Darwin's Finches: Readings in the Evolution of a Scientific Paradigm

by Kathleen Donohue

Overview

Two species come to mind when one thinks of the Galapagos Islands—the giant tortoises and Darwin’s fabled finches. While not as immediately captivating as the tortoises, these little brown songbirds and their beaks have become one of the most familiar and charismatic research systems in biology, providing generations of natural historians and scientists a lens through which to view the evolutionary process and its role in morphological differentiation.

            
In Darwin’s Finches, Kathleen Donohue excerpts and collects the most illuminating and scientifically significant writings on the finches of the Galapagos to teach the fundamental principles of evolutionary theory and to provide a historical record of scientific debate. Beginning with fragments of Darwin’s Galapagos field notes and subsequent correspondence, and moving through the writings of such famed field biologists as David Lack and Peter and Rosemary Grant, the collection demonstrates how scientific processes have changed over time, how different branches of biology relate to one another, and how they all relate to evolution. As Donohue notes, practicing science today is like entering a conversation that has been in progress for a long, long time. Her book provides the history of that conversation and an invitation to join in. Students of both evolutionary biology and history of science will appreciate this compilation of historical and contemporary readings and will especially value Donohue’s enlightening commentary.

Product Details

ISBN-13:	9780226157719
Publisher:	University of Chicago Press
Publication date:	06/15/2011
Pages:	512
Product dimensions:	6.00(w) x 8.90(h) x 1.20(d)

About the Author

Kathleen Donohue is associate professor of biology at Duke University.

Read an Excerpt

Darwin's Finches

READINGS IN THE EVOLUTION OF A SCIENTIFIC PARADIGM

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The University of Chicago Press

Copyright © 2011 The University of Chicago
All right reserved.
ISBN: 978-0-226-15771-9

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Chapter One

Grounding a Legend

We've probably all heard the legend of Darwin and his finches: Darwin sails into the Galápagos, notices a little bird, then another that looks like it but doesn't look like it because it has a huge bill and is cracking huge seeds with it. Then he sails off to another island, sees yet another bird, looking like the first two but not looking like the first two because this one is probing cactus flowers with its straight, pointy bill. Then he sails off to another island, sees another bird looking like the others, but not looking like the others, because this one, astonishingly, has picked up a small twig and is probing the holes and crevices in the trees looking to evict for itself a larval snack. There on the spot, or perhaps after some quiet time examining his neatly organized and meticulously labeled specimens, or perhaps in conversation with the Beagle's Captain Fitzroy, Darwin realizes he is looking at the perfect evidence for his revolutionary theory of Evolution by Natural Selection. In all versions of the myth, one assumption is salient: the evidence the birds present, particularly their beaks, was the most important source of inspiration for Darwin's ideas, and none was more important to the development of his theory of natural selection. That's the legend as presented in several elementary biology textbooks from earlier decades. We do love a good story.

In myth and legend birds talk to people. They may even say, "I'm very closely related to that other bird but morphologically and behaviorally differentiated in a most fascinating manner." But in real life one needs time and patience to watch the birds eat, sing, court, raise young, and die. If all the birds behave alike at first glance, we must stay for another six months until vegetation is scarcer, or more abundant, and keep watching. If we are in the right spot at the right time in the right weather, we may glimpse a woodpecker finch, and if we are lucky, we may see it pick up a twig. If we are very lucky, we'll see it poke the twig into a hole and a grub come squirming out to be picked off for lunch. Then we'll be happily excited and think: "That bird just poked with a stick, and a grub came running out, and the bird ate it!" And after the first delight at witnessing any odd action at all, we may realize that the bird just used a tool. And this excitement may get us thinking.

The legend edits out these intervening steps—the learning—in favor of creating another mythical creature—The Scientist. By doing so, it makes Darwin's conclusions and insights that much more inaccessible. While making a dramatic read, such legends can be a rather demoralizing form of pedagogy. In truth, Darwin's most important ideas were less the product of sudden inspiration than of slow gestation, and his crucial insights came long after he left the islands, not to be published until two decades after his voyage. The readings included in this chapter disclose in Darwin's own words the chronology of his reflections on the finches and how these birds informed his major theories. The readings depict his first encounters with the finches, then follow his references to them in his notes and letters and into his published works, early and late. Dealing a final blow to the legend of epiphany in the voices of finches is Frank Sulloway's engaging detective work. His article, in which he hunts along the threads of a richer narrative, chasing bird skin after bird skin, synthesizes Darwin's notes, labels, and correspondence into a coherent narrative of his changing interpretations of the Galápagos finches and how they figured in his major theories. Together, the excerpts in Darwin's words and Sulloway's article give a detailed picture of how Darwin initially collected his finches—not in any very organized manner; what he initially thought of them—not very much; and two developments that changed his mind about them—John Gould's classification of the birds into distinct species as opposed to varieties, and new discoveries of interisland variation in taxa other than the finches.

