How and Why Species Multiply: The Radiation of Darwin's Finches

Charles Darwin's experiences in the Galpagos Islands in 1835 helped to guide his thoughts toward a revolutionary theory: that species were not fixed but diversified from their ancestors over many generations, and that the driving mechanism of evolutionary change was natural selection. In this concise, accessible book, Peter and Rosemary Grant explain what we have learned about the origin and evolution of new species through the study of the finches made famous by that great scientist: Darwin's finches. Drawing upon their unique observations of finch evolution over a thirty-four-year period, the Grants trace the evolutionary history of fourteen different species from a shared ancestor three million years ago. They show how repeated cycles of speciation involved adaptive change through natural selection on beak size and shape, and divergence in songs. They explain other factors that drive finch evolution, including geographical isolation, which has kept the Galpagos relatively free of competitors and predators; climate change and an increase in the number of islands over the last three million years, which enhanced opportunities for speciation; and flexibility in the early learning of feeding skills, which helped species to exploit new food resources. Throughout, the Grants show how the laboratory tools of developmental biology and molecular genetics can be combined with observations and experiments on birds in the field to gain deeper insights into why the world is so biologically rich and diverse. Written by two preeminent evolutionary biologists, How and Why Species Multiply helps to answer fundamental questions about evolution--in the Galpagos and throughout the world.

Table of Contents:
List of Illustrations xi List of Tables xv Preface xvii CHAPTER ONE: The Biodiversity Problem and Darwin's Finches 1 Biodiversity 1 The Choice of Organisms 2 Darwin's Finches 3 Diversity of Darwin's Finch Species 5 Species and Populations 8 Overview of the Book 11 CHAPTER TWO: Origins and History 13 Introduction 13 Phylogeny 14 Ancestors 16 The Time of Arrival 16 Colonization 17 The Ecological Theater 18 A Change of Scenery 21 The Evolutionary Play 22 Recent History 22 Summary 25 CHAPTER THREE: Modes of Speciation 26 The Formation of New Species 26 Two Groups from One 27 Divergence in Allopatry 28 Coexistence in Sympatry 29 Sympatric Speciation 30 Parapatric Speciation 31 Testing the Models 33 Summary 33 CHAPTER FOUR: Colonization of an Island 35 Speciation: The Initial Split 35 Establishment of a New Population 35 Founder Effects: Expectations from Theory 36 A Colonization Event 38 Inbreeding 39 Recurrent Immigration 39 An Alternative Phenology of Founder Effects 42 Conclusion 42 Species Elsewhere 44 Summary 45 CHAPTER FIVE: Natural Selection, Adaptation, and Evolution 46 Adaptation 46 Beak Sizes and Diets 47 Adaptive Evolution When the Environment Changes 50 Natural Selection 52 Evolution 54 Oscillating Directional Selection 55 Extrapolating from Short to Long Term 58 The Sources of Variation 58 How Beaks Are Formed 59 Depth and Width 59 Length 62 Summary 63 CHAPTER SIX: Ecological Interactions 65 Introduction 65 Competition 66 Patterns of Coexistence 66 Diets Inferred from Beaks 67 Interpreting the Patterns 67 Character Displacement and Release 68 Character Displacement Observed 68 The Competitive Role of G. Magnirostris 69 Selection under Contrasting Conditions 73 Evolution of Character Displacement 73 Summary 75 CHAPTER SEVEN: Reproductive Isolation 76 Pre-mating Barrier to Interbreeding 76 Factors Involved in the Discrimination between Species 76 Beaks 77 Song 77 Learning 79 Song Differences between Species 80 Song Divergence in Allopatry 81 Adaptation to Habitat 83 Change of Songs as a Consequence of Morphological Divergence 84 The Role of Chance 86 Simulating Secondary Contact 88 Summary 91 CHAPTER EIGHT: Hybridization 92 Introduction 92 Hybridization 92 Why Hybridization Occurs 93 When Hybridization Does Not Occur 96 Hybrid Fitness 97 Introgression on Daphne Major 100 Introgression in the Archipelago 103 Reinforcement 103 Reproductive Character Displacement 105 Evolutionary Significance of Introgression 106 Summary 107 CHAPTER NINE: Species and Speciation 108 Introduction 108 From Process to Product:What Is a Species? 109 A Working Definition 110 How Many Species of Darwin's Finches? 111 Certhidea olivacea: One Species or Two? 112 Geospiza difficilis: One Species or Three? 113 From Product Back to Process 114 Fission and Fusion 116 Summary 119 CHAPTER TEN: Reconstructing the Radiation of Darwin's Finches 120 Introduction 120 The Shape of the Radiation 121 Speciation and Extinction 123 Speciation 125 Extinction 126 Implications for Phylogeny 127 Adaptive Landscape 128 A Pattern of Ecological Segregation 133 Specialization 134 The Buildup of Complex Communities 134 Summary 135 CHAPTER ELEVEN: Facilitators of Adaptive Radiation 137 Introduction 137 Environmental Opportunity 138 Geographical Suitability 139 Ecological Opportunity 140 High Diversification Potential 142 Behavioral Flexibility 142 Introgressive Hybridization 145 Hybridization and Animal Breeding 146 Environmental Conditions Conducive to Introgression 146 Finches versus Mockingbirds 148 Summary 150 CHAPTER TWELVE: The Life History of Adaptive Radiations 152 Introduction 152 The First Stage of Adaptive Radiation 153 The Second Stage of Adaptive Radiation 154 Haldane's Rule 157 The Third Stage of Adaptive Radiation 158 Synthesis 160 Summary 162 CHAPTER THIRTEEN: Summary of the Darwin's Finch Radiation 163 What Happened and Why 163 What Is Missing? 165 Epilogue 166 Glossary 168 References 175 Author Index 201 Subject Index 210