RANDOM DRIFT: CHANCE AND EXPLANATION IN EVOLUTIONARY BIOLOGY by Adam Goldstein A dissertation submitted to The Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy. Baltimore, Maryland March, 2006 c Adam Goldstein 2006 All rights reserved Abstract The central claim for which I argue in this dissertation is that there are important phenomena that occur by random drift that evolutionary biologists explain using a strategy I term “process explanation.” This claim puts me at odds with an influential view about the nature of explanation that I term “Hempelianism.” Hempelianism is the view that the scientific explanation of a particular event E requires (a) showing that E was to be expected, or indicating the degree to which it would have been rational to expect E’s occurrence; and (b) laws of nature. My central claim entails that both (a) and (b) are false. A process explanation consists of a narrative describing events causally relevant to the event to be explained. These narratives need not contain laws, show that the event to explained ought to have been expected, or indicate the degree to which it would have been rational to expect the event. My position about random drift also puts me at odds with evolutionists who, influenced by Hempelianism, claim that only natural selection can explain evolution. In my argument, I articulate the strategy of process explanation and defend it against Hempelian critics; describe a mechanism of random drift known as “indiscriminate ii sampling;” and describe process explanations of phenomena of drift that occur by indiscriminate sampling. Advisor: Dr. Peter Achinstein. iii Acknowledgements I would like to warmly extend my thanks to my wife Abigail, who—in contrast with the chance fluctuations in the fortunes of this project, which she endured with grace and magnanimity—steadily provided me with encouragement and support. Additionally, I would like to acknowledge my graduate advisors, Karen Neander, Peter Achinstein, and Steven Stanley; Alexander Rosenberg of Duke University, who served as a committee mem- ber; my teachers at Johns Hopkins, Stephen Barker, George Wilson, Maura Tumulty, and Susan Wolf; the late David Sachs, for his endowment of a fellowship at Johns Hopkins, from which I benefited; former graduate students at Johns Hopkins Richard Richards, Kent Sta- ley, and Chuck Ward; my undergraduate teachers David Hoy, W. E. Abraham, L´eo Laporte, and Todd Newberry; and my colleagues at Drew University, Seung-Kee Lee, Erik Anderson, and Thomas Magnell. The Department of Philosophy at Johns Hopkins provided me with financial support and with a rich and challenging environment for research. The graduate school and university as a whole also deserve acknowledgement for these reasons. Much of this dissertation was written in the Rose Main Reading Room of the New York Public Library, and I would like to thank the staff there for research support, and for providing a quiet and dignified place in which to write. Sara Harrington of the Rutgers University iv Art Library provided facilities for scanning and editing the images reproduced in Appendix A. The open-source computing community made a variety of first-class software packages freely available, including LATEX, which I used, together with BibDesk and TeXShop for Mac OS X, to write and typeset the dissertation. v Contents Abstract ii Acknowledgements iv List of Tables viii List of Figures ix 1 Chance, Explanation, and Narrative in Evolutionary Biology 1 1.1 What Is Random Drift? ............................. 3 1.1.1 A look at the literature ......................... 3 1.1.2 The process of drift ........................... 6 1.1.3 Purpose and accident in evolution ................... 13 1.2 Process Explanation in Evolutionary Biology ................. 19 1.2.1 Hempelianism and the Hempelian evolutionists ............ 19 1.2.2 Narrative explanations of evolution ................... 27 1.3 Chapter Summary and Overview of Chapters ................. 33 1.3.1 Chapter summary ............................ 33 1.3.2 Overview of chapters ........................... 34 2 The Nature of Process Explanation 36 2.1 Hempelianism and the Hempelians ....................... 38 2.1.1 Hempelianism ............................... 39 2.1.2 Salmon .................................. 44 2.1.3 Railton .................................. 51 2.2 The Nature of Process Explanation ....................... 54 2.3 Explaining Processes ............................... 66 3 A Defense of Process Explanation and Contextualism 67 3.1 The Incompleteness Objection .......................... 68 3.2 Exorcising the Laplacian Demon ........................ 87 3.3 Against Universalism ............................... 104 3.4 Process Explanation Vindicated ......................... 