How and Why Species Multiply:The Radiation of Darwin’s Finches is the second volume in the Princeton Series in Evolutionary Biology and fully earns its place in that series.
The story of Darwin’s Finches has been chronicled in many places, beginning with Darwin’s Voyage of the Beagle and Gould’s 1837 publication on them in the Proceedings of the Zoological Society of London. In the years following Darwin the finches were discussed by Swarth, Stresemann, Lack, and Bowman, all of whom, in one form or another, contributed to our understanding of the finches. Large parts of this research have been ably recounted in Jonathan Weiner’s The Beak of the Finch and in Peter Grant’s Ecology and Evolution of Darwin’s Finches.
How and Why Species Multiply:The Radiation of Darwin’s Finches by Peter and Rosemary Grant brings together all this work in one short, compelling, examination of the speciation process undergone by the finches. As the Grants explain in the first chapter:
We choose a single group of related species for close scrutiny, and attempt to answer the following questions: where did they come from, how did they diversify, what caused them to diversify as much as they did (and no more), and over what period of time did this happen?
To answer this question they draw on a broad range of modern scientific disciplines including: geography, animal behavior, ecology and genetics. The result is a mini textbook in evolutionary theory (that reads like a gripping, page turning mystery) that is a powerful demonstration of the ability of evolutionary theory to explain biodiversity (both past and present).
Chapters Two and Three lay out some of the theoretical background for the book. Chapter Two is devoted to a discussion of a phylogeny of the finches based on molecular data. Chapter Three looks at various theoretical issues related to speciation. The next seven chapters measure the data accumulated through some thirty years of study against the theoretical considerations laid out in the first two chapters. The next three chapters examine adaptive radiations from around the world (cichlid fish in Africa, Hawian Honeycreepers, etc., unfortunately, this piranha study came out too late to be included) in light of what has been learned from studying the adaptive radiation of the finches. Perhaps one of their most important conclusions comes early on in the book when they say:
Not all the contemporary ecological niches were available when the ancestors arrived. The Galapagos islands have undergone substantial change since then. The number of islands has increased; their sizes, elevations, and distances from each other have changed as a result of fluctuating sea level; climate has changed from warm, wet and permanent El Niño-like conditions to generally cooler and drier conditions with seasonal and annual fluctuations. Therefore the drama of the finch radiation did not unfold in a single act with unchanging scenery on the Galapagos stage, but in a dynamic environment: numbers and types of opportunities for finch evolution increased as the number of islands increased and the food of the finches – plants and the arthropods that feed on them, and on each other – increased in diversity and changed in distribution.
The Grants go on to point out that the adaptive radiation of the finches occurred in several phases and based on this, as well as the comparative material mentioned above, argue that adaptive radiations occur in three phases. In the first phase, the one Darwin’s finches are in, speciation:
…proceeds with ecological divergence of populations under natural selection in allopatry and the subsequent establishment of sympatry.
Pre-mating blocks to reproduction (such as divergence in mating songs) evolve, and there is some hybridization and backcrossing. The second stage is characterized by the origination of partial but not complete genetic incompatibility and post-mating reproductive isolation and the subsequent decay of hybrid viability. The third stage is characterized by the complete loss of fertility in hybrids. One wrinkle to the scheme is that not all species in the radiation go through these phases at the same time and the ultimate fate of the adaptive radiation is determined by the rates of speciation (predominates early in the radiation) and extinction (predominates late in the radiation). Older radiations are characterized by having only one or two species per genus and they may even be replaced by radiations from another group.
The final chapter talks about future directions for research. Particularly in the area of the ecological history of the Galapagos, refining the phylogeny of the finches (especially in the area of their relationships to finches on the mainland and in the Caribbean), the interbreeding potential of allopatric populations, and limits to the rate of speciation among the finches.
Overall, the book is a fascinating discussion of Darwin’s finches that sheds interesting light on the process of adaptive radiations. It is a must read for anyone interested in the topic of evolution.