Journal of Heredity Advance Access originally published online on February 24, 2005
Journal of Heredity 2005 96(4):470-471; doi:10.1093/jhered/esi047
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Book Review |
Speciation
Jerry A. Coyne and H. Allen Orr.Sinauer Associates, Sunderland, Massachusetts. 2004. 545 pp. $89.95.
What is a species? How do new species arise? How central is the role of natural selection in the speciation process? How quickly can new species appear, and how long can species last without changing? Ever since Darwin, biologists have wrestled with these questions. Now, with our ability to examine entire genomes and to manipulate the genes that are actually involved in speciation, we are beginning to approach some firm answers. Jerry Coyne and Allen Orr have been at the center of this new movement. Their thorough and thoughtful review of speciation ranges over the entire field and examines it dispassionately. Theirs is a remarkable work of synthesis, and it belongs on every biologist's bookshelf. This is because it goes far beyond most surveys of speciation and examines the work of thousands of scientists in depth. The resulting juxtaposition of often conflicting results, examined in detail, frequently allows a surprisingly clear picture to emerge.
They begin by confronting the practical idea of what is meant by a species, skipping over much of what they accurately characterize as the "vast and stupefying" literature that argues endlessly over definitions of species. It is clear that the world is populated by distinct groups of organisms, and that even hunter-gatherers with no scientific training have been able to name these groups in a way that often corresponds surprisingly well with the scientific consensus. But science does add something to the hunter-gatherer view of the world. Coyne and Orr point out that the example that is in all the textbooks, the remarkable almost one-to-one correspondence of names of birds given by the peoples of the Arfak mountains of New Guinea to the scientific classification of those same birds, is an exception. In the competition between scientists and hunter-gatherers, the hunter-gatherers' common names for species usually score about 80% or lower next to the scientific classification. And Coyne and Orr go far beyond the textbooks, examining the literature to see whether classifications above the species level have any correspondence in the prescientific world. The answer is a resounding no—preliterate people classify animals and plants into crude categories like "big" and "small."
It is here that the ability to sift through characteristics in order to distinguish between those that are shared ancestrally and those that are derived, and then to use the characteristics as tools for classification, really illustrates how our world has been transformed by scientific modes of thought. Coyne and Orr, after considering the evidence, come down solidly on one side of a number of vexing controversies. They conclude that sympatric speciation is possible, but only when there is some kind of environmental heterogeneity that is sufficient to interfere with gene flow. They conclude that random drift plays a small role in speciation compared with natural selection. And they conclude that species-level selection, an important component of theories that assume some higher-level type of process must be invoked to explain evolution above the species level, has not yet been demonstrated. In an important chapter they trace the checkered history of genetic reinforcement, which is the superficially alluring idea that the rate of accumulation of prezygotic barriers can be accelerated when species ranges overlap. They are driven to the conclusion that evidence for reinforcement is sketchy at best and impossible to distinguish from competing hypotheses. But they hold out a ray of hope, pointing out that a test of reinforcement is possible.
It has been known for a long time that in most species, females should be more strongly affected by reinforcement than males, because females suffer more than males do when they make the wrong choice of mate. So females should show a more rapid accumulation of prezygotic mechanisms than males. Yet surprisingly, few tests of this prediction have been made. At this and other points in the book, Coyne and Orr perform a valuable service by pointing out clearly what needs to be done to fill in the gaps in our understanding of speciation.
My only complaint about the book is that it does not explore some of the really exciting possibilities for future research at the molecular and genetic levels. For example, it is now clear that small numbers of plant genes are often involved in deciding which pollinators will visit which flowers. What will happen when genetically heterogeneous plant populations are pollinated by new pollinators? How quickly can selection for adaptations to these new pollinators take place? Can individual genes in such plants be modified directly to see the impact that these specific genetic changes have? How quickly can new species, separated from others by pollinator preferences, emerge from these manipulations?
On the whole, this book is a wonderful resource and a fine example of what happens when clever scientists take a clear and unbiased look at the evidence. It will, I predict, join the pantheon of really important books about evolution.
Division of Biological Sciences University of California–San Diego La Jolla, CA 92093-0116
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