118 vi 4 The Probability Account of Indiscriminate Sampling 119 4.1 Background to Indiscriminate Sampling .................... 121 4.2 Indiscriminate Parent Sampling ......................... 127 4.2.1 Essential background ........................... 127 4.2.2 The core probabilistic equality ..................... 130 4.2.3 A further condition ............................ 143 4.3 Indiscriminate Gamete Sampling ........................ 148 4.3.1 Essential background ........................... 148 4.3.2 The core probabilistic equality ..................... 149 4.3.3 A further condition ............................ 159 4.4 Indiscriminate Sampling and Evolution ..................... 163 4.4.1 A pluralistic view of drift ........................ 164 4.4.2 Matthen and Ariew’s hierarchical realization model .......... 167 4.5 Concluding Remarks ............................... 195 5 Process Explanations of Drift 198 5.1 The Hempelian Evolutionists .......................... 200 5.1.1 The exclusivity thesis .......................... 200 5.1.2 Dennett .................................. 202 5.1.3 Dawkins .................................. 206 5.1.4 The hierarchical-realization model ................... 212 5.2 Explaining Drift Independently of N ...................... 215 5.2.1 The chance elimination of rare but favorable alleles .......... 216 5.2.2 The irreducibility thesis ......................... 221 5.2.3 Molecular evolution ........................... 232 5.3 Explaining Drift by Reference to N ....................... 236 5.3.1 Drift in small populations ........................ 237 5.3.2 The shifting balance process ....................... 239 5.3.3 The origin of species ........................... 247 5.3.4 The shape of phylogeny and punctuated equilibrium ......... 255 5.4 Explaining Drift by Process Explanation .................... 263 6 Chance and Explanation in Evolutionary Biology 264 Appendices 275 A Images of Random Drift 275 B Pluralism About Drift 282 Bibliography 287 Vita 304 vii List of Tables 4.1 Example of correlated variants ......................... 134 4.2 Example of correlated alleles .......................... 155 B.1 Example of fitness relationships in fluctuating environments ......... 283 viii List of Figures 5.1 Selection, h = 0.5, s = 0.1 ............................ 218 5.2 Wright’s adaptive landscape and the shifting balance process ........ 243 5.3 Representation of phylogenetic patterns in two dimensions .......... 256 5.4 Contrasting images of the shape of phylogeny ................. 258 A.1 Allele frequencies evolving by drift ....................... 276 A.2 Probability distribution of genotypes. ...................... 279 ix Chapter 1 Chance, Explanation, and Narrative in Evolutionary Biology This dissertation concerns a process of evolution known as random genetic drift, also known as “random drift” or just “drift.” Drift is nonadaptive, and may be thought of as a process of evolution by chance or accident. In this regard, it differs from natural selection, which is adaptive, and may be thought of as purposive. The main claim for which I argue in this dissertation is that there are important phenomena that occur by drift that evolutionary biologists explain using a strategy that I term “process explanation.” By claiming that there are important phenomena that occur by drift that evolu- tionary biologists explain, I disagree with an influential and articulate group of evolutionists who adhere to what I term “the exclusivity thesis.” The exclusivity thesis is the view that natural selection alone explains evolution—a view that entails that phenomena occurring by drift cannot be explained. The resolution of this disagreement does not depend on scientific 1 theory and observation alone; rather, it depends in part on the resolution of an important conceptual issue about the nature of explanation. This conceptual issue is the nature and justification of process explanation, the strategy that (as I state above) I claim that evolutionary biologists use to explain phenomena that occur by drift. Process explanation, which I elaborate and defend in the dissertation, is incompatible with a widely-held view about explanation. I term this view “Hempelianism” to reflect its close association with the philosopher Carl Hempel. Hempelianism is the view that explaining a particular event E requires showing why E occurred by citing laws of nature. In contrast, a process explanation of a particular event E derives its force from a narrative describing E’s causes; such a narrative describes how E occurred, and does not require any laws of nature. This disagreement with the Hempelians reflects a deeper disagreement about the nature of explanation. Hempelianism entails what