Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page i PLANT EVOLUTION AND THE ORIGIN OF CROP SPECIES Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page ii Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page iii Plant Evolution and the Origin of Crop Species Second Edition James F. Hancock Department of Horticulture Michigan State University CABI Publishing Plant Evolution - Chap 00A Prel 6/10/03 14:25 Page iv CABI Publishing is a division of CAB International CABI Publishing CABI Publishing CAB International 875 Massachusetts Avenue Wallingford 7th Floor Oxon OX10 8DE Cambridge, MA 02139 UK USA Tel: +44 (0)1491 832111 Tel: +1 617 395 4056 Fax: +44 (0)1491 833508 Fax: +1 617 354 6875 E-mail: [email protected] E-mail: [email protected] Website: www.cabi-publishing.org ©J.F. Hancock 2004. All rights reserved. No part of this publication may be reproduced in any form or by any means, electronically, mechanically, by photocopying, recording or otherwise, without the prior permission of the copyright owners. A catalogue record for this book is available from the British Library, London, UK. Library of Congress Cataloging-in-Publication Data Hancock, James F. Plant evolution and the origin of crop species / James F. Hancock.-- 2nd ed. p. cm. Includes bibliographical references (p. ). ISBN 0-85199-685-X (alk. paper) 1. Crops--Evolution. 2. Crops--Origin. 3. Plants--Evolution. I. Title. SB106.O74H36 2003 633-dc21 2003006924 ISBN 0 85199 685 X Artwork provided by Marlene Cameron. Typeset in 10pt Souvenir by Columns Design Ltd, Reading. Printed and bound in the UK by Biddles Ltd, Guildford and King’s Lynn. Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page v Contents Preface ix Acknowledgements ix Introduction 1 Part 1. Evolutionary Processes 1. Chromosome Structure and Genetic Variability 3 Gene and Chromosomal Structure 4 Types of Mutation 5 Measurement of Variability 16 Construction of Genetic Maps and Genome Evolution 28 2. Assortment of Genetic Variability 32 Random Mating and Hardy–Weinberg Equilibrium 32 Migration 35 Selection 42 Genetic Drift 52 Evolution in Organelles 54 Interaction between Forces 55 3. The Multifactorial Genome 58 Intragenomic Interactions 60 Coadaptation 64 Canalization 72 Paradox of Coadaptation 73 4. Polyploidy and Gene Duplication 77 Factors Enhancing the Establishment of Polyploids 78 v Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page vi vi Contents Evolutionary Advantages of Polyploidy 80 Genetic Differentiation in Polyploids 93 Chromosomal Repatterning 95 Genome Amplification and Chance 98 5. Speciation 100 What is a Species? 100 Reproductive Isolating Barriers 103 Modes of Speciation 110 Genetic Differentiation during Speciation 117 Hybridization and Introgression 119 Hybridization and Extinction 124 Crop–Weed Hybridizations 125 Risk of Transgene Escape into the Environment 127 Part 2. Agricultural Origins and Crop Evolution 6. The Origins of Agriculture 129 Rise of our Food Crops 129 Emergence of Homo 131 Evolution of Homo 137 Appearance of Modern Humans 139 Spread of H. sapiens 141 Agricultural Origins 143 Early Crop Dispersals 145 Transcontinental Crop Distributions 148 7. The Dynamics of Plant Domestication 151 Evolution of Farming 153 Early Stages of Plant Domestication 154 Origins of Crops 157 Characteristics of Early Domesticants 159 Changes During the Domestication Process 161 Genetic Regulation of Domestication Syndromes 163 Evolution of Weeds 166 Genetic Diversity and Domestication 169 Domestication and Native Diversity Patterns 170 8. Cereal Grains 174 Barley 174 Maize 176 Millets 181 Oats 183 Rice 185 Rye 187 Sorghum 188 Wheat 190 Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page vii Contents vii 9. Protein Plants 195 Chickpea 195 Cowpea 196 Pea 198 Lentil 200 Phaseolus Beans 203 Faba Beans 206 Soybean 206 10. Starchy Staples and Sugars 209 Banana 209 Cassava 212 Potato 214 Sugar Cane 217 Sugar Beet 219 Sweet Potato 220 Taro 222 Yam 223 11. Fruits, Vegetables, Oils and Fibres 226 Fruits Apples 226 Citrus 228 Grape 230 Peach 231 Strawberry 232 Vegetables Cole Crops 234 Squash and Gourds 236 Chilli Peppers 238 Tomato 240 Fibres and Oils Cotton 241 Groundnut 243 Sunflower 244 12. Postscript: Germ-plasm Resources 246 Ex situ Conservation 248 In situ Conservation 249 References 251 Index 307 Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page viii Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page ix Preface The first edition of this book was published in 1992 by Prentice-Hall. This second edition incorporates the wealth of new information that has emerged over the last decade on plant evolution. The advent of molecular markers has generated a cascade of new information on evolutionary processes, the struc- ture of plant genomes and crop origins. Ideas about the evolutionary role of introgression, hybridization and polyploidy have been dramatically altered, and the species origins of many recalcitrant crops