The Poverty of the Linnaean Hierarchy A Philosophical Study of Biological Taxonomy MARC ERESHEFSKY University of Calgary published by the press syndicate of the university of cambridge The Pitt Building, Trumpington Street, Cambridge, United Kingdom cambridge university press The Edinburgh Building, Cambridge CB2 2RU, UK 40 West 20th Street, New York, NY 10011-4211, USA 10 Stamford Road, Oakleigh, VIC 3166, Australia Ruiz de Alarcón 13, 28014 Madrid, Spain Dock House, The Waterfront, Cape Town 8001, South Africa http://www.cambridge.org © Marc Ereshefsky 2001 This book is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2001 Printed in the United States of America Typeface Times Roman 10.25/13 pt. System QuarkXPress [BTS] A catalog record for this book is available from the British Library. Library of the Congress Cataloging in Publication data Ereshefsky, Marc. The poverty of the Linnaean hierarchy : a philosophical study of biological taxonomy / Marc Ereshefsky. p. cm. – (Cambridge studies in philosophy and biology) Includes bibliographical references and index. ISBN 0-521-78170-1 (hardcover) 1. Biology – Classification – Philosophy. I. Title. II. Series. QH83 .E73 2000 578¢.01¢2 – dc21 00-028945 ISBN 0 521 78170 1 hardback Contents Preface page ix Introduction 1 PART I THE HISTORICAL TURN 1 The Philosophy of Classification 15 1.1 Essentialism 16 1.2 Cluster Analyses 24 1.3 The Historical Approach 28 1.4 Taxonomic Monism and Pluralism 39 1.5 Hierarchical Classification 46 2 A Primer of Biological Taxonomy 50 2.1 Evolutionary Taxonomy 51 2.2 Pheneticism 60 2.3 Process Cladism 66 2.4 Pattern Cladism 75 2.5 Species Concepts 80 3 History and Classification 94 3.1 The Death of Essentialism 95 3.2 The Failure of Cluster Analysis 102 3.3 Taxa as Historical Entities 109 3.4 Species as Individuals 112 3.5 Theory and Explanation 119 PART II THE MULTIPLICITY OF NATURE 4 Species Pluralism 129 4.1 The Possibility of Pluralism 130 4.2 The Ontological Argument 138 vii Contents 4.3 Varieties of Pluralism 145 4.4 Monist Objections 154 5 How to Be a Discerning Pluralist 163 5.1 Normative Naturalism 164 5.2 The Aim of Biological Taxonomy 170 5.3 Primary and Secondary Rules 178 5.4 A Case Study: Species Concepts 185 5.5 Methodological Unity and Disunity 193 PART III HIERARCHIES AND NOMENCLATURE 6 The Evolution of the Linnaean Hierarchy 199 6.1 Linnaeus’s Original System 200 6.2 Darwin and the Modern Synthesis 208 6.3 Cladistic Changes 214 6.4 Remaining Problems 221 6.5 Pragmatic Considerations 232 7 Post-Linnaean Taxonomy 238 7.1 Hierarchies 239 7.2 Naming Taxa 247 7.3 Defining Taxon Names 256 8 The Future of Biological Nomenclature 267 8.1 The Annotated Linnaean System 268 8.2 The Post-Linnaean System 274 8.3 Theory and Practice 280 Notes 292 References 300 Index 313 viii Introduction THE LINNAEAN SYSTEM RECONSIDERED Three hundred years ago biological taxonomy was a chaotic discipline marked by miscommunication and misunderstanding. Biologists dis- agreed on the categories of classification, how to assign taxa to those categories, and even how to name taxa. Fortunately for biology, Linnaeus saw it as his divinely inspired mission to bring order to taxonomy.The system he introduced offered clear and simple rules for constructing classifications. It also contained rules of nomenclature that greatly enhanced the ability of biologists to communicate. Linnaeus’s system of classification was widely accepted by the end of the eigh- teenth century. That acceptance brought order to a previously dis- organized discipline. Furthermore, it laid the foundation for “the unprecedented flowering of taxonomic research” of the late eighteenth and early nineteenth centuries (Mayr 1982, 173). Linnaeus himself seemed assured of his place in the history of biology. Consider the frontispiece of his Hortus Cliffortianus (1737) (Figure I.1). Linnaeus’s youthful face is seen on the body of Apollo. In one hand he holds a light, in the other he pushes back the clouds of ignorance from crowned Mother Nature. With his foot Linnaeus tram- ples the dragon of falsehood. In the foreground, plants are brought for identification and two cherubs admire Linnaeus’s centigrade ther- mometer.An exuberant illustration, and an immodest one – it was com- missioned and approved by Linnaeus.The metaphors of the illustration are not completely unfounded; Linnaeus’s work did usher in a golden era of biological classification. In the last two hundred years, the theoretical landscape of biology has changed drastically. The foundation of Linnaeus’s system was his 1 Introduction Figure I.1 The frontispiece of Linnaeus’s Hortus Cliffortianus (1737). Courtesy of Hunt Institute for Botanical Documentation, Carnegie Mellon University, Pittsburgh, PA. 2 Introduction biological theory – in particular, his assumptions of creationism and essentialism. These assumptions have gone by the wayside and have been replaced by evolutionary theory. Still, the vast majority