THE last twenty years have been a turning point in the history of man's knowledge and thinking about organic evolution. Great advances have been made in the fields of genetics, cytology, and the statistical study of populations, as well as in the more traditional descriptive fields of systematics and morphology. These coupled with increasing co-operation and interchange of ideas between scientists with different training and background have made possible a far broader conception of the processes of evolution as a whole than any which was open to evolutionists of even a generation ago. There is no need now for seeking hidden causes of evolutionary diversification or evolutionary progress, except in regard to certain specific processes, like mutation. Instead, the attention of scientists has been focused on evolution as a series of problems in dynamics. The direction and speed of the evolution of any group of organisms at any given time is the resultant of the interaction of a series of reasonably well-known factors and processes, both hereditary and environmental. The task of the evolutionist, therefore, is to seek out and evaluate all these factors and processes in respect to as many different groups of organisms as possible, and from the specific information thus acquired to construct such generalizations and hypotheses as he can. This requires the broadest possible knowledge of biology, which, if it cannot be acquired through direct contact with original research, must be built up vicariously through communication with biologists in different fields.

The present book is intended as a progress report on this synthetic approach to evolution as it applies to the plant kingdom, and in particular to the seed plants. It does not intend to offer any new hypotheses, except on certain limited phases of plant evolution. On the other hand, some hypotheses, speculations, and generalizations are offered whenever they seem to serve as stimuli for further research. No attempt has been made to give a final answer to any of the major problems confronting evolutionists, but the information and ideas are presented here in the hope that they will help to open the way toward a deeper understanding of evolutionary problems and more fruitful research in the direction of their solution.

Nevertheless, certain concepts of the nature of evolution seem to be now so well founded that they are taken as major premises on which the arguments of this book as a whole are based. The first of these is that evolution must be considered on three levels; first, that of individual variation within an interbreeding population or within a single colony of an asexual organism; second, that of the distribution and frequency of variants within a system of actually or potentially interbreeding populations, that is, the variation pattern on the population level within a species; and, third, the separation and divergence of populations or population systems as a result of the building up of isolating mechanisms, or the origin of species and consequently of separate evolutionary, lines. If terms are desired, these could be called individual variation, microevolution, and macroevolution. The dominant evolutionary processes are different at each of these levels. Individual variation is dominated by gene mutation and genetic recombination, microevolution by natural selection, and macroevolution by a combination of the effects of selection and of the development of isolating mechanisms, chiefly of a genetic-physiological nature. The second major premise is that at all three levels evolution has progressed chiefly by the accumulation of small changes, each with a relatively slight effect, rather than by single great jumps. The third and final one is that the speed and the direction of these changes have not been constant in any one line, as might be expected if they had been predetermined for that line. On the contrary, evolution appears as a sort of progressive opportunism, which in both direction and speed is regulated at any one time by the genetic constitution of the population and the environmental influences acting upon it at that time. Progressive evolution, when it has taken place, has been caused either by progressive alterations of the environment, by the action of selection in canalizing the genetic potentialities of the evolving population, or by both of these factors. This type of evolution has been much less prominent in plants than in animals and and therefore has received comparatively little attention in the present book. In brief, evolution is here visualized as primarily the resultant of the interaction of environmental variation and the genetic variability occurring in the evolving population.

Much of this book is based on a series of Jesup Lectures delivered at Columbia University in October and November, 1946. Parts of the manuscript were read by Edgar Anderson. Ernest B. Babcock, W. S. Boyle, F. G. Brieger, Spencer W. Brown, David G. Catcheside, Ralph W. Chaney, Jens Clausen, Roy E. Clausen, Ralph E. Cleland, Lincoln Constance, Pierre Dansereau, Everett R. Dempster, Th. Dobzhansky, Ralph Emerson, Carl Epling, Rudolph Florin, Adriance S. Foster, Edward Garber, H. Bentley Glass, Ake Gustafsson, E. Heitz, M. J. Heuts, W. Horowitz, James A. Jenkins, David D. Keck, I. Michael Lerner, H. E. McMinn, Herbert L. Mason, George Papenfuss, Harold H. Smith, Leon A. Snyder, A. H. Sturtevant, O. Tedin, Juan I. Valencia, Mogens Westergaard, Thomas W. Whitaker, and Ira L. Wiggins.

