THE UNIFYING WEDGE

The idea that ecology and evolution can influence each other simultaneously in a feedback loop has become the lynchpin of the new field of eco-evolutionary dynamics (Pelletier et al. 2009). However, under a different name, eco-evolutionary dynamics is actually a venerable idea. Biologists, starting with Darwin (1859) and later Brown and Wilson (1956), have long recognized that competitive interactions between species lead to evolutionary divergence that facilitates coexistence, an eco-evolutionary process known as character displacement. Despite all the attention lavished on eco-evolutionary feedbacks in recent years, character displacement is often neglected. This neglect likely reflects an uneasy combination of over-familiarity (hasn’t it all been done many times over?) and a lingering skepticism after rancorous debates over ecological character displacement and reinforcement. With renewed interest in eco-evolutionary feedbacks, it is time for a synthetic review to resuscitate the original eco-evolutionary process. Pfennig and Pfennig (2012), who have produced a series of elegant touchstone studies on both ecological and reproductive character displacement in spadefoot toads (Spea), are ideally qualified to write such a review. With Evolution’s Wedge: Competition and the Origins of Diversity (2012), they have succeeded. The book posits that competition is a ubiquitous ecological process and that competition’s consequence, character displacement, is an evolutionary wedge that generates biodiversity. Pfennig and Pfennig argue that this wedge provides a unifying theory for fundamental questions throughout evolutionary biology and ecology. In the first chapter, Pfennig and Pfennig cover the basics: definitions, alternative outcomes, and criteria for demonstrating character displacement. The subsequent three chapters provide thorough, and at times speculative, discussions of why, when, and how character displacement occurs. Although each section addresses reproductive and ecological character displacement separately, Pfennig and Pfennig also provide insightful discussions of how the two types of character displacement facilitate and impede evolution of the other. The second half of the book examines how character displacement influences other ecological and evolutionary patterns and processes, like the formation of intraspecies diversity, niche formation, community composition, sexual selection, speciation, and macroevolution. Pfennig and Pfennig bring valuable perspective to aspects of character displacement that are usually overlooked. For example, they argue that phenotypic plasticity is a key component of character displacement. Plasticity can reveal cryptic genetic variation and consequently may delay competitive exclusion long enough for genetically canalized differences to evolve. Moreover, variation in plasticity itself can be heritable, and thus the strength of plasticity can evolve during character displacement. In addition, the authors attempt to dissolve the dichotomy between reproductive character displacement and reinforcement. Finally, the book includes many examples from the plant literature, something rarely done in the discussion of character displacement. Evolution’s Wedge is not an exhaustive summary of all relevant examples of each phenomenon discussed. Instead, Pfennig and Pfennig focus on historically important and iconic empirical examples with helpful diagrams to cover the essential concepts. The authors seem to have made a conscious decision not to discuss the extensive theoretical research on character displacement in detail. They often refer to general conclusions derived from mathematical models, but never discuss the mathematical details. Some readers may find the lack of equations inviting in a book primarily concerned with general concepts, whereas others may see this as an unfortunate missed opportunity to define and discuss