113219754 amundson the changing role of embryo in evolutionary thought

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The Changing Role of the Embryoin Evolutionary Thought

In this book, Ron Amundson examines 200 years of scientific views on theevolutiondevelopment relationship from the perspective of evolutionary devel-opmental biology (evodevo). This new perspective challenges several popularviews about the history of evolutionary thought by claiming that many earlierauthors made history come out right for the Evolutionary Synthesis.

The book starts with a revised history of nineteenth-century evolutionarythought. It then investigates how development became irrelevant to evolutionwith the Evolutionary Synthesis. It concludes with an examination of thecontrasts that persist between mainstream evolutionary theory and evodevo.

This book will appeal to students and professionals in the philosophy ofscience, and the philosophy and history of biology.

Ron Amundson is Professor of Philosophy, University of Hawaii at Hilo.

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cambridge studies in philosophy and biology

General EditorMichael Ruse Florida State University

Advisory BoardMichael Donoghue Yale University

Jean Gayon University of ParisJonathan Hodge University of Leeds

Jane Maienschein Arizona State UniversityJesus Mostern Instituto de Filosofa (Spanish Research Council)

Elliott Sober University of Wisconsin

Alfred I. Tauber The Immune Self: Theory or Metaphor?Elliott Sober From a Biological Point of View

Robert Brandon Concepts and Methods in Evolutionary BiologyPeter Godfrey-Smith Complexity and the Function of Mind in Nature

William A. Rottschaefer The Biology and Psychology of Moral AgencySahotra Sarkar Genetics and Reductionism

Jean Gayon Darwinisms Struggle for SurvivalJane Maienschein and Michael Ruse (eds.) Biology and the Foundation

of EthicsJack Wilson Biological Individuality

Richard Creath and Jane Maienschein (eds.) Biology and EpistemologyAlexander Rosenberg Darwinism in Philosophy, Social Science, and Policy

Peter Beurton, Raphael Falk, and Hans-Jorg Rheinberger (eds.)The Concept of the Gene in Development and Evolution

David Hull Science and SelectionJames G. Lennox Aristotles Philosophy of Biology

Marc Ereshefsky The Poverty of the Linnaean HierarchyKim Sterelny The Evolution of Agency and Other Essays

William S. Cooper The Evolution of ReasonPeter McLaughlin What Functions Explain

Steven Hecht Orzack and Elliott Sober (eds.) Adaptationism and OptimalityBryan G. Norton Searching for Sustainability

Sandra D. Mitchell Biological Complexity and Integrative PluralismJoseph LaPorte Natural Kinds and Conceptual ChangeGreg Cooper The Science of the Struggle for Existence

Jason Scott Robert Embryology, Epigenesis, and EvolutionWilliam F. Harms Information & Meaning in Evolutionary Processes

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The Changing Roleof the Embryo in

Evolutionary ThoughtRoots of EvoDevo

RON AMUNDSONUniversity of Hawaii at Hilo

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Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, So Paulo

Cambridge University PressThe Edinburgh Building, Cambridge , UK

First published in print format

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Ron Amundson 2005

2005

Information on this title: www.cambridge.org/9780521806992

This book is in copyright. Subject to statutory exception and to the provision ofrelevant collective licensing agreements, no reproduction of any part may take placewithout the written permission of Cambridge University Press.

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Cambridge University Press has no responsibility for the persistence or accuracy ofs for external or third-party internet websites referred to in this book, and does notguarantee that any content on such websites is, or will remain, accurate or appropriate.

Published in the United States of America by Cambridge University Press, New York

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hardback

eBook (NetLibrary)eBook (NetLibrary)

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To my father, Art Amundson. He taught me curiosity, and the pleasureof taking things apart to see how they work.

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Contents

Acknowledgments page xiii

1 Introduction1.1 EvoDevo as New and Old Science 11.2 EvoDevo and the Windfall of the 1990s 41.3 How I Came to Write this Book 91.4 Historical Format 111.5 Epistemological Concepts in Historical Context 14

1.5.1 Inductivist Caution 141.5.2 Idealism 161.5.3 Two Essentialisms 18

1.6 Explanatory Relativity 201.7 Historical Conventions 221.8 Historical Precis 23

part i. darwins century: beyond the essentialism story

2 Systematics and the Birth of the Natural System2.1 Introduction 312.2 The Discovery of Species Fixism 342.3 Linnaeus and His Contemporaries 392.4 French Systems: Jussieu and Cuvier 412.5 British Systems and the Growth of Taxonomic Realism 452.6 Review of Species Fixism, Essentialism, and Real

Groups 50

3 The Origins of Morphology, the Science of Form3.1 Morphology and Natural Theology 53

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3.2 Form as a Topic of Study 553.2.1 Goethe 553.2.2 The Great Cuvier-Geoffroy Debate 563.2.3 Von Baer and Development 583.2.4 The Study of Form Summarized 61

3.3 Natural Theologians on Unity of Type 623.3.1 William Paley 633.3.2 William Buckland 643.3.3 Charles Bell 643.3.4 William Whewell 653.3.5 Peter Mark Roget 66

3.4 The Structural Turn 673.4.1 Martin Barry 683.4.2 William Carpenter 703.4.3 Rudolph Leuckart 72

3.5 What is Natural Theology? 734 Owen and Darwin, The Archetype and the Ancestor

4.1 Introduction 764.2 Typology Defined: Kinds of Types 784.3 Owen Builds the Archetype 824.4 Owen on Species Origins 884.5 Anti-Adaptationism 934.6 Darwins use of Morphological Types 964.7 Misunderstanding Darwin on Owen 994.8 Darwin on Unity of Type 1024.9 A Structuralist Evolutionary Theory? 1034.10 How Darwin Differed 104

5 Evolutionary Morphology: The FirstGeneration of Evolutionists5.1 The Program of Evolutionary Morphology 1075.2 Evolutionary Morphology as Non-Darwinian and as

Darwinian 1085.3 The Biogenetic Law 1125.4 Early Origins in Phylogeny and Ontogeny 1145.5 Explaining Form 1185.6 The Struggles of Evolutionary Morphology 1215.7 The Conflict between Adaptation and Structure 125

6 Interlude6.1 Two Narratives of the History of Evolutionary Biology 1306.2 One Theory or Two? 130


Contents xi

6.3 Grounds for Species Fixism 1316.4 Darwins other Primary Achievement: The Tree of

Life 1326.5 The Significance of Gappiness 1346.6 And Forward 136

part ii. neo-darwins century: explaining the absenceand the reappearance of development in

evolutionary thought

7 The Invention of Heredity7.1 Truisms of Heredity 1397.2 Epigenetic Origins of Heredity 1407.3 Epigenetic Heredity During the Nineteenth Century 143

7.3.1 Martin Barry 1437.3.2 Charles Darwin 1447.3.3 August Weismann 144

7.4 The Cleavage between Heredity and Development 1487.5 Reinforcing the Dichotomy: Rewriting Weismann and

Johannsen 1527.6 Broad and Narrow Heredity 155

8 Basics of the Evolutionary Synthesis8.1 A Long Story Made Short 1598.2 The Struggles of Natural Selection 1608.3 Problems in Characterizing the Evolutionary

Synthesis 1618.4 The Evolutionary Synthesis Characterized 1638.5 By-Products of the Core of Synthesis Thought 166

8.5.1 Systematics 1668.5.2 Phylogeny 1678.5.3 Mechanisms 167

9 Structuralist Reactions to the Synthesis9.1 Experimental Embryology and the Synthesis 1699.2 The Program of Experimental Embryology 1709.3 The Embryological Critique of the Synthesis 175

9.3.1 Critique 1: The Causal Completeness Principle 1759.3.2 Critique 2: The Developmental Paradox 1779.3.3 Critique 3: Fundamental versus Superficial

Characters 1809.3.4 Cytoplasmic Inheritance versus Darwinian

Extrapolation 185


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9.4 Points of Contact among Developmental and GeneticBiologists, and Synthesis Evolutionists 1899.4.1 Sewall Wright 1909.4.2 Oxford Morphology 1919.4.3 Waddington and Schmalhausen 1939.4.4 Richard Goldschmidt 195

9.5 Historical Reflection: Explanatory Goals 1969.5.1 Form-Theoretic Evolutionary Theory 1969.5.2 Change-Theoretic Evolutionary Theory 197

10 The Synthesis Matures10.1 The Darwin Centennial Celebration 19810.2 Uses of Dichotomies 20110.3 Proximate versus Ultimate: Context 20310.4 Population Thinking versus Typological Thinking:

Context 20410.5 Ernst Mayr as a Structuralist? 20910.6 The Enlarged Quiver of Dichotomies 211