I am much indebted to all of these men for valuable suggestions. In particular, Drs. Babcock, Dobzhansky, Anderson, and Epling have discussed these problems extensively with me both before and during the writing of this book, and have given much needed assistance and encouragement at all times. Naturally, however, the complete responsibility for the factual statements and the theoretical conclusions presented here is mine, except where other authors are cited.

G. LEDYARD STEBBINS, JR.
Berkeley, California
April, 1949

Contents

DESCRIPTION AND ANALYSIS OF VARIATION PATTERNS

1. Exploring and Charting Patterns of Variation
2. Old and New Morphological Characters in the Higher Plants
3. Cytological, Serological, and Distributional Characters Qnantitative Methods in Descriptive Systematics Analyzing the Variation Pattern
4. Some Principles of Variation
5. Taxonomic Categories and Their Genetic Significance Some Genetic Terms and Their Taxonomic Significance

EXAMPLES OF VARIATION PATTERNS WITHIN SPECIES AND GENERA

1. The Ecotype Concept
2. Ecotypic and Clinal Variation
3. Ecotypes and Subspecies
4. Variation on the Level of the Species and Genus
5. Patterns in the Family Ranunculaceae
6. Variation Patterns in the Genus Potentilla
7. The Genus Quercus
8. Patterns in the Family Compositae
9. The Family Gramineae
10. General Conclusions

THE BASIS OF INDIVIDUAL VARIATION

1. Environmental Modification and Its Effects The Importance of Recombination
2. Types of Mutation and Their Significance Genetic Effects of Mutations
3. Rates of Mutation

NATURAL SELECTION AND VARIATION IN POPULATIONS

1. Experimental Evidence for Natural Selection
2. Historical Evidence for Natural Selection
3. The Adaptive Value of Diagnostic and Distinguishing Characteristics
4. The Indirect Action of Natural Selection
5. The Genus Camelina as an Example of the Action of Selection
6. Selective Correlation and Seed Characteristics
7. Kiv Contents
8. Selection and Differentiation in the Compositae and the Gramineae
9. Natural Selection and Morphological Differences: Summary
10. The Dynamics of Selection and Random Variation

GENETIC SYSTEMS AS FACTORS IN EVOLUTION

1. Mutation and Evolution in Asexual Organisms
2. Haploidy and the Evolution of Diploidy
3. Heterokaryosis as a Genetic System
4. Cross-Fertilization and Self Fertilization in the Higher Plants
5. The Chromosomal Mechanism and the Genetic System
6. The Selective Value of Genetic Systems
7. The Origin and Degeneration of Sex as Affected by Natural Selection
8. A Genetic Theory for the Origin of the Diploid State
9. Causes for the Degeneration toward Self Fertilization
10. The Selective Value of the Recombination Index
11. Genetic Systems in Plants and Animals

ISOLATION AND THE ORIGIN OF SPECIES

1. A Comparison of Differences within Species and between Species
2. The Evolutionary Significance of Species Formation
3. Types of Isolating Mechanisms
4. Spatial and Ecological Isolation in Relation to Species Formation
5. Spatial and Ecogeographic Isolation as Species-Forming Barriers
6. Ecological Separation of Sympatric Types
7. Seasonal Isolation
8. Mechanical Isolation
9. Prevention of Fertilization
10. Hybrid Inviability or Weakness
11. Hybrid Sterility
12. Degeneration of Hybrid Progeny
13. The Interrelationships between Isolating Mechanisms and Interspecific Differences
14. Some Typical Patterns of Isolating Mechanisms
15. The Origin of Isolating Mechanisms

HYBRIDIZATION AND ITS EFFECTS

1. The Frequency of Hybridization in Plants and Animals
2. Degrees of Hybridity
3. Some General Principles Concerning Hybridization
4. Introgressive Hybridization and Its Effects

FOSSILS, MODERN DlSTRIBUTION PATTERNS AND RATES OF EVOLUTION

1. The Nature and Value of Paleobotanical Evidence
2. Modern Patterns of Distribution and Their Interpretation
3. Disjunct Distributions and Their Significance
4. Distributional Patterns Suggesting Rapid Evolution
5. The Possible Basis for Differential Evolutionary Rates
6. Environmental Conditions Promoting Rapid Evolution
7. Biological Conditions Promoting Rapid Evolution
8. The Basis of Evolutionary Rates: Summary and Conclusions

WORKS CITED