesis of Modern Evolutionary Theory (1991).
The colloquium honoring Mayr’s book and its legacy featured 17
presentations, including one by E. O. Wilson, “Introductory Essay: Systematics and the Future of Biology” (Chapter 1), which appears immediately after this preface. Wilson makes the point that the tremendous growth in molecular and cellular biology will be hampered if it is not balanced by similar progress in our understanding of biological diversity. He argues forcefully for growth in systematics and biodiversity research and for the establishment and increasing use of Internet-based virtual collections so that researchers and laypersons can freely access the resources of museums worldwide.
The remaining chapters in this volume are distributed in four parts: Part I, The Origins of Species Barriers; Part II, Discerning Recent Divergence; Part III, The Nature of Species and the Meaning of “Species”; and Part IV, Genomic Approaches and New Insights on Diversity.
Ayala, F. J. & Fitch, W. M., eds. (1997) Genetics and the Origin of Species: From Darwin to Molecular Biology 50 Years After Dobzhansky (Natl. Acad. Sci., Washington, DC).
Ayala, F. J., Fitch, W. M. & Clegg, M. T., eds. (2000) Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins (Natl. Acad. Press, Washington, DC), pp. xi, 340.
Bock, W. J. (1994) Ernst Mayr, naturalist: His contributions to systematics and evolution. Biol. Philos. 9, 267–327.
Darwin, C. (1859) On the Origin of Species by Means of Natural Selection (Murray, London).
Dobzhansky, T. (1937) Genetics and the Origin of Species (Columbia Univ. Press, New York).
Fisher, R. A. (1930) The Genetical Theory of Natural Selection (Clarendon, Oxford).
Fitch, W. M. & Ayala, F. J., eds. (1995) Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson (Natl. Acad. Press, Washington, DC), pp. viii, 325.
Haldane, J. B. S. (1932) The Causes of Evolution (Longmans, Green & Co., London).
Mayr, E. (1942) Systematics and the Origin of Species (Columbia Univ. Press, New York).
Mayr, E. (1980a) Prologue: Some thoughts on the history of the evolutionary synthesis. In The Evolutionary Synthesis, eds. Mayr, E. & Provine, W. (Harvard Univ. Press, Cambridge, MA), pp. 1–48.
Mayr, E. (1980b) How I became a Darwinian. In The Evolutionary Synthesis, eds. Mayr, E. & Provine, W. (Harvard Univ. Press, Cambridge, MA), pp. 413–423.
Mayr, E. (2004) What Makes Biology Unique? Considerations on the Autonomy of a Scientific Discipline (Cambridge Univ. Press, Cambridge, U.K.).
Mendel, G. (1866) Versuche über Pflanzen-Hybriden. Verhandlungen des Naturforschenden Vereines in Brünn 4, 3–47.
Simpson, G. G. (1944) Tempo and Mode in Evolution (Columbia Univ. Press, New York).
Stebbins, G. L. (1950) Variation and Evolution in Plants (Columbia Univ. Press, New York).
Wright, S. (1931) Evolution in Mendelian populations. Genetics 16, 97–159.
Introductory Essay: Systematics and the Future of Biology
Inter-Locus Antagonistic Coevolution as an Engine of Speciation: Assessment with Hemiclonal Analysis
Chromosome Speciation: Humans, Drosophila, and Mosquitoes
Developmental Plasticity and the Origin of Species Differences
Speciation in Birds: Genes, Geography, and Sexual Selection
Critical Review of Host Specificity and Its Coevolutionary Implications in the Fig/Fig-Wasp Mutualism
Evolutionary Animation: How Do Molecular Phylogenies Compare to Mayr’s Reconstruction of Speciation Patterns in the Sea?
Mayr, Dobzhansky, and Bush and the Complexities of Sympatric Speciation in Rhagoletis
On the Origin of Lake Malawi Cichlid Species: A Population Genetic Analysis of Divergence
A Multidimensional Approach for Detecting Species Patterns in Malagasy Vertebrates
Examining Bacterial Species Under the Specter of Gene Transfer and Exchange
Ernst Mayr and the Modern Concept of Species
Decoding the Genomic Tree of Life
Prospects for Identifying Functional Variation Across the Genome
Genetics and Genomics of Drosophila Mating Behavior
Genomes, Phylogeny, and Evolutionary Systems Biology