Plant speciation, the book: perspectives and paradigms

Authors


The purpose of this Special Issue of New Phytologist is to discuss plant speciation, the subject, and remember Plant speciation, the book (Grant, 1971, 1981). I have been asked to describe the climate in which the book was produced, first in 1971, in a medium sized edition, then in 1981, in a much expanded second edition.

Plant speciation, the book

One day in 1968 I sat down and drew up an outline of a thesis that I thought should be developed in book form. The thesis was the reticulate mode of plant evolution. Plant evolution proceeds in cycles: divergence to the species level, interspecific hybridization, and the origin of new species in the progeny of the hybrids. This mode of evolution had been recognized by a number of previous botanists, including Gustafsson (1946–1947), Stebbins (1950) and Clausen (1951), and in an early paper of my own (Grant, 1953). But it seemed worthwhile to put the scattered elements of the thesis together in a single book. The title of the manuscript at the start was: The divergence and refusion of plant species. That soon became shortened to Plant speciation.

The intrinsic interest of the subject matter was reason enough for writing the book, but there was a second reason. Most of the books on evolution in the early synthesis period of modern evolution – the 1930s and 1940s – were written by animal evolutionists. Most animals are diploid, sexual and outcrossing; consequently, evolutionary theory as it developed in the synthesis period emphasized a diploid sexual genetic system. But many or most species of higher plants have other kinds of genetic systems. Plants are different! There was a place for a study of evolutionary modes in a kingdom of organisms that exhibits a wide variety of genetic systems.

Three books had major influences on Plant speciation: Recent advances in cytology (Darlington, 1932, 1937); Apomixis in higher plants (Gustafsson, 1946–1947); and Variation and evolution in plants (Stebbins, 1950). Of course many other workers and works contributed ammunition to the Plant speciation book – it was a synthesis. Jens Clausen, William Hiesey and Edgar Anderson should be mentioned. The others are too numerous to list.

Another, and more general, influence on Plant speciation was the scientific paradigm of the time – the synthetic theory of evolution. This is evolution explained in terms of the forces that operate in populations. Plant speciation was written within the framework of the synthetic theory.

Scientific paradigms

Thomas Kuhn's concept of scientific paradigms is helpful and relevant (Kuhn, 1962, 1970). A scientific paradigm is a conceptual and logical framework that provides an explanation for the empirical evidence in a field. In any given era, workers in a field tend to subscribe to a standard paradigm. So-called ‘normal research’ is conducted within the prevailing paradigm. Workers tend to react negatively to unorthodox research papers that challenge the standard paradigm. However, over time, old paradigms are often replaced by new ones.

In Kuhn's treatment, paradigm shifts take place when new evidence contradicts major tenets of an old paradigm – then a new, more satisfactory paradigm is constructed. The shift in paradigms constitutes a scientific revolution in Kuhn's sense.

In the history of science, paradigm shifts have undoubtedly been stimulated by the inadequacy of an old framework, as Kuhn postulates. However, in evolutionary biology, at least, shifts also occur for other reasons. For example, work in an old field may reach a point of diminishing returns. New equipment and techniques may open up new fields. Two paradigms which exist concurrently in a state of isolation may at some point merge – hybridize – and ideas of one paradigm may introgress into the other paradigm. Paradigm shifts may occur in a variety of ways. Four different paradigms and several paradigm shifts come into our present story.

Paradigm shifts in evolutionary biology

What is the medium of communication that activates a paradigm shift? Is it journal articles? Lectures? Symposia? In evolutionary biology during the nineteenth and twentieth centuries the catalyst for a shift to a new paradigm has usually (but not always) been a book – a book that develops a thesis on a pivotal subject, synthesizes evidence for it, and appears in the early stages of the subject. In the paradigm of the synthetic theory, one can think of the role played by the books of Fisher (1930), Haldane (1932), Dobzhansky (1937, 1941), and Mayr (1942). This role was also played by the long, 62-page journal article of Wright (1931), almost a book.

My first introduction to the synthetic theory came in the spring of 1946 when I returned to the University of California, Berkeley, after the war, came into contact with Stebbins, took his course in evolution, and read Dobzhansky's (1941) book. I became a convert of the synthetic theory. All of my subsequent work, including the Plant speciation book, was done within this framework.

Now flashback to the 1930s. I first became acquainted with Darwin in 1934, in my high school years in Oakland, California. First I read The origin of species (1872); it was a joy to see theory and facts laid out so clearly. Then on to The voyage of the Beagle and The descent of man (Darwin, 1845, 1871, 1872). Then the books of Arthur Wallace (1889) and others. This was the paradigm of nineteenth century Darwinism. It was like the synthetic theory in several important respects, but it lacked genetics and was hazy about heredity. Partly for this reason, it went into a decline in the early twentieth century.

The books of Darwin, Wallace, Lyell, Haeckel, and other great nineteenth-century evolutionists were all available in the Oakland public library and in good second-hand book stores in the San Francisco Bay area in the mid 1930s. But the dates of the books were old then and there were few or no newer books on the subject. This was a sign of the decline of Darwinism and evolutionism in general in the early decades of the twentieth century, though I failed to pick up the sign at the time.

I started undergraduate studies in botany at UC Berkeley in 1936 in order to learn more about evolution. I was in for some surprises. I became immersed in another and different paradigm for the next 4 years – the study of the structure and function of individual organisms.

