By Peter R. Grant and B. Rosemary Grant Princeton University Press, 2014 Hbk, 400pp; 44 colour photos and illustrations, 129 black-and-white illustrations ISBN 978-0-691-16046-7 - Subbuteo code M24276 - Â£34.95 Although he was more impressed with the diversity of giant tortoises among the Galapagos Islands, it is the finches with which Charles Darwin is most associated. There have been several studies of these birds: by Harry Swarth in the 1930s, David Lack in the 1940s and Robert Bowman in the early 1980s. But by far the most detailed investigations into their evolution and diversification has been made by the husband and wife team of Peter and Rosemary Grant. Much of their research was undertaken on the small island of Daphne Major, situated between the larger islands of Santa Cruz and Santiago. This island was chosen partly because it held only two species of finch - Medium Ground Finch (MGF) Geospiza major and Cactus Finch (CF) G. scandens - and the ecological differences and evolutionary divergence of these species might thus be easier to elucidate. Fortunately for the Grants (and for a generation of evolutionary biologists), the situation was complicated by droughts, exceptionally heavy rainfalls, colonisation of the island by the Large Ground Finch (LGF) G. magnirostris and the occasional presence of the Small Ground Finch (SGF) G. fuliginosa, which sometimes hybridised with MGF. Forty years and 90 scientific publications later, a picture has emerged of how strongly the environment can modify the genetic structure of finch populations, and the critical role that hybridisation and competition can play in their evolution. This book is an overview of the results of these studies. To say that it is a summary would be insulting. It draws together the most important findings, setting them against a background of evolutionary, population, quantitative and behavioural genetics, with a smattering of molecular ecology thrown in for good measure. It is not a gentle read, but the Grants make every effort to explain the scientific background to their discoveries. They started by colour-ringing almost all of the finches on the island (taking the usual biometrics) and finding nests, which allowed parent-offspring analysis to reveal a high heritability of these body traits. They confirmed that beak morphology is important in determining the speed and efficiency with which MGF can manipulate seeds of varying size and hardness. The absence of other ground-feeding finches meant that MGF was somewhat smaller than on larger islands (where it co-existed with SGF), and so on Daphne it could utilise smaller seeds that elsewhere were the preferred food of its congener. A drought came, caused by La NiÃÂ±a, and the seed crop failed: in particular the smaller seeds of grasses and annual herbs. The deep-rooted trees and cacti were less affected by the drought and continued to produce their larger, harder seed - albeit at lower quantities than normal. Larger-billed individuals within the MGF population were able to utilise this resource better than the smaller individuals. A larger proportion of the smaller birds died, and the Grants actually found the bodies of many of those that perished. The following year, the rains returned and so did the seeds. However, the genetic structure of the population had been changed and the larger size of the survivors was passed on to their progeny. Evolution by natural selection! Subsequently, the MGF morphology reverted to the pre-drought position; the reappearance of the smaller seeds gave an advantage back to the smaller-billed birds. However, when an El NiÃÂ±o followed, the rains were heavy and the grasses and herbs produced exceptionally large crops. Furthermore, many of the bushes and cacti became overgrown with vines so that they produced fewer of the larger seeds that the bigger birds utilised. The situation was thus reversed and the smaller birds survived better. To add to the excitement, there was a tiny population of SGF on the island. Typically, when these were unable to find a conspecific, they hybridised with MGF, and the progeny were physically smaller than most of the MGF population. Normally, these smaller hybrids were out-competed by their larger congeners and their survival was correspondingly low. However, during the El NiÃÂ±o, the hybrids were equally (or even more) efficient at processing the plentiful supply of grass and herb seeds and survived as well as (or even better than) their MGF cousins. Natural selection in the reverse direction! It has long been known that small, isolated populations are subject to close inbreeding, which can lead to the loss of genetic variability and the potential for inbreeding depression. Although there was evidence of this in the finches, the admixture of genetic material through hybridisation with SGF resulted in an increase in heterozygosity of MGF. But equally importantly, the SGF were largely wandering individuals from adjacent islands and by hybridising successfully they injected novel genetic variation into the MGF population. Not only did this reinforce the 'small bill genes' on the island, but it also introduced other allelic variation, which might explain why inbreeding depression was less marked than might have been expected. Another event was the establishment of a small population of LGF. These birds were superior to MGF in their ability to process large, hard seeds. The larger MGF were outcompeted and gradually their bill size decreased. There is so much more. The importance of vocalisations in species recognition. The significance of hybridisation to species formation. The evolution of isolating mechanisms and character displacement. The theoretical implications of their findings take up the final chapters. At a parochial level, the relevance of the Grants' research to the evolution of diversity among Crossbills Loxia is so obvious. This is a quite remarkable book. I wish it had been around when I was teaching evolutionary biology at Nottingham University. As I said at the outset, in places it may be a difficult read, but it is one of the most important books that I have been privileged to read for BB. Book of the Year? To my mind, absolutely. David Parkin Buy this book from the British Birds bookshop, which is run by Subbuteo Natural History Books This means that 5% of all sales generated by British Birds subscribers, whether it is books reviewed in the journal, featured on its book page or listed on the Subbuteo website, will be paid to British Birds - and will directly support the production of the journal.