11 Recent Debates and the Continuing Tension11.1 Diversity versus Commonality: Starting with Genes 21311.2 The Four Dichotomies Defend the Synthesis 218

11.2.1 Maynard Smith: The Germ Line-Soma Critique 21811.2.2 Hamburger and Wallace: The Typological and

Germ Line-Soma Critiques 21911.2.3 Mayr: The Proximate-Ultimate and

Genotype-Phenotype Critiques 22211.2.4 Refutation by Slogan? 224

11.3 Populations, Ontogenies, and Ontologies 22511.4 Adaptationist Ontology: How the Focus on Diversity

Affects Ontology 22611.5 Structuralist Ontology: Commonality and

Developmental Types 22911.6 Concepts of Homology 238

11.6.1 The Historical Concept of Homology 23811.6.2 The Developmental Concept of Homology 240

11.7 A Philosophical Ontology of EvoDevo 24411.8 A Newer Synthesis? 250

References 259Index 275


Acknowledgments

This book was written with the aid of a very large number of colleaguesand friends. I owe special thanks to historian Polly Winsor, to biologistsScott Gilbert, Gunter Wagner, and Brian Hall, and to philosophers RichardBurian and Rasmus Winther. Additional help and advice came from Gar Allen,Andrew Brook, Giovanni Camardi, Michael Dietrich, Rafael Falk, FrietsonGalis, James Griesemer, Larry Heintz, Jane Maienschein, Staffan Muller-Wille, Lynn Nyhart, Rudy Raff, Marsha Richmond, Sara Scharf, David Wake,Sherry Amundson, the librarians of Mookini Library, and many others.

I live on the most remote island archipelago on earth. My physical impair-ments make overseas travel difficult, and everywhere is overseas from here.My research is successful only because of these colleagues patience with myincessant emailed questions and my demands for obscure sources. (Dont getme wrong Im not complaining about life in Hawaii. Im merely thankingthe people who have helped to make it productive as well as pleasant.)

Since the mid-1980s I have also received generous support and advice fromErnst Mayr and David Hull, in full recognition of the fact that my conclusionswere much at odds with theirs. I owe special thanks to my editor Michael Ruse,without whose blunt encouragement this book would never have been written.(Conversation in the beer garden at the 1995 Leuven meetings of ISHPSSB:Do you mean to tell me youre about to turn fifty and you havent written abook? You should be ashamed of yourself.)

Portions of Chapter 4 will appear as an introductory essay to a facsimilereprint of Richard Owens On the Nature of Limbs (Owen 1849) forthcomingfrom the University of Chicago Press. Research was supported by the NationalScience Foundation under Grant SES-0135451.

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Introduction

1.1 evodevo as a new and old science

At the annual meeting of the Society for Integrative and Comparative Biol-ogy in January of the year 2000, a new Division was formed: the Division ofEvolutionary Developmental Biology. This new organization would serve asa home for a lively field by the same name: evolutionary developmental biol-ogy, popularly known as evodevo. In the minds of many of its practitioners(especially the more junior ones), evodevo was new. It was a product of theexplosive growth in knowledge about molecular developmental genetics dur-ing the 1990s. In a sense they were right; evodevo really was new. Withoutthe new molecular knowledge, evolutionary developmental biology wouldnot have gathered the number of researchers or achieved the remarkable re-sults that it could boast by the year 2000. Nevertheless, the subject is morethan 150 years old. The conceptual connection between the development ofan individual (ontogeny) and the evolution of a lineage (phylogeny) predatesthe 1859 publication of Darwins Origin of Species. However, if evolutionarydevelopmental biology is an old study, how could it be thought to be new inthe year 2000?

The answer is that for most of the twentieth century only a minority ofevolutionary biologists believed that ontogenetic development had any rele-vance at all to evolution. The Evolutionary Synthesis of the 1930s and 1940sestablished the mainstream of evolutionary biology (Mayr and Provine 1980).Population genetics was regarded as a causally adequate model of the evo-lutionary process. Natural selection was the sole direct-giving mechanismof evolutionary change, and the phenomena of macroevolution (patterns ofevolution above the species level) were simply extrapolated from microevo-lution (natural selection within populations). The ontogenetic developmentof individual organisms had no place in this framework.

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The Changing Role of the Embryo

I am a philosopher and self-styled historian of biology. I am primarily in-terested in theoretical and methodological debates between scientific views,rather than in scientific theories themselves. I am interested in the recentrevival of evolutionary developmental biology for a special reason: The evo-lutionary irrelevance of developmental biology was argued on the basis ofphilosophical, methodological, and sometimes even historical grounds dur-ing the mid-twentieth to late twentieth century. The basic concepts of evolu-tionary theory were said to preclude the relevance of development to evolu-tion. These principles were described, examined, and (mostly) approved byphilosophers as well as scientists. They were used by historians and scientistsin reporting the history of evolutionary biology both before and after Darwin.Narratives of the history of biology depicted the predecessors of todays evodevo practitioners as metaphysically confused and scientifically regressive.Pre-Darwinian biology was described in ways that detracted from the impor-tance of developmental thinkers and that categorized them, along with almostall other opponents of Darwin, as religious reactionaries.

In other words, many philosophers and historians during the mid-twentiethto late twentieth century produced work that showed neo-Darwinism in afavorable light and developmental evolutionary theories in an unfavorablelight. This is perfectly understandable, and I would have it no other way.Philosophers of science ought to take contemporary scientific knowledgeas their starting point, and they ought not to feign wisdom that is superiorto that of their scientific colleagues. In fact, I intend to do the very samething in this book. I intend to look at the history and philosophy of biologyfrom the standpoint of contemporary science. However, I will take a differ-ent standpoint from those who assumed the adequacy of the EvolutionarySynthesis.

Nothing succeeds like success. Evodevo is a flourishing enterprise,notwithstanding the arguments and historical narratives of earlier days. I climbon this bandwagon here. I conduct my philosophical and historical examina-tion from the standpoint of evodevo rather than the Evolutionary Synthesis.Thus, the difference between this book and writings associated with the Evo-lutionary Synthesis is that this book has a different vantage point, a vantagepoint that has gained new legitimacy from recent science.

This book assumes the basic legitimacy of evodevo. It examines cer-tain traditional narratives of nineteenth-century biology with a view towardidentifying and replacing the biases that made neo-Darwinian theory seem in-evitable and alternative (especially developmental) theories seem regressive.It then examines the history of the twentieth-century interactions between

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Introduction

evolutionary and developmental biology. Why was developmental biologyabsent from the early versions of neo-Darwinism? Why was it not laterincorporated? Most importantly, what brought about the historical narrativesand philosophical arguments that implied that development was in principleirrelevant to evolutionary biology?

It may seem that I am starting with a controversial assumption, that evodevo and neo-Darwinism really are inconsistent. Surely they are not . . . well,probably they are not. Very few evodevo practitioners doubt that natural se-lection within populations is responsible for the changes that occur withinspecies. Evodevo advocates merely believe that additional mechanisms,mechanisms involved with ontogeny rather than population genetics, mustcontribute to a full understanding of evolution. The problem is that the ar-guments constructed by neo-Darwinians that imply the irrelevance of devel-opment to (neo-Darwinian) evolution are very convincing! They entail thatone can accept either evodevo or neo-Darwinism, but not both; thus it is notmy words, but the words of the neo-Darwinian commentators, that entail theinconsistency of evodevo and neo-Darwinism. I hope, and most evodevopractitioners believe, that a way can be found to accommodate both evodevoand neo-Darwinism. There is a genuine tension between these viewpoints. Ido not know how to refute the irrelevancy arguments of the neo-Darwinians.It is not yet clear how this dilemma will be resolved.1

Some readers will doubt that neo-Darwinians actually argued that devel-opment is irrelevant to the understanding of evolution, or that those argumentsapply equally well to modern evodevo. I document both assertions and domy best to explicate the tensions between the two views of evolution. I mustleave it to others to resolve the tensions.

1 Frankly, many evodevo practitioners are not aware of these tensions. Most are aware of thepractical barriers between the fields, such as the reliance of evodevo on a relatively small numberof model organisms and the lack of population-level studies. There are a range of opinions withinthe discipline regarding its relation to neo-Darwinian theory. Some practitioners, such as BrianHall, consider evodevo to be a new synthetic field of study that has no particular conflict withneo-Darwinism (Hall 2000). I discuss the contrast between Halls own approach and that of neo-Darwinism in Chapter 11. Others recognize the conflicts but are optimistic about their resolution(Gilbert 2003b). One valuable approach to the history of evodevo is to recognize its agenda, thecontrast with the agenda of neo-Darwinism, and the various scientific disciplines that kept theevodevo agenda alive during the twentieth century. These include comparative and experimentalembryology, morphology, and paleontology (Love and Raff 2003). In this book I am primarilymotivated by the specific methodological arguments that arose around 1980 concerning whetheror not development was relevant to the understanding of evolution, and the philosophical andhistorical doctrines that gave rise to those arguments.