have been elucidated. In addition, the major loci associated with domestication have been mapped and it has been shown that crop genomes can be quite fluid. To my knowl- edge, no other book on plant evolution has attempted to combine the last decade of molecular information with conventionally acquired information. In this edition, I have tried very hard to show how natural and crop evo- lution are intimately associated. Much more of the crop information is incor- porated in the early evolutionary discussions, and I take greater pains to describe the evolutionary mechanisms associated with crop domestication. The previous discussion about prehuman plant and animal evolution has been greatly abbreviated so that more space can be devoted to variation patterns associated with crop domestication and dispersal. All in all, I think this edition does a better job of describing the continuum between natural and crop evolution. Acknowledgements Numerous people contributed directly and indirectly to the book: first and foremost, my wife, Ann, who has been unflagging in her support over the ix Plant Evolution - Chap 00A Prel 3/10/03 9:26 Page x x Preface years and has served as a model of creativity and drive. Marlene Cameron added greatly to the text with her exceptional artwork. Norm Ellstrand and Paul Gepts made numerous helpful suggestions. I am also indebted to the students who have pushed me in my crop evolution class over the last 20 years, and the Plant Breeding and Genetics Journal Club, which keeps uncovering important publications that I have missed. Several previous texts have had particularly strong influences on me and been tremendous resources: J.R. Harlan’s Crops and Man, C.B. Heiser’s Seed to Civilization, V. Grant’s Plant Speciation and Organismic Evolution, J.D. Sauer’s Historical Geography of Crop Plants: a Select Roster, J. Smartt and N.W. Simmond’s Evolution of Crop Plants, B. Smith’s The Emergence of Agriculture, and D. Zohary and M. Hopf ’s Domestication of Plants in the Old World. Plant Evolution - Chap 00B Intr 3/10/03 9:26 Page 1 Introduction This book has been written for advanced undergraduates and graduate stu- dents in the biological sciences. It is meant primarily as a text for crop evolu- tion courses, but should serve well in a wide range of plant evolution and systematics courses. It is also intended as a resource book on individual crop histories. I have worked hard to combine the recently emerging molecular data with archaeological, morphological and cytogenetic information. The book is arranged in two sections: Chapters 1–5 cover the genetic mechanisms associated with plant evolution, and Chapters 6–12 deal with the domestication process and the origin of crop species. In the first half of the book, little effort is made to distinguish between natural and crop evolu- tion, since both kinds of change occur through the reorganization of genetic variability. The processes of change are the same, regardless of whether we are dealing with wild or domesticated populations; only the selector differs. In the first five chapters, I rely heavily on the evolutionary literature, but try to incorporate relevant crop species where appropriate. The goal of these chapters is to describe the overall framework of species change and demon- strate the intimacy of nature and crop evolution. In the second half of the book, I focus on when and where crops were domesticated and the types of changes associated with their domestication. Chapters 6 and 7 give an overview of the emergence and diffusion of agri- culture, and the ways species were changed during domestication. The next four chapters deal with the evolution of individual crops, representing grains, legumes, starchy roots, fruits, vegetables and oils. Whenever possible, the genetic mechanisms described in the first five chapters are highlighted. The last chapter contains a brief discussion of germ-plasm resources and why they need to be maintained. Clearly, if we are to continue to feed the human © J. Hancock 2004. Plant Evolution and the Origin of Crop Species, 2nd edn (J.F. Hancock) 1 Plant Evolution - Chap 00B Intr 3/10/03 9:26 Page 2 2 Introduction population, we must develop greater respect for our natural populations and their ongoing evolution. The overall goal of this book is to describe the processes of evolution in native and cultivated populations and to provide a blueprint for the system- atic study of crop origins. It is hoped that when the student completes this book she or he will understand the factors involved in species change and will have a greater appreciation of the coadaptive nature of plants and people. Plant Evolution - Chap 01 3/10/03 9:26 Page 3 Chromosome Structure and 1 Genetic Variability Introduction Evolution is the force that shapes our living world. Countless different kinds of plants and animals pack the earth and each species is itself com- posed of a wide range of morphologies and adaptations.