READINGS

To begin are Darwin's earliest impressions of the finches, formed while he collected them and shortly thereafter. They include brief passages from his "Beagle Diary," field notebooks, and his "Ornithological Notes," which, according to Nora Barlow, were probably written aboard the Beagle between early 1834 and sometime prior to March 1837. Excerpts from the "Ornithological Notes" presented here include a list of Darwin's collected specimens (including the misidentified "wren," which John Gould later classified as Certhidea olivacea, the warbler finch), and some observations on the finches' appearance and habits. Following these is a brief communication to the Zoological Society of London. All references to the finches here are conspicuously brief. As a hint of future interest, however, Darwin casually mentions in a letter written in Sydney in January 1836, to his mentor, J. S. Henslow, that while in the Galápagos he "paid also much attention to the Birds, which I suspect are very curious."

It becomes clear somewhat later that the mockingbirds, not the finches, alerted Darwin to the possibility that species vary from island to island. At the time Darwin was collecting his specimens, he did not recognize these interisland differences, and in his Journal of Researches he publicly bemoans his failure to heed the fine-scale variations among the island taxa. His new preoccupation with interisland variation is reflected also in his repeated pestering of Henslow, one such letter being included here, and his inquiries to J. D. Hooker about whether the plants also indicate that closely related species are found on different islands. Indeed, he had guessed as much and was rewarded by Hooker's affirmation and insightful interpretation of the finding: "I was quite prepared to see the extraordinary difference between the plants of the separate Islands from your journal, a most strange fact, & one which quite overturns all our preconceived notions of species radiating from a centre & migrating to any extent from one focus of greater development" (Hooker 1844). Interisland differences among taxa were a preoccupation of Darwin and his colleagues because they challenged the notion of a restricted number of locations for creation (to be discussed in chapter 3).

Darwin's cogitations on how differences among taxa arise and are maintained are inextricably tied to his analysis of the geographic distribution of taxa. Next are excerpts from Darwin's notebooks on the transmutation of species, thought to be composed upon returning from his voyage in 1836. These notes reveal some of Darwin's earlier thoughts on how geographic isolation maintains differences that would otherwise disappear through interbreeding. Rather than focusing on interisland variation, the first excerpt shows the beginnings of his analysis of the relationship between continental and island affinities and differences, which was to figure so prominently in his major works. The remaining excerpts from these notebooks elaborate upon the causes of the changes that occur in isolation, as well as causes of the isolation itself. In particular, he focuses on dispersal and isolation, in contrast to divine creation, as causes of the geographic patterns of affinity and difference among taxa—the "horizontal history" that he was able to read so fluently from the spatial distribution of species. This notion of "horizontal history" marks a major insight, as it reveals Darwin's historical (eventually genealogical) interpretation of spatial patterns of distribution. While the Galápagos biota are mentioned several times, the finches do not figure in these notes.

The year before the second edition of his Journal of Researches, Darwin composed a 250-page essay (his 1844 Essay) in which he described the process of "a natural means of selection." An excerpt from this essay appears next. Here Darwin gives considerable attention to the geographic distribution of variation among taxa and postulates geographic isolation as a central factor in the accumulation of species differences. He cites affinities and differences between the Galápagos and the American mainland as evidence, but now he also cites smaller differences between closely related species from different islands, appearing to recognize these two patterns as reflections of the same process on different scales. Linking these spatial scales was major progress toward considering evolution to be a continuous process. In this narrative of the evolution of species differences, Darwin masterfully describes the role of isolation first as it occurs on oceanic islands, and then as it occurs on continents. In this manner he spells out explicitly how evolution as observed on islands pertains to the whole of the globe.

This same year, Darwin sent his "confession" to Hooker in his beliefs in transmutation, included as the next selection, crediting the Galápagos for his insight into the transmutation of species. Apparently, he had become comfortable enough with his theory by this time to discuss it with other correspondents, including Leonard Jenyns, a letter to whom is included here. Significantly, though, in this letter he tempers his theory of species transmutation via the differential death of individuals by describing it as one of the "intermediate laws" of creation.

