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Introduction

Variation and evolution in plants and microorganisms: Toward a new synthesis 50 years after Stebbins

Francisco J. Ayala*, Walter M. Fitch*, and Michael T. Clegg

* Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697-2525; and College of Natural and Agricultural Sciences, University of California, Riverside, CA 92521

The present book is intended as a progress report on [the] synthetic approach to evolution as it applies to the plant kingdom” (ref. 1, p. ix). With this simple statement, G. Ledyard Stebbins formulated the objectives of Variation and Evolution in Plants (1), published in 1950, the last of a quartet of classics that, in the second quarter of the 20th century, set forth what became known as the “synthetic theory of evolution” or “the modern synthesis.” The other books are Theodosius Dobzhansky's Genetics and the Origin of Species (2), Ernst Mayr's Systematics and the Origin of Species (3), and George Gaylord Simpson's Tempo and Mode in Evolution (4). The pervading theory of these books is the molding of Darwin's evolution by natural selection within the framework of rapidly advancing genetic knowledge. Stebbins said it simply: “In brief, evolution is here visualized as primarily the resultant of the interaction of environmental variation and the genetic variability recurring in the evolving population” (ref. 1, p. xi).

Variation and Evolution in Plants distinctively extends the scope of the other books to the world of plants. Dobzhansky's perspective was that of the geneticist. Mayr 's was that of the zoologist and systematist. Simpson's was that of the paleobiologist. All four books were outcomes of the famed Jesup Lectures at Columbia University. Plants, with their unique genetic, physiological, and evolutionary features, had been left out of the synthesis until then. In 1941, the eminent botanist Edgar Anderson was invited to write botany's analogue to Mayr's Systematics and the Origin of the Species and to publish it jointly with Mayr's book. Anderson did not fulfill the task, and Stebbins was thereafter invited to deliver the Jesup Lectures in 1947. Variation and Evolution in Plants is the outgrowth of those Lectures.

The mathematical underpinnings of the modern synthesis were set between 1918 and 1931 by R. A. Fisher (5) and J. B. S. Haldane (6) in Britain, and Sewall Wright (7) in the United States. According to Darwin, evolutionary change occurs by natural selection of small individual differences appearing every generation within any species. Any change effected by selection is typically small, but it may amount to major change over time. In contrast, Thomas Huxley and Francis Galton, among Darwin's most dedicated supporters, argued that evolution occurs by selection of discontinuous variations, or sports, and that evolution proceeds rapidly by discrete leaps. In Huxley's view, natural selection, operating on only gradual differences among individuals, could hardly account for the gaps between existing species evident in the paleontological record. According to Galton, evolution proceeds by “jerks,” some of which imply considerable organic change, rather than as a smooth and uniform process.

In the latter part of the 19th century, the biometricians Karl Pearson and W. F. R. Weldon believed, like Darwin, in the primary importance of common individual differences. William Bateson, like other geneticists, argued for the primary importance of discontinuous variations. The controversy was acrimonious. The rediscovery of Mendelian inheritance in 1900 might have served as the common grounds to resolve the conflict. Instead, the dispute between biometricians and geneticists extended to continental Europe and to the United States. Bateson was the champion of the Mendelians, many of whom accepted the mutation theory proposed by De Vries (8), and denied that natural selection played a major role in evolution. The biometricians argued that Mendelian characters were sports of little significance for the evolutionary process. Fisher, Haldane, and Wright advanced theoretical models of evolutionary processes based on the natural selection of genetic changes (mutations) that are small when considered individually but are cumulatively of great consequence.

Theodosius Dobzhansky was the first to complete the mathematicians ' theoretical propositions with a wealth of biological knowledge and empirical support, and Mayr, Simpson, and Stebbins (and, less notably, many others) soon followed. Stebbins was particularly suited to bring in the evidence from plants. He was born in 1906 and became interested in natural history during his childhood. He started studying botany in his early teens while he was a student at Cate School in Santa Barbara, CA. As an undergraduate at Harvard (1924–1928), he came under the influence of Merritt Lyndon Fernald (1873–1950), a charismatic teacher and distinguished botanist, whom Stebbins accompanied on field trips to study the New England flora. In 1928, Stebbins became a graduate student at Harvard and worked on the cytology, geographic variation, and seed development of Antennaria, a genus that bore several apomictic species that could be collected in nearby localities. He would focus on the distinctive evolutionary role of vegetative reproduction in plants throughout his life.

The 17 papers that follow were presented at the colloquium “Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins.” The colloquium celebrated the 50th anniversary of the publication of Stebbins' classic book. Professor Stebbins, although frail for the last few years, intended to attend the colloquium. Alas, he became ill about 1 month before the colloquium was held and died on January 19, 2000, 2 weeks after his 94th birthday. The “Appreciation” that follows this introduction was delivered by Peter Raven, on January 28, at the time that had been reserved for Stebbins ( 9). The 16 papers following the “Appreciation” are organized into five successive sections: Early Evolution and the Origin of Cells, Viral and Bacterial Models, Protoctist Mod

This paper was presented at the National Academy of Sciences colloquium "Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins," held January 27-29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.

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To whom reprint requests should be addressed at: Department of Ecology and Evolutionary Biology, University of California, 321 Steinhaus Hall, Irvine, CA 92697–2525. E-mail: fjayala@uci.edu.



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