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1.2 evodevo and the windfall of the 1990S

Most nineteenth-century evolutionists and several twentieth-century evolu-tionists have argued for the importance of the processes of development inunderstanding evolution. These early views receive more attention in thisbook than the stunning molecular discoveries that stimulated the growth ofevodevo in the 1990s. I now briefly report on some of the discoveries of the1990s to illustrate how new life was breathed into evolutionary developmentalbiology.

The 1970s and 1980s saw a number of iconoclastic challenges to the well-established Evolutionary Synthesis. Some of the criticisms have since beendropped (e.g., the alleged unfalsifiability of adaptationism), and some havebecome internal matters within mainstream evolutionary theory (e.g., thepunctuation vs. gradualism issue in paleontology). The role of development inevolution is the single persistent dispute. It first took the form of an argumentover adaptation versus developmental constraints (Maynard Smith et al.1985; Amundson 1994; Schwenk 1995). That debate will be discussed later.For present purposes, the debate was important because it raised awarenessof the significance (for the prodevelopment side) of the concept of homology.This new interest in homology coincided with the discovery by molecularbiologists that protein molecules could be sequenced, and the similarity ofsequences of different protein molecules could be measured. Like traditionalanatomical homology, these molecular homologies could be compared intwo ways: different forms of a certain category of protein within an indi-vidual (e.g., and globin molecules) is similar to anatomical serialhomology, and comparison between corresponding proteins in two speciesreveals special homology. Like anatomical special homologies, closenessof match of molecular cross-species homologies was correlated with evolu-tionary relatedness. The serial homologies strongly suggest an evolutionaryscenario in which the genetic basis of a single original protein had dupli-cated in some ancestors genome, after which the duplicates independentlydiverged. Even these early molecular discoveries showed an intriguing sim-ilarity between nineteenth-century morphology and modern molecular biol-ogy (Gilbert 1980). Nothing radical is implied; both serially and speciallyhomologous proteins merely exhibit evolutionary divergence.2

The molecular homologies among globin molecules were not at thetime seen as developmental phenomena. The globin genes did not instructdevelopment; rather their activation was seen as the consequence of the

2 The brief narrative in this section follows Gilbert, Opitz, and Raff (1996).

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Introduction

interactions that caused certain cells to become red blood cells. They were theendpoint of differentiation, not its cause. Developmental implications began totake shape when the molecular techniques began to be applied to the genes thatcontrolled the nature of specialized insect segments. The genetic experimentswere inspired by homeotic mutations, a class of mutations discovered early inthe twentieth century in which an insect segment, together with its ordinaryappendages, was transformed into another type of segment; a Drosophilahaltere could be transformed into a wing, or an antenna into a leg. Thesehad been favorites of developmental evolutionary theorists such as WilliamBateson and Richard Goldschmidt. It was first discovered that the genes thatproduced the various homeotic mutations in Drosophila were themselves se-rially homologous. Moreover, they were located tandemly on a small regionof a particular chromosome, and they were expressed on that chromosome inthe same sequence as along the anteriorposterior axis in a flys body. Eachof these gene sequences contained a certain DNA sequence called the home-obox. These homeobox-containing genes came to be called Hox genes. So farso good. We were learning about the developmental genetics of Drosophilaby identifying the genes that encode the proteins that determine segmentidentity.

The excitement really started when genes homologous to insect Hox geneswere found in vertebrates. Insects and vertebrates are both segmented, but noone for the past century had seriously argued that segmentation was homolo-gous between the two phyla. Then vertebrate genes similar in sequence to flyHox genes were isolated. They proved to be arranged in the same order onthe chromosome, and they were expressed in the same order in the body asthe insect genes. And last, it was shown that the enhancer region of a humanhomeotic gene, such as deformed, can function within Drosophila to activategene expression in the same relative position as in the human embryo inthe head (Gilbert et al. 1996: 364). Genes that act during development in ahumans head can do their usual job in a developing flys head. This was onlythe beginning of a sequence of shocking genetic homologies homologiesthat firmly demonstrated phylogenetic relationships between groups whoseanatomical characters almost no one had been so bold to identify as homolo-gies. For example, the development of both the insect eye and the vertebrateeye is begun by the expression of homologous genes. The same is true withthe hearts of insects and vertebrates, and with the limbs not only of insects andvertebrates but almost all other metazoan groups. More and more basic (andoften analogous) body parts in diverse groups of organisms were found to betriggered by homologous genes. The implications are very hard to sort out, ofcourse. Anatomical homologies have traditionally been identified either by

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their patterns of connections with other body parts or by their embryologicalorigins. These initiating causes do not necessarily make the anatomical struc-tures homologous (although they certainly challenge the traditional conceptsof homology). Insect and vertebrate eyes are developed and structured in ex-tremely different ways, even though they are the same with respect to the genethat begins their development. The difficult job for developmental geneticsremains to show how the corresponding genes could serve as the originaldevelopmental triggers for such structurally distinct body parts. Tracing thegenetic pathways and interactions downstream toward the eventual adultbody part is an ongoing process; surprising new commonalities are revealedat every step.

These discoveries hearken to bygone days, and many developmental bi-ologists knew it. One of the wildest homological speculations in history wasput forth by Etienne Geoffroy St.-Hilaire in the 1820s. Geoffroy proposedthat arthropods and vertebrates had identical body plans. The obvious prob-lem (to knowledgeable anatomists) was that arthropods have their circulatory(haemal) system on their dorsal side and their neural system on their ventralside. Vertebrates are the reverse, with their neural spine along their back.This forced Geoffroy to suggest that the identical body plans were flippedupside -down with respect to the dorsalventral axis. Vertebrates travel withtheir neural spine toward the sun, whereas arthropods travel with their neuralspine toward the earth. There was laughter all around. Toby Appels 1987book The CuvierGeoffroy Debate is quite sympathetic to Geoffroy. Still, shedescribes the arthropodvertebrate body plan reversal as preposterous, andshe assures the reader that such comparisons seemed no less fanciful to hiscontemporaries than they appear to us today (Appel 1987: 111).

Geoffroy may have had the last laugh. Seven years after Appels pub-lication, it was discovered that the dorsalventral axes of vertebrates andarthropods are determined by homologous genes but that their expressionpatterns were reversed in the two groups (De Robertis and Sasai 1996). Indeed,the expression patterns of an entire suite of genes used to specify the dorsaland ventral structures were inverted. These discoveries were not business asusual. Commonalities of animal structure that had previously been regarded asstarry-eyed speculation were suddenly being traced to their molecular geneticroots.

The details of modern molecular developmental genetics are much morecomplex and fast changing than can be described here (Morange 1998). Genesare identified not in terms of the phenotypic effects that they produce in theadult. They are rather defined in terms of their roles in a genetic toolkit that

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Introduction

is used, in different ways, in the embryological construction of the bodies ofdifferent kinds of organisms. The Hox system operates quite differently ininsects and in vertebrates, but it operates in largely the same manner withinthe groups. The basic aspects of organic form are attributed to similar devel-opmental processes, employing homologically similar developmental genes,or to similar tools (e.g., genetic processes). The repeated use of not simplygenes but also genetic pathways has caused Scott Gilbert to speak of homolo-gies of process rather than traditional anatomical homologies (Gilbert andFaber 1996; Gilbert and Bolker 2001). Attempts to understand how identicaldevelopmental genes can produce such diversity have led to an interpretationof developmental gene interactions as a kind of circuit, and major evolutionarychanges as matters of the rewiring of genetic networks (Carroll, Grenier,and Weatherbee 2001; Wray 2001). Diversity is created by different appli-cations of the same old tools. By applications, I mean the use of the samegenetic systems in the actual building of the individual bodies of organismsof incredible diversity. This diversity is the product of the varying applica-tions of shared developmental processes. Evodevo itself goes well beyondthe discoveries of deep homologies. It constructs evolutionary explanations;it doesnt just discover developmentalgenetic causes. Most of the evodevoexplanations are consistent in spirit with developmental theories of past years.The dramatic new genetic homologies count as promises that there is muchyet to be discovered. I discuss some of the evodevo explanations, and theirhistorical predecessors, later in the book. The dramatic new genetic homolo-gies themselves will play no further role. I examine historical arguments, notmodern discoveries.