Differences between the 1837 and 1845 editions of his Journal of Researches are revealing, so the Galápagos chapters from both editions are presented here in tandem, the later in excerpted form. By 1837 Darwin had been enlightened in much of his thinking about the finches by John Gould, who by that time had published his description of the finches and other birds of the Galápagos. Darwin knew that, according to the most eminent ornithologist of his time, the finches represented good species, rather than being, as he had earlier thought, "only varieties." He knew also that they were indeed very closely related, though vastly different in morphological characters that typically define genera—that is, their bill morphology. Between the two editions not only had Hooker completed an initial analysis of the plants of the Galápagos, but, as his notebooks reveal, Darwin had been struggling also with the interpretation of these patterns. What he chose to add and change in the 1845 edition shows the progression of his thoughts and his timid sharing of this progress with a public audience. By then, the Galápagos finches had impressed Darwin enough that he included another plate—at his own expense—depicting the heads of four of them.

When Darwin discusses the Galápagos in his major published works, however, his emphasis is on the overall relation of the biota of the Galápagos with that of South America—not the interisland variation. Although the Galápagos make it into his stunning Origin of the Species, the finches do not, nor do they make it into any of his other major works.

Darwin's later publications, correspondence, and memoirs highlight the Galápagos flora and fauna as influential to his thinking about the transmutation of species. When writing to his various correspondents, he repeatedly invoked the Galápagos as a source of the crystallization of his ideas on the transmutation of species. For example, in reference to "the law of succession"—the observation that fossil organisms resemble, but are not identical to, extant ones—he writes: "In fact this law with the Galapagos Distribution first turned my mind on origin of species" (C. Darwin 1859) and "It would have been a strange fact if I had overlooked the importance of isolation, seeing that it was such cases as that of the Galapagos Archipelago, which chiefly led me to study the origin of species" (Charles R. Darwin to Moritz Wagner, Down, October 13, 1876).

This tendency to refer to the Galápagos as the source of his insights is reflected in his "little history of the Origin" that he sent to Ernst Haeckel and in his autobiography (cited in F. Darwin 1909), excerpts of which are included. These publications and communications contribute in their own way to the legend that the Galápagos species held a special and rather immediate place in the development of his theory of evolution by natural selection, but, again, they contain no specific mention of the finches.

In fact, Darwin himself never used the Galápagos finches as a case example for any of his major principles of the transmutation of species by natural selection. Instead, he frequently cited the more general pattern of the Galápagos flora and fauna resembling the South American biota as central to his theories. This larger pattern illustrated affinity through descent and divergence from an ancestral stock—his major message. The finer pattern of interisland variation was suggestive of the ubiquity of the transmutation process itself, as he implies in his 1844 Essay: indispensable for insight, but far more difficult to show definitively. Sulloway speculates that the finches never figured in Darwin's discussion because of the incomplete information he had on their geographic distribution and the guesswork involved in assigning the specimens to different islands. No doubt this is true, but the fine variation among the taxa, and the difficulty of their classification into species versus varieties, was problematic in itself. Darwin simply didn't have the answers to explain how interspecific differences among the finches arose and were maintained.

Why then have the finches become such an enduringly important example of the evolutionary process? In part because they surprised even Darwin and challenged him directly. A letter from Darwin to Osbert Salvin in 1875, included last, expresses Darwin's surprise that the finches from different islands do not vary in the manner he predicted, with a single species isolated on each island. Yet, he says, studying the birds "on the spot" would "throw a flood of light upon variation." Still very much unanswered for Darwin is how species differences in these finches are maintained at such close quarters. Enough said. If these finches can illuminate the problem that stumped the master, then they are worth the trouble we take to study them.

NOTE ON TRANSCRIPTIONS

I have omitted strike-throughs and line breaks from the original transcriptions to facilitate reading. I have also inserted periods in place of transcriptional dashes where it seemed there was an end to the sentence or thought. Transcriptional corrections were also incorporated without brackets. The original transcriptions, without these molestations, can be obtained from the sources cited.

Many of these readings are available online on the Web site of the University of Cambridge's "Complete Works of Charles Darwin Online" (http:// darwin-online.org.uk/) and are reproduced with their permission, as indicated. All correspondence is also available on the Web site of the University of Cambridge's "Darwin's Correspondence Project" (http://www.darwin project.ac.uk/).

