The Galapagos Islands are justly famous for their beauty and extraordinary wildlife. They are also bound up in the public imagination with the story of Charles Darwin's voyage in HMS Beagle that inspired his theory of evolution by natural selection. When he visited the islands in 1835, he found the wildlife so unaccustomed to people that he was able to push a hawk off a branch with the muzzle of his gun. He took advantage of this tameness to amass a large collection of specimens, including some rather drab birds that he provisionally divided into separate subfamilies because of their range of beak sizes and shapes. He paid little attention to differences among islands, even when a resident pointed out that tortoises from each island had characteristic shells.
The significance of the small birds, now known as Darwin's finches, became apparent only when John Gould of the British Museum catalogued the Beagle expedition specimens. Struggling to sort out Darwin's inadequate labelling, Gould concluded that the finches all belonged to a single, related group with distinct species present on different islands. Now Darwin realised their significance: they had apparently diverged from a single stock, adapting to occupy a range of ecological niches, and evolution had been facilitated by the isolation between islands.
The story was taken up by the gifted ecologist David Lack, who visited the Galapagos in 1938-39 and published Darwin's Finches in 1947. His hypothesis was that a group of finches colonised the islands from the South American continent and diversified into 14 extant species. Divergence, he argued, was driven by competition for food, which led to adaptations in beak morphology that increased the range of resources the birds could exploit.
Peter and Rosemary Grant have spent the past 34 years in an exceptionally detailed dissection of Lack's proposals. The result is one of the most compelling documentations of the operation of natural selection. In many respects Lack has been proved right but the story is now much richer. For example, the evolution of beak morphology fluctuates with changes in food supply driven by El Niño climatic oscillations, the developmental basis of beak morphology is being unravelled, hybridisation is now known to play a role in maintaining the supply of genetic variation, and song learning has been shown to play an essential part in the origin of reproductive isolation between finch species.
In this book, the Grants aim to capture the key insights provided by Darwin's finches into mechanisms of adaptation and speciation generally. They succeed in making a complex topic accessible without losing the excitement inherent in tackling a difficult problem. They are sometimes provocative. There is enough depth to stimulate serious students of evolutionary biology, enough explanation for general readers and an approachable style that will please both.
Some have suggested that Darwin's finches should be renamed Lack's finches in honour of his pioneering work. Readers of this book will wonder whether they should actually be called Grants' finches.
How and Why Species Multiply: The Radiation of Darwin's Finches
Author - Peter R. Grant and B. Rosemary Grant
Publisher - Princeton University Press
Pages - 2
Price - £19.95
ISBN - 9780691133607
Published - 9 January 2008