DARWIN’S ORIGIN OF SPECIES (1859, p. 1) begins: “When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.” We followed in his footsteps 138 years after his visit to the Galápagos, with the same quest in mind. Our book describes what we learned by studying Darwin’s finches (fig. P.1) for 40 years on Daphne Major (fig. P.2), a small island in the center of the Galápagos archipelago (fig. P.3).
Genetic variation is the raw material for evolution. Although much has been learned about genetic variation from theoretical and laboratory research, long-term field studies in natural environment have been relatively neglected. Knowledge of how genetic and phenotypic variation interacts with environmental variation is fundamentally important for understanding evolution in nature. It can be achieved by long-term field studies of evolution with well-chosen organisms in well-chosen environments when coupled with the benefits of laboratory science. This is what we have attempted to do. By taking a multidimensional approach to questions of evolution and speciation, carefully documenting genetic, ecological, and behavioral factors responsible for evolution of finches across 40 years, we have made discoveries about speciation far beyond our initial expectations. And although finches are the focal organisms of the study, the principles of their evolution apply broadly to all organisms.
These are exciting times to be an evolutionary biologist. Changes in technology are proceeding at an ever-increasing pace, giving us the tools for expanding our knowledge and understanding of how evolution occurs in the natural world. In view of this it is worth reflecting on the state of knowledge in 1973 (Mayr 1970, Dobzhansky 1970), on the methods and tools available for evolutionary investigations at that time, and the transformation that studies of speciation have undergone since then. Techniques we take for granted now did not exist. Electrophoresis (Hubby and Lewontin 1966) was only just becoming widely available as a method of detecting allozyme variation if the appropriate buffers could be worked out. It required a power source, which ruled it out for most studies on uninhabited islands. Cladistics (Hennig 1966) had yet to shake the foundations of phylogenetic reconstruction and interpretation. Personal computers had yet to be invented; we were the handmaidens of university mainframes. Statistical programs for the analysis of complex data had not been invented either, and the revolution in molecular biology, including evodevo, lay far off in the future. Whole genomes were conceivable, but their sequences were not. Now we do things we could not anticipate doing at the outset; for example, we study genes, their expression patterns and regulation, and we infer from molecular data the time when events took place in the past. Our account of evolution on Daphne reflects the expanding knowledge brought about by the development of new methods as the study progressed.
Fig. P.1 The four species of finches. Upper left: Small Ground Finch, G. fuliginosa. Upper right: Medium Ground Finch, G. fortis. Lower left: Large Ground Finch, G. magnirostris. Lower right: Cactus Finch, G. scandens. From Grant and Grant 2008a).
Fig. P.2 The two Daphnes. Left: Daphne Major, with Daphne Minor (Chica) in the background (D. Parer and E. Parer-Cook). Right: Daphne Minor, 1976. Daphne Minor has been climbed once, with ropes. Two immature fortis banded on Daphne Major were seen at the top (Grant et al. 1980).
In the absence of fossils, answers to questions about speciation in the past have to be sought with living organisms by looking backward in time. Typically this is done by using information on contemporary populations to test the assumptions of historical hypotheses: a retrospective analysis. We followed this path with a combination of field studies of short duration on many islands and a decade-long study on Genovesa. We concentrated almost entirely on the six ground finch species in the genus Geospiza because they are distributed widely in the archipelago, generally abundant, and easy to observe (Lack 1947). We wrote three books on our findings: two on the total finch radiation (Grant 1986 [1999], Grant and Grant 2008a) and one on the finch populations on Genovesa (Grant and Grant 1989a).
Here we describe what we have found by adopting a different approach to questions of speciation: studying populations through time, a prospective analysis. This provides insights into the process of speciation even if the progress we observe toward complete reproductive isolation is small. The present volume completes our program of converting research results into more accessible and synthetic book form, with new and unanticipated insights into Darwin’s question on the origin of species.
We have designed this book for students, educators, and others to read for enjoyment and inspiration. Throughout we have tried to express our views in jargon-free language, with technical terms given only where necessary. We have adopted a few conventions. First, we have given numerical results of statistical tests only when not given before in the original papers we cite. Second, we have supplied details of methods of study and analysis in boxes and not in the main text. Third, we have used appendixes for some background information and discussion of topics that are relevant to the material in chapters but not central to it. In some cases these discussions have been supplemented with numbers in tables and figures.
Acknowledgements. Our research has been supported financially by the National Science and Engineering Research Council (Canada), National Science Foundation (USA), and McGill and Princeton Universities (Class of 1877 Fund), and logistically by the Charles Darwin Foundation and the Galápagos National Parks Service. More than twenty assistants helped us in fieldwork. Foremost among them were Ian and Lynette Abbott as postdoctoral fellows, and Peter Boag, Laurene Ratcliffe, Trevor Price, and Lisle Gibbs as PhD graduate students. They and we were helped by Phillip de Maynadier, James Gibbs, Lukas Keller, Greg Keys, Steve Latta, Irby Lovette, David McCullough, Stephen Millington, David Moore, Ken Petit, Ken Petren, Bob Podolsky, Wally Rendell, Gerry Retzlaf, Uli Reyer, Dan Rosenberg, Keith Tarvin, Ayse Unal, Carlos Valle, Jonathan Weiland, David Wiggins, and our two daughters, Nicola and Thalia. We thank Lisle Gibbs for reading chapter 7, and Trevor Price, Dolph Schluter, and an anonymous reviewer for reading previous drafts and offering numerous suggestions for improvement. They helped us write a significantly better book. For transforming the potential into the realized book we thank the editorial staff at Princeton University Press: Alison Kalett and Quinn Fusting for guidance in the early stages, Mark Bellis for shepherding it through production, and Dimitri Karetnikov for giving us the benefit of his artistic skills.
Data used in constructing the figures have been deposited in Dryad (doi:10.5061/dryad.g6g3h). Two educational films illustrate features of the book. One, made by the Howard Hughes Medical Institute for high schools, illustrates natural selection on Daphne. It is entitled The Origin of Species: The Beak of the Finch (www.biointeractive.org). We are preparing another that is designed to illustrate all the main topics of the book.
February 2013