Why are these new discoveries a problem for the neo-Darwinian critiqueof development? By announcing these dramatic discoveries at the beginningof this book, I may have made it difficult for the reader to imagine howanyone could doubt the importance of development to evolution. However,neo-Darwinism had its origins not in developmental genetics but in trans-mission genetics. Transmission genetics identifies individual genes not bytheir molecular sequence but by tracking phenotypic features through gen-erations of organisms in breeding experiments. Genes are hypothesized onMendelian principles in order to account for the patterns of the phenotypicfeatures in offspring generations. The genes of transmission genetics are de-signed to explain the sorting of traits through generations; they expresslydo not explain how traits are ontogenetically created within the individualorganism. Population genetics, at the core of neo-Darwinian evolutionarytheory, requires transmission genetics alone. It has absolutely no need for

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developmental genetics. This fact, together with the neo-Darwinian evo-lutionists dislike for the developmental theorizing of the time, led to theantidevelopmental arguments.

And the arguments made sense. If populational processes are the onlymechanism of evolutionary change, what difference does it make that hu-man eyes and insect eyes originate from expression of the same gene?

The difference has to do with the significance of homology. As we willsee, Darwin and his twentieth-century followers treat homology as a mereby-product of past evolutionary change, the leftover residue of ancestral char-acters that have not (yet) been selected out of the lineage. Homologies giveevidence of past ancestry, but they are causally inert. Developmental evolu-tionists treat homology as an indicator of underlying causal processes of de-velopment that continue to exert their effects in contemporary species. Theseprocesses are the constraints in the adaptation versus constraints debates.The importance of the discovery of the deep genetic homologies is not justthat one more homology has been detected. The discoveries were very spe-cial ones. The new deep homologies are causally active in the developmentof bodies, and that fact cannot be doubted. They are not mere residue. Thevery different bodies that are built by these genetic processes still show deepcommonalities. Even the bilateral symmetry that characterizes such a widevariety of animal groups is no longer regarded as merely an efficient way tobuild bodies. It is a developmental heritage from an ancient common ancestor:Urbilateria.

The widespread sharing of developmentally important genes justifies acentral assertion of evodevo. It is that one must understand how bodiesare built in order to understand how the process of building bodies can bechanged, that is, how evolution can occur. The same arguments have beenmade since the early nineteenth century. The new genetic homologies offernew evidence that evolution cannot be understood without understandingdevelopment.

I examine the difference between Darwinian and developmental views ofevolution during the course of this book. The book shows how an evodevosensibility produces a different narrative of the history of biology than aneo-Darwinian sensibility. I could not have written this book in 1990, priorto the discoveries of deep genetic homologies. The reason is not that myown arguments and historical narratives rely on the molecular discoveriesthemselves. They do not. The reason is that I intend to assume the legitimacy ofevodevo. I do not intend to argue for it. Such an assumption would have beencontroversial in 1990. The deep genetic discoveries allow me the same luxurythat the neo-Darwinian commentators had between 1959 and the 1970s, when

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Introduction

the philosophical and historical stage was being set. Like them, I can nowreasonably assume that my favorite theory pretty much tells it like it is.

1.3 how i came to this book

I began studying these debates in the early 1980s, in the midst of the anti-Synthesis criticisms. I was just finishing an extended historical study ofmethodological conflicts in the history of experimental psychology betweenbehaviorist and early cognitive psychologists (Amundson 1983, 1985, 1986).The two sides often seemed to argue past one another in these debates. How-ever, I found that it was possible to discover hidden methodological conflictsby a close reading of the argumentation. Some features of the evolutionarydebates of the 1970s and 1980s seemed very similar to me, especially thosecentering on development.3 Adaptation versus developmental constraint wasa function-versus-structure debate. The proadaptation side favored functionover structure, and the prodevelopmental side favored structure (constraint)over function. I had just worked through a similar debate in psychology:the cognitivists were structuralists and the behaviorists were functionalists(Amundson 1989). I began reading in the history of evolutionary biology tosee how deeply the structurefunction contrast could be traced. It ran verydeep indeed (Russell 1916). It seemed likely that the conflict between adap-tation and developmental constraint was not only a phenomenon of the 1970sand 1980s.

My reading in the history of evolutionary biology has been guided bysecondary historical sources. As I read through reports about pre-DarwinianBritish naturalists, I began to get the feeling that the deck had been stacked.Even in the secondary literature I could recognize structurefunction debatesbetween pre-Darwinian scientists. Their disagreements paralleled those of the1980s. However, most historical commentaries failed to take that distinctionseriously. They classified all pre-Darwinians into a single category of antievo-lutionists, and they glossed over the differences between functionalists andstructuralists. This was my first hint that an examination of the methodologi-cal debates of the 1980s would extend into an examination of how the historyof evolutionary biology had been written. Important pre-Darwinian conflictshad been historiographically minimized in a way that obscured the parallel be-tween the pre-Darwinian structurefunction debates and those of the 1980s.

3 A sabbatical year in 19851986 spent in Stephen Jay Goulds lab, and regular discussions withPere Alberch and Richard Lewontin, aided these thoughts.

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To top it all off, the conceptual errors that were attributed to the (generic)pre-Darwinians were exactly those conceptual errors being attributed to themodern-day structuralist critics of the Synthesis!

The game is afoot, thought I. Someone is cooking the books! (thoughperhaps not in those very words).

This was my first evidence that many histories of evolutionary biologyhad been written by people who considered the Evolutionary Synthesis tobe essentially correct about evolutionary biology, including its opposition tomodern alternative theories that involve development. The commitment to aparticular modern theory had colored the reportage of historical science.Historical narratives could be read simultaneously as explanations of Darwins100-year-old success over his critics, and of the parallel success of the Evolu-tionary Synthesis over its modern critics. I realized that a historian who tookthe constraints side of the modern adaptationconstraints debate wouldwrite a very different history of evolutionary biology.

This is that history. I have cooked my own book.I have since come to understand that writing the history of science is

seldom an objective facts-only report of events. Scientists, especially whenwriting about the history of their own science, are simultaneously conductingcontemporary research and argumentation. This is true of philosophers aswell, who often have philosophical as well as scientific theories in the backs oftheir minds. Historians (especially recently) are somewhat less influenced bymodern science, apparently because their discipline has provided them withother frameworks for their studies (e.g., the influence of social institutionsor the self-interests of scientists on the practice of science). However, aswe will see, historians too have a tendency to provide narratives that comeout right. A narrative comes out right when the predecessors of approvedmodern theories appear (in the narrative) to have made more sense than theircontemporaries who turned out to be predecessors of theories that are nowregarded as fallacious.

I will not attempt to avoid this problem of bias, but I will try to make it astransparent as possible. We (philosophers especially) do not do history froman abstract love of history. David Hull and I have come to quite differentconclusions in our historical writings. In a discussion of our differences,David pointed out to me that my own writing was as biased as I claimed thetraditional Synthesis histories to have been. He said that his work of the 1960sand 1970s was history done in a good cause. He made me realize that mineis exactly the same. But its now thirty years later. His good cause was won(with his able help), and my good cause is a different cause.

10


Introduction

1.4 historical format

I had originally conceived this book as a chronological history of the relationsbetween development and evolution. The chronological sequence has beenroughly maintained, but the book is now separated into two parts that aredistinct in methodology and in the centuries they cover. The two parts couldvery well be read separately. Although both parts are historical narratives,they are set against different backgrounds. In an odd way, the first part of thebook is dependent on the second part.

The first part covers roughly the nineteenth century. The narrative is self-consciously revisionist; it is set against the background of traditional narra-tives that grew up with the Evolutionary Synthesis. I refer to the traditionalnarratives as Synthesis Historiography, or SH. I try to show that developmen-tal approaches to evolution were scientifically progressive before Darwin, thatthey benefited Darwins program, and that Darwin recognized that fact. In ad-dition, the program of evolutionary morphology that immediately followedDarwin was itself well motivated and reasonable. It failed, but not becauseof the ideological flaws alleged by SH authors. Understanding the nineteenthcentury from this point of view, there is nothing that would lead one to believethat development is irrelevant to evolution except the practical difficulties ofunderstanding how embryogenesis actually works. This is all contrary to thetradition of SH, which finds ideological flaws and metaphysical errors in everynineteenth-century advocate of developmental evolution.

The second part of the book covers the twentieth century. It is not par-ticularly revisionist. Indeed, it supports many of the claims of SH authors(e.g., Ernst Mayrs claim that embryologists were not originally interested inparticipating in the Synthesis). The goal of the second part is to understandhow the Evolutionary Synthesis came to be opposed to development in evo-lution how things changed so very much from the nineteenth century. Apart of this narrative is how SH (and its associated philosophical arguments)came into being. The antidevelopmental views of neo-Darwinian theoristsare associated with philosophical and historical views that were articulatedaround the 1959 centennial of the publication of Darwins Origin. Thesewere not purely philosophical notions, of course. They involved central as-pects of the population genetic understanding of evolution and the theoryof heredity on which it was based. Nevertheless, I also trace the continuedchallenges from advocates of development through the century, and the alter-native understanding of the nature of evolution on which those views werebased.