It is interesting to look back now and see how different the teaching of biology was in a major university in that era, the 1930s. The College of Letters and Science at UC Berkeley had two biological departments, Zoology and Botany. Both emphasized anatomy, comparative morphology, major groups and physiology. At the introductory level, there was no general biology course in the College of Letters and Science or anywhere else on the campus. Botany and Zoology each had its own introductory course which was a prerequisite for other courses. Students were channeled into one major or the other with little traffic between departments.

The College of Agriculture and the School of Forestry in UC Berkeley offered a number of good biological courses that were available as electives. Among these were general genetics, cytogenetics, and forest ecology.

Neither general genetics nor forest ecology was a required or even a recommended course for the botany major. The Botany faculty felt that ecology was a field lacking in rigor. As for genetics, we received 1 week of uninteresting instruction in Mendelian inheritance in freshman botany, and this was thought to be enough. I took forest ecology as an elective and profited greatly from it. I didn’t realize that I should have been interested in genetics and didn’t take it, and this was a big mistake.

Topics that the nineteenth century evolutionists dealt with were never discussed in the old Botany department. Natural selection was never mentioned. I never heard Darwin's name mentioned. Adaptation was regarded with skepticism and was only rarely mentioned. It was too close to the view of purpose in nature, and people made a special point of opposing teleology. Anti-teleology is all right, but it was a nonissue in the 1930s; it had been made a dead issue by Darwinism in the preceding century.

As I was to learn much later, good plant evolutionary work was going on in the Genetics department, College of Agriculture, UC Berkeley, under Babcock and Roy Clausen; and in the nearby Carnegie Institution station at Stanford, under Jens Clausen and William Hiesey. Neither I nor classmates I talked with were aware of these research groups at that time.

The situation in biology at Berkeley in the 1930s was paralleled in other universities around the country. The botany curriculum at Berkeley was standard to judge from the textbooks which were written in the eastern and midwestern US. However, pioneering work in evolutionary biology was also going on at scattered sites in Europe and the US in the 1930s. Two paradigms existed side by side for awhile, but the new evolutionary paradigm spread as the 1930s phased into the 1940s.

I left Berkeley in the summer of 1940 to go to Central and South America to see the tropical forests and Andes mountains and to try my hand at plant collecting. Later I served in the War Department in the Panama Canal Zone. Consequently I was isolated from academic botany and biology from 1940 to late 1945. When I returned to Berkeley in early 1946 I found great changes. Stebbins had joined the faculty and had introduced a new course in evolution in about 1941 or 1942. The botany and genetics faculties were collaborating on interdisciplinary studies in plant evolution. The paradigm of the synthetic theory had spread in Berkeley and elsewhere. This was a case of introgression from one paradigm into another.

Now a fourth paradigm has moved to center stage. This is the paradigm of cladistics, particularly molecular cladistics. Its center of focus is branching phylogeny and the construction of trees. It exists side by side with the older synthetic theory, which continues to be focused on evolutionary processes in populations and population systems.

The synthetic theory is still valid. Breeding populations still exist. So do reproductively isolated population systems, or biological species. Various modes of speciation are known. Various patterns of evolution of groups at levels above species are known.

Perspectives

So we have two paradigms dealing with evolution now. The interaction between them takes various forms. Sometimes they don’t interact. Many cladists practice cladistics as a self-sufficient system, with, for example, its own vocabulary and its own special definition of monophyly and other terms.

However, some exchange of ideas between the paradigms has taken place. Cladistics now accepts, in principle at least, nondichotomous branching brought about by two common modes of speciation – hybrid speciation and quantum or peripatric speciation. Conversely, molecular cladistic evidence now permeates evolutionary biological research. These exchanges have little effect if any on the core philosophies of the two paradigms, which remain distinct.

The two paradigms are far apart in the area of minor systematics. Speciation studies and taxonomy have long had beneficial effects on one another. Together they have engendered the early versions of the biological species concept and the geographical theory of speciation in the 1890s and early 1900s (Jordan, 1896, 1905; Rothschild & Jordan, 1903; Poulton, 1908; see Grant, 1994), and the mode of quantum or peripatric speciation in the 1950s (Mayr, 1954). On the other hand, cladistics and taxonomy have a history of rivalry. Cladistics was originally designed (Hennig, 1950) and is often presented (e.g. Moritz & Hillis, 1996; Judd et al., 1999) as a method to replace traditional taxonomy. However, cladistics deals with hierarchies of branch lines; has no criteria for distinguishing between divergence at the species level and at the racial level or level of species group; and is not concerned with the forces that drive these divergences. Cladistics is not designed for studies of speciation as a process.

Nevertheless, there is a bridge between the two paradigms. It involves the molecular evidence. Cladistics has been in the forefront in using this extremely valuable line of evidence. Good results can be obtained by taking up molecular cladograms, but not necessarily the cladistic interpretations of them, and fitting the cladograms into the new-Darwinian framework of plant speciation studies. This is known as recombination.

Acknowledgements

I wish to express my appreciation to Loren H. Rieseberg and Jonathan F. Wendel for organizing the symposium ‘Plant Speciation’ in 2003, to Loren H. Rieseberg and Karen A. Grant for help with the manuscript, and to Billie L. Turner for presenting the ideas outlined here at the symposium. It is a great pleasure to see so many younger workers going on with the study of plant speciation.

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