SYNDACTYLUS PHYLOGENIES: THE ROLE OF GENE EXCHANGE IN ADAPTIVE EVOLUTION

Review of Evolution through Gene Exchange. Michael L. Arnold, 2006. Oxford University Press, Oxford, UK, 252 pp., PB $75.00, ISBN 978-0-19-922903-1. This book reviews the theory, methods, and evidence for gene exchange between species and examines the importance of its role in the evolution of adaptive novelty and speciation. In this insightful summary of hybridization research, Arnold, a botanist, first reviews the history of concepts in this area, going back to the classical works by Stebbins, Anderson, and others on introgressive hybridization as a source of adaptive variation for the sieve of natural selection. Because the concepts of genetic exchange and species tend to be interdependent, with the latter determining the one's view of the former, Arnold reviews species concepts with an emphasis on their tolerance for hybrid exchange in the second chapter. In the third chapter, he reviews the methods available for testing evolutionary hypotheses of gene exchange between taxa, before addressing in the following chapter the multiplicity of processes interrupting gene flow that are common place in many groups. The chapter on hybrid fitness components, wherein natural selection amplifies or restricts successful interspecific reproduction, is very extensive, covering six or seven empirical examples each, from microorganisms, plants, and animals. Arnold makes a convincing case that web-like as opposed to tree-like phylogenies are not restricted to the domain of plants or solely a concern for botanists. Here, the data clearly indicate that hybrid fitness is as much determined by environmental contingency as it is by intrinsic genetics. A brief chapter on gene duplication and the “genetic shock” that may attend the hybridization of divergent genomes precedes a more interesting chapter on hybridization and the origin of new evolutionary lineages. The book ends with chapters on the significance of hybridization for endangered and domesticated species.