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The invention of SH is one central topic of Part II of the book. SH is alsothe ideological background against which Part I is set. The reader may wantto read Part II first. It explains how SH came into being that same SH thatformed the problematic of Part I.

My identification of Synthesis Historiography as an actual trend of schol-arship in history of biology may be controversial. The very fact that I give ita name suggests that I harbor a paranoid conspiracy theory. In fact, I dontbelieve in a conspiracy at all. The early examples of SH really were in agood cause as David Hull reports. In the 1960s, modern neo-Darwinismwas still poorly understood by nonbiologists, and (it appears) especially byphilosophers.4 SH served the purpose of explaining the modern theory in aclear and concise way, revealing its philosophical richness, and expressing itsopposition to theoretical alternatives. There is no doubt that the public andthe academic community were educated by those writings. The subsequentgrowth of the fields of both history of biology and philosophy of biology canbe largely credited to the efforts of SH authors. My own education in historyand philosophy of biology owes a very great deal to these writings, as doeseveryone elses. Nevertheless, I believe that central features of those writingshave outlived their usefulness. They are a barrier to a better understandingof the history of evolution theory, and possibly to an integration between theSynthesis and evodevo. To take development seriously, we must challengethe historical interpretations that made it seem so irrelevant.

My defense against the accusation of paranoia can come only by the ex-amples I discuss in Part I. The primary source of Synthesis Historiography isErnst Mayr, one of the architects of the Evolutionary Synthesis and a giantin twentieth-century history and philosophy of biology. Other authors whohave contributed to (what I see as) the Synthesis interpretation of history arephilosopher David Hull and historians Peter Bowler and, to a lesser extent,William Coleman. Michael Ruse will hold a neo-Darwinian bias until the dayhe dies, but his historical writing poses somewhat fewer interpretive prob-lems. I cannot identify SH with a definite list of authors, because some of themost egregious concepts have even been adopted by authors who are generallysympathetic to development. For example, both Dov Ospovat and Stephen JayGould follow Bowler in describing pre-Darwinian transcendental morphologyas an idealist version of the Argument from Design (Bowler 1977). I con-sider this a serious misrepresentation, detrimental to structuralist interests,

4The philosophers, in particular, were almost unanimously opposed [to Darwinian selectionism]until relatively recent years (Mayr 1980a: 3). Mayr cites Ernst Cassirer, Marjorie Grene, andKarl Popper.

12


Introduction

and I argue against it in Chapter 3. Nevertheless, Ospovat was the first his-torian to reveal the importance of pre-Darwinian structuralism, and Gouldwas among the strongest structuralist critics of the Synthesis (Ospovat 1981;Gould 2002).

The final reason I find it difficult to pin down SH precisely is that someof its effects are so global as to be virtually untraceable. Here is an example:Between Linnaeus and Darwin, most intellectuals believed in species fixism.What were their reasons? As Chapter 2 documents, species fixism was anempirically founded discovery of the mid-eighteenth century, not an ancientdoctrine from Greek philosophy and Christian theology as SH would haveit. Ernst Mayr acknowledges this fact in his early writing, although seldomafter the crucial year of 1959. However, very little scholarly writing exists onthis topic today (exceptions are cited in Chapter 2). This lack of scholarshipis almost certainly due to SH itself. No one has looked for the reasons forspecies fixism because they were already known! The explanation given byMayr in 1959 (mirrored by A. J. Cain in 1958, and echoed by Hull andMichael Ghiselin shortly after) was that pre-Darwinians were essentialists,and essentialism implies species fixism.

This is the Essentialism Story, the central pillar of SH (named followingWinsor 2003). It holds that typology and essentialism (said to be identical doc-trines) were to blame for the pre-Darwinian belief in species fixism, and laterto blame for evolutionary saltationism and other theoretical alternatives to theEvolutionary Synthesis. The historical claim was not challenged until 1990,and it is still reported as historical fact (Ereshefsky 2001). We will see that itis both conceptually inadequate to account for species fixism and historicallyirrelevant to species fixism and other pre-Darwinian beliefs. Nevertheless, itseems to me, the reason that we have so little historical information about thereasons for the belief in species fixism is that the Essentialism Story was as-sumed to have answered that question. If this is correct, then SH is responsiblefor the lack of historical research on species fixism.

The Essentialism Story is so central to SH that I am almost willing todrop the SH concept itself and let my case stand or fall on the EssentialismStory. My reason for continuing to talk about SH is that the EssentialismStory sounds like a narrow issue. It is not. It is tied to substantive theoreticalaspects of the Evolutionary Synthesis, to the Synthesis reconstruction of thehistory of evolution theory, and to Synthesis critiques of modern structuralist,developmentalist alternatives. Other aspects of historiography, like the treat-ment of transcendental morphology and the alleged idealist version of theArgument from Design also fall under the heading of Synthesis Historiog-raphy. Therefore, I will continue to allude to Synthesis Historiography.

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1.5 epistemological concepts in historical context

Three issues of general epistemological orientation recur in this discussion.The book is not intended to justify one or another philosophical position; thephilosophical issues are secondary to those of scientific method and content.However, philosophy played a role in the debates, and these concepts were(and are) involved. Two of the concepts roughly correlate with the traditionalempiricismrationalism dichotomy. They are what I will call inductivist cau-tion, and idealism. The third is essentialism itself, and the two distinct versionsof essentialism that have been in play during the past half-century.

1.5.1 Inductivist Caution

This view is the often-empiricist conservative methodological position thatencourages the scientist to avoid speculation, and especially avoid to concep-tual speculation, speculation that invents new concepts. The institutions ofscience seem to vacillate between extremes on the conservativeliberal spec-trum. Larry Laudan contrasts epochs when the object of science is seen asdiscovering empirical laws and epochs when the stress is upon discoveringexplanatory, deep-structural theories (Laudan 1980: 178179). Laudan saysthat the early twentieth century was conservative, with logical positivism andthe Copenhagen interpretation of quantum mechanics (I would add behavior-ist psychology). The late twentieth century is far more liberal, with quarks andblack holes (and cognitive psychology with dozens of hypothesized mentalmodules). David Hull has documented the conservative and empiricist atmo-sphere of Britain in the pre-Darwinian period (Hull 1973, 1983; one of manydebts I owe to SH authors).

Inductivist caution is the attitude that values high-probability inferencesthat remain close to the scientists direct observations. Phenomenal laws,those that are expressed in terms of observable properties, should be the or-dinary scientists goal. Once an adequate understanding of the world in termsof phenomenal laws was available, one might try to discern an underlyingcausal law, which designates a true cause or vera causa (Ruse 1979). Thedistinction between phenomenal laws and causal laws seems always to havebeen illustrated with Kepler and Newton. Kepler discovered the phenomenallaws that describe the motions of planets; it took a Newton to determine thevera causa of the law of universal gravitation (Hopkins 1860). Even thoughcausal laws were sometimes reachable, it was considered immodest for anordinary scientist to claim to have reached one. This view of vera causa wascertainly an influence on the severe reaction that most British scientists had

14


Introduction

to the anonymous publication in 1844 of Vestiges of the Natural History ofCreation, and it had similar affects on reactions to Darwins Origin in 1859.Darwins claim of common descent was much less objectionable than hisclaim to have found its cause in natural selection.

The empiricism behind inductivist caution will play a role in other contextsthan that leading up to Darwin. The view is often associated with an antireal-istic attitude toward unobserved theoretical entities or processes. As Laudanreminds us, we live in a time of lively interest in such inferences. However,conservative eras are frequent in the history of science, when the payoffs ofhigh speculation are believed to be outweighed by its risks. Conservativesinsist that scientists stick to observations and predictions, and abandon hy-potheses and explanations. Although this kind of operationalism or positivismis out of fashion today, it is historically important. It can smooth the transitionbetween old established concepts and radical new ones. This is not an inten-tional strategy of scientists in these cases; their positivism is an honestly heldmethodological principle. Nevertheless, the positivism of the transitional sci-entist often gives way to realism about the radical new theory. A declarationthat a bizarre new idea is just a calculating device, not a description of realitycan calm the resistance against a new idea long enough for the progress ofscience to work its way past the barrier. Newtons hypotheses non fingo washis way of dodging the fact that gravity was action at a distance, believed to bemetaphysically impossible in his day. Copernicuss editor Osiander inserted apreface to De Revolutionibus that said the sun-centered system was not abouta real earth moving through space around a real sun; it was a mere calculatingdevice. After the shock had worn off, the notions of a gravitational force and amoving earth were interpreted realistically. In the episodes to follow, we willsee that the origins of both embryology and genetics were aided by similarlyconservative, antirealistic stances that buffered the objections that otherwisemight have been fatal to radical new theories.