From Darwin's Beagle Diary (1831–1836)

* CHARLES R. DARWIN

* 1835 Sept: 17th [page 606]

* Galapagos Isds

The birds are Strangers to Man & think him as innocent as their countrymen the huge Tortoises. Little birds within 3 & four feet, quietly hopped about the Bushes & were not frightened by stones being thrown at them. Mr King killed one with his hat & I pushed off a branch with the end of my gun a large Hawk.

* Oct 1st [page 614]

Since leaving the last Island, owing to the small quantity of water on board, only half allowance of water has been served out (ie 1/2 a Gallon for cooking & all purposes). This under the line with a Vertical sun is a sad drawback to the few comforts which a Ship possesses. From different accounts, we had hoped to have found water here. To our disappointment the little pits in the Sandstone contained scarcely a Gallon & that not good, it was however sufficient to draw together all the little birds in the country. Doves & Finches swarmed round its margin. I was reminded of the manner in which I saw at Charles Isd a boy procuring dinner for his family. Sitting by the side of the Well with a long stick in his hand, as the doves came to drink he killed as many as he wanted & in half an hour collected them together & carried them to the house.

(Continues...)

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Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Acknowledgments

General Introduction

Part 1 The Finches in Place 1

1 Grounding a Legend 3

From Darwin's Beagle Diary (1831-1836) Charles R. Darwin 8

From The Galapagos Notebook (1835) Charles R. Darwin 9

From Darwin's Ornithological Notes (1835) Charles R. Darwin 9

Remarks upon the Habits of the Genera Geospiza, Camarhynchus, Cactornis and Certhidea of Gould (1837) Charles R. Darwin 11

Letter to John S. Henslow (1838) Charles R. Darwin 12

From Notebook [B] on "Transmutation of Species" (1837-1838) Charles R. Darwin 13

From Notebook [C] on "Transmutation of Species"(1838) Charles R. Darwin 16

From The Foundations of "The Origin of Species," Two Essays Written in 1842 and 1844 (1909) Francis Darwin 16

Letter to Joseph D. Hooker (1844) Charles R. Darwin 24

Letter to Leonard Jenyns (1844) Charles R. Darwin 26

From Journal of Researches into the Geology and Natural History of the Various 28

Countries Visited by H.M.S. Beagle (1839) Charles R. Darwin

From The Voyage of the Beagle (1845) Charles R. Darwin 45

Letter to Ernst P. A. Haeckel (1864) Charles R. Darwin 53

Letter to Osbert Salvin (1875) Charles R. Darwin 53

Darwin and His Finches: The Evolution of a Legend (1982) Frank J. Sulloway 55

2 Place: Historical Expeditions to the Galápagos 98

Time Line of Visits to the Galápagos Archipelago 101

Letter to Emperor Carlos V of Spain (1535) Fray Tomás 108

From A New Voyage round the World (1683-1688) William Dampier 111

From The Encantadas (1856) Herman Melville 116

From Innocence in the Galapagos: Voyage to a Time Machine in the Pacific (1975) Annie Dillard 120

3 Land: A Thousand Accidents 133

The Galapagos Islands George Baur 138

From On the Avifauna of the Galapagos Archipelago (1876) Osbert Salvin 141

Drowned Islands Downstream from the Galapagos Hotspot Imply Extended Speciation Times (1992) David M. Christie Robert A. Duncan Alexander R. McBirney Mark A. Richards William M. White Karen S. Harpp Christopher G. Fox 142

4 A Confusion of Finches 149

Table 1 Classification of Darwin's Finches over Time from Gould to the Present 154

Table 2 Lability of Genus Designations of Darwin's Finches over Time 157

From The Zoology of the H.M.S. Beagle (1841) Charles R. Darwin 158

From On the Avifauna of the Galapagos Archipelago (1876) Osbert Salvin 167

From Birds of the Galapagos Archipelago (1896) Robert Ridgway 168

The Finches of the Galapagos in Relation to Darwin's Conception of Species (1936) Percy R. Lowe 176

From Darwin's Finches (1947), Chapters 2 and 11 David Lack 187

From Comparative Landscape Genetics and Adaptive Radiation of Darwin's Finches: The Role of Peripheral Isolation (2005) Kenneth Petren Peter R. Grant B. Rosemary Grant Lukas F. Keller 203

Part 2 Adaptation and the Evolution of Diversity 207

5 What Matters? Variation and Adaptation 209

The Relation of the Food to the Size and Shape of the Bill in the Galapagos Genus Geospiza (1902) Robert E. Snodgrass 212