of biolo- gists use the Linnaean hierarchy and its system of nomenclature. Unfortunately, the system’s outdated theoretical assumptions under- mine its ability to provide accurate classifications. Furthermore, its rules of nomenclature, once prized for their ability to bring order to biological classification, are no longer practical.These problems are far from minor, for the Linnaean system is the backbone of biological clas- sification and much of biology.The Linnaean system prescribes how to name and represent taxa and, in doing so, provides the template for displaying life’s diversity. Moreover, the terms and concepts of the Linnaean system play a central role in biological theorizing. They frame all theoretical questions concerning groups of organisms above the level of the local population. To get a better idea of the problems facing the Linnaean system, let us take a closer look at its theoretical assumptions. Among them is Linnaeus’s conception of biological taxa. Linnaeus thought that species and other taxa are the result of divine intervention. Once a taxon is created, each of its members must have the essential properties of that taxon. The evolution of a species was foreclosed by God’s original creation. Needless to say, Darwinism gives us a different picture of the organic world. Taxa are the products of natural rather than divine processes. Species are evolving lineages, not static classes of organisms.This conceptual shift in biological theory is well discussed in the literature and comes under many banners. Some authors talk of the “death of essentialism”; others refer to the “species are individu- als” thesis. In broader perspective, this conceptual shift falls under “the historical turn” in biology, or what Ernst Mayr calls “population thinking.” Essentialism concerning taxa has fallen out of favor among evolu- tionary biologists, so this tenet of Linnaeus’s original system has been dropped. Nevertheless, many of Linnaeus’s original principles remain in place, and those assumptions, I will argue, are equally problematic. Consider Linnaeus’s conception of the species category. The species category is the group of all species taxa, whereas species taxa, such as Homo sapiens and Drosphila melangaster, are groups of organisms. For Linnaeus, not only do species taxa have essences, but so does the species category. In the Linnaean system, all species taxa are compa- rable and distinguishable from all other types of taxa. The assumption 3 Introduction that there is an essence to the species category is still widely held. Many biologists believe that species are groups of organisms that can suc- cessfully interbreed and produce fertile offspring. Being a group of organisms with those properties defines membership in the species cat- egory. So while biologists have rejected the Linnaean assumption that species taxa have essences, they have, for the most part, retained the assumption that there is an essence to the species category. That latter assumption, however, should be questioned as well. The idea that species are groups of organisms that successfully interbreed and produce fertile offspring is just one of many prominent definitions of the species category. (Biologists often refer to such definitions as “species concepts.”) Another definition asserts that a species is a group of organisms bound by their unique phylogeny,and still another defines a species as a group of organisms that share a unique ecological niche. What are we to make of this variety of species concepts in the litera- ture? According to some authors, there is a single correct description of the species category.The existence of more than one species concept, they suggest, merely reflects a lack of consensus among biologists on the nature of species. Against this view, I will argue that there is no single correct definition of the species category. The species category lacks an essence and is in fact multifarious. This second view is species pluralism and runs counter to the Linnaean assumption that there is an essence to the species category. If one accepts species pluralism, then not only must essentialism at the level of species be abandoned, but so must essentialism at the level of the species category. Similar arguments have been used against the other Linnaean categories. Many biologists note that the higher Linnaean categories – genus, family, order, and so forth – are hetero- geneous collections of taxa. Families, for example, vary in their ages as well as their degrees of inclusiveness. Calling a taxon a “family” indi- cates only that within a particular classification that taxon is more inclusive than genera and less inclusive than classes. This meaning of “family” has no ontological significance. Similar observations apply to the rest of the Linnaean higher categories. If the higher Linnaean cat- egories lack significant defining features, then another major tenet of the Linnaean system is obsolete.