One complication must be introduced into the discussion of empiricism andinductivist caution. In periods during which new theories are being developed,scientists are often willing to commit themselves to a phenomenal law, and tothe claim that the law points to an important underlying causal explanation, butthey are not be ready to commit themselves to the nature of the underlying ex-planation itself. I call this position cautious realism. Cautious realists believein a reality underlying a phenomenal law, but they are not yet ready to nameit. This was surely the position of Kepler with respect to his planetary laws.I will argue that it was also the position of many pre-Darwinian naturalistswith respect to the organic patterns they were discovering. The SH tradi-tion tends to label pre-Darwinian authors as antievolutionists (essentialists,

15



special creationists, etc.) when they merely fail to assert evolution as the causeof the regularities that they studied. I will argue that many of these individu-als were instead cautious realists. When we recognize cautious realism as arespectable scientific stance, we need no longer divide the nineteenth centuryinto evolutionists and creationistspecies fixists.

1.5.2 Idealism

An important theoretical orientation in early-nineteenth-century biology isvariously labeled idealistic morphology, transcendental anatomy, and some-times philosophical anatomy (Rehbock 1985). Like many philosophicalterms, idealism can mean many different things. In this case, it refers to theKantian variety. The topic is treated with true derision by many authors inthe SH tradition.5 One SH approach that is not helpful is to define idealismin terms of essentialism or typology. For one thing, the assertion is simplyfalse. For another, the Essentialism Story attributes essentialism to almost allpre-Darwinians, so it doesnt help us to understand idealists when we hearthat they are essentialists. Finally, it is very difficult to explain the philosoph-ical context of Kant and his relation to the continental morphologists withoutbecoming embroiled in arcane vocabulary and disputes that are far from ourtopic.

I am frankly going to duck this issue. I will discuss the philosophicalunderpinnings of the continental morphologists only enough to allow theirscientific views to be presented with minimum prejudice. In my view, thescientific work has value enough to incline the reader toward tolerance for themetaphysical idiosyncrasies of its authors. For a deeper understanding ofthe philosophical complexities of the movement, I must refer the reader toother sources (Lenoir 1982; Sloan 1992, 2002; Larson 1994; Asma 1996;Richards 2002). Now a few comments to comfort the squeamish reader, whowill soon be asked to take transcendental anatomy and idealistic morphologyseriously.

5 Brief allusions include the lofty fallacies of idealist philosophy (Mayr 1976: 258) and idealistmoonshine (Bowler 1984: 125). Slightly more useful is the description of idealists as thosewho tended to explain the order in nature by reference to ideal types (Hull 1973: 67). Themost extended discussion is by Bowler, who associates idealism with Hegelian political theory(subordinating the individual to the state) and traces this view through to Karl Marx (Bowler 1984:99102). Bowler cites Karl Popper on his Hegel commentary; Popper is also the source of theessentialism epithet that will loom large in my discussion. The Popperian origins of the notionof essentialism suggest that the Essentialism Story itself is a holdover from the epistemologicallyconservative first half of the twentieth century.

16


Introduction

The extreme SH antagonism toward Kantian idealism seems to be aholdover from the epistemologically conservative first half of the twentiethcentury. Both Newton and Kepler had far zanier metaphysical ideas than theKantian idealists, but their scientific reputations are unsullied. The Essential-ism Story was introduced around 1959, just as logical positivism was drawingits last breath. It has kept alive an animosity toward biological idealism that isunlike anything seen in other areas of the history of science. This is especiallystriking when one recognizes how very different, and very Kantian, both thephilosophy of science and cognitive psychology had become by the 1970s.Condemnation of the idealists was easy for the positivists but much harder fora post-Kuhnian philosopher of science. Kant held the mind to be active, pro-ducing ampliative judgments that went well beyond the input of the senses.What modern philosopher of science could disagree? Modern psychology iseven more Kantian than philosophy of science. Several prominent commen-tators have argued that cognitive psychology, in describing the mind as activeand constructive, has empirically confirmed the basic framework proposedby Kant (Guyer 1987; Kitcher 1990; Brook 1994).

As an aid to the reader, I therefore propose a version of idealism intendedonly to open his or her mind to the morphological theories called idealist.It is Amundsons Minimal Idealism.

1. Human epistemic abilities are not limited to inductively generalizing fromsensory experiences; they are able to create hypothetical constructions ofunobserved reality.

2. Let us call these hypothetical constructions ideas.3. Once constructed, these ideas can be compared with sensory experience,

which may confirm or deny the value of the ideas.

On this account, ideas are nothing but hypotheses (although they are un-bound by empiricist restrictions on their content). There is no requirement forideas to have a distinct ontological mode of existence, in Gods mind or any-where else. Ideas are merely what we (i.e., our minds) use in understandingthe world. This is almost all that we need for so-called idealist morphol-ogy to make sense. I am skipping many details, of course. Some of Kantsideas were automatic instincts of the mind that themselves formed (ratherthan following from) experience. Idealists differed on the status of an ideaonce it had been constructed, and there were great differences about whichideas provided (or constituted) knowledge, and which ideas were only heuris-tic, regulative principles that might be indispensable for research. Kant heldthat teleology, for example, was an indispensable but regulative (heuristic)assumption, needed for the practice of biological research. Others adopted

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what Sloan calls the Schelling revision, and declared that Kantian ideaslike teleology were constitutive of nature (Sloan 1992: 33). These werethe Naturphilosophen. They have received immense abuse through historyfor their metaphysical merriments, beginning as early as the 1820s. How-ever, even this distinction will not concern us. We will be concerned withthe value of the scientific concepts that Naturphilosophen and other idealistscontributed to biological thought. We will fret over their metaphysics no morethan we fret over Keplers.

Amundsons Minimal Idealism distorts the history of philosophy. Never-theless I hope it allows the reader who might be inclined to dismiss idealistmorphological theories to pay them heed. SH authors are not concerned withthe purely philosophical aspects of idealism anyhow. They oppose idealismbecause it leads (they say) to faulty scientific theories. I disagree. Whateverthe value of idealism as a philosophical doctrine, it has given rise to extremelyimportant scientific theories. These theories played a crucial role in the his-tory of evolutionary thought, and they do not deserve the disdain to whichthey have so long been subject.

1.5.3 Two Essentialisms

Essentialism is a doctrine about natural kinds. Natural kinds are assumed topossess essences. The nature of the commitment to essences differs in the tra-ditional concept of essentialism (embedded in the Essentialism Story) and amodern version discussed in the paragraphs that follow. In its traditional form,essences are definitional sets of intrinsic necessary and sufficient conditionsthat logically determine an entitys membership in the kind. Kinds are (bytheir definitional essences) discrete from one another. Because the definitionsare timeless, the kinds are eternal and unchanging. The essential charactersof some kinds (e.g., biological taxa) may require scientific discovery. How-ever, essentialism itself is not a scientific doctrine but an a priori doctrine ofmetaphysics. It is not an empirical but a conceptual truth that natural kindsexist, and that they are fixed, eternal, and changeless.

Since about 1970, philosophers of science (though usually not philosophersof biology) have discussed essentialism from a very different perspective.Philosophers have increasingly been attracted to scientific realism, consistentwith Laudans observation about our epistemological era. Realism has be-come associated with a kind of essentialism that is quite distinct from that ofthe Essentialism Story. Natural kinds are seen as categories that play a role ina law of nature (after Quine 1969). Both the determination of the kinds them-selves and the discovery of the causal facts underlying their kindhood (their

18


Introduction

essences) are seen as a posteriori empirical achievements. Essences are con-ceived not as definitional stipulations but as underlying causal structures thatexplain the observed, discovered lawlike facts about the kind. The standardexample of a natural kind is a chemical element such as gold. The essence ofa chemical element is the atomic structure of an atom of that element. RichardBoyd has prominently argued for a version of this kind of essentialism (Boyd1991). He distinguishes two different kinds of realistessentialist definitions.One definition is suitable to the early stages of the development of a scien-tific theory. This is a programmatic definition, which specifies the role thata particular natural kind is intended to play in a theory. An example is theposition on the periodic table of a chemical element, indicative of its combi-natorial relations with other elements. The other definition is explanatory,and it typically occurs later in the development of a science. It specifies theessence as the causal properties in virtue of which the kind is able to fulfillits programmatic role in the scientific theory. The programmatic definition ofchemical elements involves their valences and combinatorial relations. Theexplanatory definition specifies the atomic structure that explains the valenceand combination relations. Chemical elements show the combinatorial rela-tions that they do in virtue of their different atomic structures.