Evolution of the Galapagos Finches (1940) David Lack 227

6 Diversity as Adaptation 233

From Darwin's Finches (1947), Chapters 6-8 David Lack 237

From Morphological Differentiation and Adaptation in the Galapagos Finches (1961) Robert I. Bowman 258

Ecological Character Displacement in Darwin's Finches (1985) Dolph Schluter Trevor Price Peter R. Grant 272

7 Darwin's Finches as a Case Study of Natural Selection 281

Intense Natural Selection in a Population of Darwin's Finches (Geospizinae) in the Galapagos (1981) Peter T. Boag and Peter R. Grant 286

Unpredictable Evolution in a 30-Year Study of Darwin's Finches (2002) Peter R. Grant and B. Rosemary Grant 293

Evolution of Character Displacement in Darwin's Finches (2006) Peter R. Grant and B. Rosemary Grant 306

8 Sexual Selection in Darwin's Finches 314

From The Evolution of Song in Darwin's Finches (1983) Robert I. Bowman 317

Correlated Evolution of Morphology and Vocal Signal Structure in Darwin's Finches (2001) Jeffrey Podos 339

Part 3 The Origin and Maintenance of Species 349

9 Microevolution and Macroevolution: Does One Explain the Other? 351

From Darwin's Finches (1947), Chapter 14 David Lack 356

Morphological and Phylogenetic Relations among the Darwin's Finches (1984) Dolph Schluter 361

10 The Evolution of Reproductive Isolation 375

Reproductive Isolation of Sympatric Morphs in a Population of Darwin's Finches (2007) Sarah K. Huber Luis Fernando De León Andrew P. Hendry Eldredge Bermingham Jeffrey Podos 378

High Survival of Darwin's Finch Hybrids: Effects of Beak Morphology and Diets (1996) B. Rosemary Grant and Peter R. Grant 389

From Darwin's Finches (1947), Chapter 15 David Lack 408

11 Hybridization 415

Hybridization in the Recent Past (2005) Peter R. Grant B. Rosemary Grant Kenneth Petren 418

The Secondary Contact Phase of Allopatric Speciation in Darwin's Finches (2009) Peter R. Grant and B. Rosemary Grant 439

12 The Genetic Basis of Variation: Molecular Genetics, Development, and Evolution 459

Bmp4 and Morphological Variation of Beaks in Darwin's Finches (2004) Arhat Abzhanov Meredith Protas Peter R. Grant B. Rosemary Grant Clifford J. Tabin 463

The Calmodulin Pathway and Evolution of Elongated Beak Morphology in Darwin's Finches (2006) Arhat Abzhanov Winston P. Kuo Christine Hartmann B. Rosemary Grant Peter R. Grant Clifford J. Tabin 474

Bibliography 487

Editorial Reviews

This anthology provides a fascinating record of how a small assemblage of innocuous birds living on a remote island chain became the cornerstone for studies of adaptation and the origin of species. The papers Donohue has compiled, together with her descriptions of their biological and historical importance, give us an unusually clear picture of the evolution of species and of science itself.”

Douglas W. Schemske

The finches of the Galapagos are an iconic example of evolution in action, even more important to the research of scientists today than they were to the development of Darwin’s ideas.   Kathleen Donohue has done a wonderful job in bringing together historical documents on the finches with contemporary papers from the cutting edge of research, illustrating how science progresses and how modern evolutionary biology is a synthetic, interdisciplinary field.  Interspersed with her own insightful commentary, Donohue has provided a great service to both students and scholars.”

Jonathan Losos

Biologists, historians, and anyone with an interest in Darwin’s finches and their place in evolutionary biology and ecology will appreciate this wonderful compendium.

Quarterly Review of Biology - Frederick R. Davis

Darwin’s finches are famous for what they have revealed about evolution. Understanding of how, when, and why they evolved has increased enormously since Darwin’s visit to the Galapagos in 1835. Kathleen Donohue has creatively helped the reader to learn how this happened by assembling an extensive and fascinating collection of writings spanning more than 150 years. The result is a treasure, a stimulating history for the biologist and biology for the historian.”

B. Rosemary Grant

Biologists, historians, and anyone with an interest in Darwin’s finches and their place in evolutionary biology and ecology will appreciate this wonderful compendium.

— Frederick R. Davis, Florida State University

Quarterly Review of Biology