Boyds version of essentialism is quite distinct from that of the Essen-tialism Story. He carefully explains the difference: The Essentialism Storydescribes a metaphysical, a prioristic account that implies fixed and unchange-able kinds and is based on strict necessary and sufficient definitions. Boydsversion is thoroughly a posteriori. It freely uses fuzzy definitions (which cor-respond to homeostatic property clusters rather than strict necessary andsufficient conditions) and has no implications regarding fixity. In the Essen-tialism Story, essences are not offered as causally explanatory. Essentialism istreated instead as a pre-existing, a priori, rationally unacceptable metaphysi-cal commitment that entailed species fixism in the entire absence of empiricalevidence.

I introduce Boyds version of essentialism not because it is relevant to theEssentialism Story. As Boyd insists, it is very different. Boyd does not chal-lenge the story (as I will); instead he merely claims that those essentialistswere doing something different than he is doing (Boyd 1999: 146). I introduceBoyds version because it expresses a form of scientific realism that will playa genuine role in the discussions that follow. Scientific talk about essencesneed not imply the Essentialism Story (metaphysical, semantic, a prioris-tic, and species fixist). It may instead indicate a commitment to scientificrealism regarding a process or entity, even when the deep or ultimate na-ture of the entity is not yet understood. I will argue that the Essentialism

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Story is false. However, we will see in nineteenth-century biology somesigns of a Boydian essentialism, and the use of programmatic definitionsof kinds. This fits well with what I have already called cautious realism. Real-istic but a posteriori commitments to certain entities (including both speciesand the Natural System itself) were an important and progressive feature ofpre-Darwinian thought. SH makes it impossible to recognize this importantscientific development.

1.6 explanatory relativity

Scientific explanation is an epistemological topic of a different nature thanthe relation between theory and evidence. Explanations are context relativein a way that theories are not. They are best understood as answers to why-questions (Bromberger 1966). If a person asks you Why x?, you may re-spond not with an immediate answer but with another question: What is itabout x that you dont understand? An explanation of x that is satisfactoryto one person might not be satisfactory to someone else, because what theydidnt understand about x was different. Sometimes these differences canbe discovered by rephrasings of the why-question. Why did Adam eat theapple? might mean

Why was it Adam who ate the apple (rather than someone else)?Why was it the apple Adam ate (rather than some other food)?Why did Adam eat the apple (rather than throwing it, or cooking it into a pie)?(after van Fraassen 1980)

The Adam example illustrates the fact that explanations occur against a back-ground of presuppositions that can be seen as a contrast class of alternativepossible answers assumed by the questioner. The questions {Why x ratherthan a, b, or c} and {Why x rather than d, e, or f} call for different answers.Another illustration comes from the apocryphal story about the notoriousbank robber Willie Sutton. A priest visited Willie in his cell and asked himwhy he robbed banks. Willie replied, Because thats where the money is.The priest assumed the contrast class of robbing banks versus making an hon-est living. Willie assumed the contrast class of robbing banks versus robbingother establishments.

The interest-relativity of explanations has led many philosophers, includ-ing Bas van Fraassen, to the view that explanation is a matter of practical orapplied science, not of pure science. Van Fraassen ends his elegant discus-sion of explanatory relativity by claiming that a successful explanation gives

20


Introduction

no special support to the scientific theory that underwrote that explanation(van Fraassen 1980: 156). Evidence that the theory is true must come inde-pendent of the fact that it satisfies our interests. I have no desire to argue thatpoint: What is and is not pure science is not my concern. Instead, I am inter-ested in understanding the conflicts between contrasting scientific traditions.Here, explanation is crucial.

If you want to understand a scientific theory or research tradition, lookat what the scientists want to explain, and what they think counts as an ex-planation of it. If you want to understand the difference between a pair ofcompeting scientific theories, look at why each theorys advocates believethat the others explanations are faulty.

Scientific theory changes are often associated with changes in whatis thought to require explanation. Aristotle said that motion requiredexplanation; Newton said that change of motion required explanation. Phlog-iston theory attempted to explain the observable qualitative characteristicsof chemical compounds in terms of the qualities of their elements: Metalsare shiny and malleable because they contain phlogiston, the element of fire.Atomic theory did not replace the phlogistic explanation of the qualities ofcompounds. Instead, it abandoned the goal of explaining the qualities ofcompounds entirely, and it replaced it with the new goal of explaining theproportionate weights of elements and compounds a poor substitute, to mymind. Nevertheless, the new explanatory goal was achievable, and the oldgoal was not. To understand the differences between these theories, we mustunderstand the differences in the questions they asked which is to say, theexplananses for which they sought explananda.

The methodological differences between research programs are often hid-den behind the verbal similarities in the questions they ask. This can lead toapparent conflicts where no conflict really exists. (Imagine a debate betweenpriests: Willie robs banks because thats where the money is! No, he robsbanks because he has no moral conscience!) It is important to be aware ofthese potential confusions. Aristotles four causes are really different kindsof explanation; each may apply to the same explanans (or rather to fourdifferent explananses that have the same verbal expression). Ernst Mayrsdistinction between proximate and ultimate causation (discussed in detail inChapter 10) is similar. Proximate and ultimate explanations need not conflict.The choice between them depends on ones explanatory interests.

Explanatory relativity is important for this project because the dichotomyof function versus structure is a contrast of explanatory modes. It is possible tohave both a functional and a structural explanation of the same phenomenon.Nevertheless, functionalist and structuralists tend to clash, and the clash runs

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through our entire 200-year historical narrative. Function versus structure isa contrast not simply about the facts of the world but about what phenomenarequire explanation. The advocates of one side will frequently announce thattheir opponents do not give explanations, but only descriptions. This is asure sign of explanatory relativity. A large part of my task will be to teaseapart these disputes and try to separate those that involve actual matters of factfrom those that are based on disagreements about what needs to be explained.

Explanatory relativity does not imply a relativity of truth, or even a rela-tivity of theoretical adequacy. Some theories really are better and some areworse. Structuralist theories are better in some domains, and functionalisttheories in others (Amundson 1989). It is difficult to understand the contrastuntil the differences in explanatory goals are understood. The differences inexplanatory goals between neo-Darwinism and evodevo have not been ade-quately recognized. I will develop an interpretation of these differences thatis based on their respective historical origins, but I must revise the historiesbefore such a comparison will be useful.

1.7 historical conventions

Professional historians of science will soon recognize that a philosopher wrotethis book. It sometimes reads as if history is made up of the interactions ofideas rather than of people. It will sometimes overgeneralize the similaritiesand underestimate the complexities of historical events and debates. This thehazard of doing history and philosophy at the same time. I have had to ignoresome particulars in order to recognize the general and repeated aspects ofmethodological debates through history. I have tried not to use this as anexcuse to misrepresent the debates, but it has forced me to neglect somecomplexities.

I have also, for conveniences sake, used some terms in ways that arehistorically inaccurate. Here I apply the term Lamarckian to the theory ofthe inheritance of acquired characteristics, even though there is very littlehistorical justification for that label. (Lamarck didnt invent the notion, andprior to August Weismann almost everyone including Darwin believed in itanyhow.) A second misused term is neo-Darwinism. This was first appliedto Weismanns evolutionary theory, which accepted natural selection andrejected Lamarckian (ahem) inheritance. Some historians argue that neo-Darwinism should not be used to label twentieth-century evolutionary theory(Reif, Junker, and Hossfeld 2000). Notwithstanding, here I use it to labelthe theory associated with the Evolutionary Synthesis. Finally, historians are

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Introduction

careful not to apply invented terminology to historical cases that existed priorto the invention. So, for example, Newton would not be called a scientistbecause William Whewell invented the term scientist in the 1830s. I violatethis convention with impunity, but I try to avoid the errors that the conventionis designed to prevent.

1.8 historical precis

A central theme of the book is structure versus function. I once had hopes ofreducing two centuries of history of biology to this theme (a typical philoso-phers ambition), but it didnt work. Two additional factors make the storymuch more complex. One is a second difference in explanatory goals (besidesfunctionstructure) between Darwinian and structuralist evolutionary theo-ries. The other is a radical change in the concept of heredity that occurred inthe early twentieth century.

Even aside from their commitment to functionalism (adaptationism),Darwin and the Evolutionary Synthesis share an explanatory goal that sepa-rates each from structuralist theories. I begin to examine this contrast inChapter 4 and continue throughout the book: Darwinian theories are changetheoretic whereas structuralist theories are form theoretic. Very roughly, thisimplies that Darwinian theories do not accept responsibility to explain form(or anything else) in the evolutionary entity; rather they explain change in theentity through evolutionary time (change in form, or change in anything else).The ancestral features are assumed as a background condition. In contrast,structuralist theories accept responsibility to explain form both in ontogeneticand phylogenetic time, and to associate the two aspects of form-explanations.So ancestral form is not merely a background condition for structural theories;it must receive an appropriate (e.g., ontogenetic) explanation. The change-theoretic explanatory goal is shared by Darwin and the Evolutionary Syn-thesis. This legitimates the traditional claim that the Synthesis is Darwinian(more than the questionable claim that Darwin was a population thinker).

The second factor needed to understand the twentieth-century history ofthe relation between evolution and development is a certain radical changein the concept of heredity. I literally remember the hour I came to realizethis fact. It was in 1997 at the Seattle meetings of the ISHPSSB, at a sessionhonoring historian Fred Churchill.6 During the discussion, one of Churchills

6 ISHPSSB is the International Society for the History, Philosophy, and Social Studies of Biology,pronounced Ishkabibble.

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former students said something to this effect: As Fred has shown, during thenineteenth century heredity was considered to be an aspect of embryologicaldevelopment. My head swam. I quickly chased down Jane Maienschein toget the proper citation (Churchill 1974). This was how a structuralist view ofevolution could seem almost self-evident in the nineteenth century but be la-beled as incoherent in the twentieth. (I had already rejected the EssentialismStory that blamed structuralist beliefs on silly metaphysical views.) A re-lated but independent historical discovery was made by Jean Gayon (Gayon1998). Gayon showed that natural selection in the nineteenth century wasinconsistent with any known theory of heredity. Moreover, no one even knewhow to specify what must be true about heredity in order to make naturalselection work as an evolutionary mechanism! So the nineteenth-century re-jection of natural selection as the primary mechanism of evolution was basednot on typological dogma but on hard-nosed experimental science (and theEssentialism Story fails again). Thomas Hunt Morgan introduced the newkind of heredity that distinguishes it from development (Morgan et al. 1915).This new entity (nondevelopmental heredity) would turn out to enable naturalselection as a long-term evolutionary mechanism but at the same time disal-low the relevance of development to evolutionary change. This story and theconsequent debates comprise Part II of this book.

I intend the overall narrative of the book to come out right for evodevo.Evolutionary developmental biology has a respectable history of intellectu-ally productive predecessors from the early nineteenth century through thetwentieth. Unfortunately, so does neo-Darwinism. I feel that I have success-fully refuted the SH belittlement of nineteenth-century structuralist evolutiontheory. I have defended twentieth-century structuralists against their method-ological critics. I have even shown some interesting details about methodolog-ical contrasts between neo-Darwinism and evodevo. But I have not shownneo-Darwinism to be wrong about evolution, and I have not shown how tomeld the two traditions into one. That must remain a job for the scientists.

Here is a sketch of the historical narrative to follow.

part i: darwins century: beyond the essentialism story

1. Species fixism was a discovery of the eighteenth century. It was scientif-ically progressive because it enabled the construction of a classificatoryNatural System. A Natural System would have been incoherent underprefixist understandings of organic relationships.

24


Introduction

2. Contrary to SH, essentialism and typology were neither part of thegrounds for species fixism nor involved in the early systematic attemptsto classify organisms into the Natural System.

3. Morphology (including embryology) grew out of systematics, but it de-veloped into an autonomous discipline. It originated the important ex-planatory goal of the explanation of form. The concept of morphologicaltypes grew from this enterprise. Morphological types are not appropri-ately characterized by the SH condemnation of typology for reasons tobe discussed.

4. Early understandings of the Natural System were conventionalist in thesense that taxonomic categories were seen not to reflect objective realitybut instead to organize data for human uses. As time passed, the NaturalSystem came more and more to be seen as an accurate depiction of realrelationships in the natural world. Morphology was centrally involved inthis increasing reification of the Natural System.

5. Darwins achievement must be seen in two parts. Part 1 (by his ownclassification) was the argument for natural selection as the force behindevolutionary change. Part 2 was his proof of descent from common an-cestry. Part 2 was successful almost immediately. The general acceptanceof Part 1 awaited the Evolutionary Synthesis seventy years later.

6. Darwins proof of Part 2 depended on the existence of a well-establishedand reified Natural System. It also relied heavily on the morphologists(including their brand of typology), and Darwin acknowledged that fact.SH authors assert that Darwin rejected the importance of, for example,Richard Owens typological morphology. These reports stem from sys-tematic misreadings of Owen and of Darwin.

7. Darwins argument for Part 1 introduced a new explanatory goal forevolutionary biology. It is the explanation of change. Darwin did notaccept the morphologists (e.g., Owens) goal of explanation of form(though the distinction was not well recognized by either side). In chang-ing the explanatory goal, Darwin abandoned the explanatory aspects ofthe morphologists concept of types. This innovation (more than popula-tion thinking) justifies viewing Darwin as forefather of the EvolutionarySynthesis.

8. Evolutionary morphology was the first evolutionary research tradition. Itaccepted Darwins Part 2 but marginalized Part 1. It incorporated commondescent into the traditional morphological goal of the explanation of form.The program failed but was not conceptually flawed by typology in theway that SH commentators report it to be.

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part ii: neo-darwins century: explaining the absence andthe reappearance of development in

evolutionary thought

9. The concept of heredity grew in importance during the nineteenth cen-tury, largely because of interest in Darwins theory of natural selection.Heredity was conceived as an aspect of embryological development. Thisis very different from the modern conception of heredity, and the signif-icance of that difference is enormous.

10. Natural selection remained scientifically anomalous throughout the nine-teenth century. This was (not because of the bad influences of typologybut) because the known facts of heredity could not be shown to be consis-tent with natural selection as a long-term cause of evolutionary change.

11. The separation of heredity from development was achieved around 1915by T. H. Morgan and associates. It required a quasi-positivistic argumentthat hereditary properties of the germ could be said to cause adult traitsat a distance (i.e., ignoring the intervening causes that construct the adultfrom the zygote). Even though we all know that ontogenetic processes areinvolved in the construction of adult traits, those processes are henceforthto be ignored when we are discussing heredity.

12. Most embryologists rejected Morgans quasi-positivistic gene on thegrounds that it cannot, in principle, explain development. In effect, theyrejected the new antidevelopmental definition of heredity.

13. In contrast, population genetics was based entirely on Morgans posi-tivistic transmission gene. For the first time, a theory of heredity wasconsistent with natural selection as a cause of long-term evolutionarychange. (This required changing the meaning of heredity of course.)Thus, Morgans quasi-positivistic gene enables natural selection as anevolutionary cause and at the same time disables the participation ofembryology in the resulting evolutionary theory.

14. The Evolutionary Synthesis is formed on the basis of population genetics.It shares Darwins goal of explaining change. Meanwhile, experimentalembryology flourishes while largely ignoring genetics (including the non-positivistic developmental genetics). It retains the morphological goalof explaining form. A few individuals (such as Goldschmidt, Wadding-ton, and Schmalhausen) attempt to integrate the explanation of form intothe Synthesis, but they fail.

15. By the 1950s, support for the molecular genetic studies of developmentis growing. This has two independent effects. The first effect is thatnongenetic experimental embryology fades.

26


Introduction

16. The second effect of the rise of molecular genetics is that, around 1959,Ernst Mayr, leader of the naturalist contingent within the EvolutionarySynthesis, is moved to write several methodological papers on the impor-tance of naturalistic studies in evolution. These are aimed not particularlyat refuting the relevance of development, but at enhancing the status ofnaturalists in the Synthesis (as against mathematical geneticists), de-fending the Synthesis against critics, and aligning it with Darwin as anintellectual forefather. This is the origin of SH. (Needless to repeat, SHmisreports several of the historical episodes listed herein.)

17. Mayr in 1974 organizes two symposia on the history of the EvolutionarySynthesis (reported in Mayr and Provine 1980). During the 1970s and1980s a series of attacks are mounted against the Synthesis, allegingmethodological flaws. One of these is the structuralist claim that theSynthesis ignores development. The Synthesis is writing its own historyjust at the time that it is being attacked.

18. In the 1980s and 1990s, Synthesis advocates deploy a series of argumentsto defend against the continuing structuralist criticisms. These includeconcepts derived from Morgans 1915 distinction between heredity anddevelopment, and others from Mayrs important methodological writingsaround 1959 (Mayr 1959b, 1959c, 1961). It is argued that developmentis conceptually irrelevant to evolution. These arguments can be seen torevolve around the difference in explanatory goals between explainingchange and explaining form. For this reason they are inconclusive.

19. Tremendous growth of molecular genetics in the 1990s provides muchnew developmental data. These data show far more unity within theembryological creation of organic form than the boldest structuralists hadexpected. Evodevo is generated. Nevertheless, the explanatory contrastswith Synthesis evolutionary theory remain.

20. Amundson begins to write his revisionist history of evolutionary theory,offering an alternative to SH and attempting to understand the method-ological conflicts during the twe

113219754 amundson the changing role of embryo in evolutionary thought

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