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ix this book examines how a pervasive feature of living systems—competition—drives evolution and generates diversity. Competitive interactions take place whenever any two individuals, populations, or species affect each other adversely. these interactions might involve direct contests for limiting resources or reproductive opportunities. alternatively , they might assume more subtle forms, such as depleting a shared resource or interfering with each other’s ability to identify high-quality conspecific mates. regardless of how it is expressed, competition is central to the darwinian theory of evolution by means of natural selection. indeed, darwin recounts in his autobiography (Barlow 1959, p. 120) how he developed his theory only after reading an essay on population growth by malthus (1797 [1990]). in this essay, malthus argued that, if unchecked, the rate of human population growth would exceed the rate of increase in the food supply , resulting in a “struggle for existence.” malthus’ emphasis on overpopulation and competition immediately inspired darwin’s thinking regarding the mechanism that drove evolution—natural selection. Upon reading malthus, darwin’s critical insight was to realize that all organisms tend to produce far more offspring than could be supported by the available food supply and habitat, thereby fueling perpetual competition among organisms for these crucial resources. such competition, darwin (1859 [2009]) held, favors individuals that are least like their competitors in how they obtain resources. in this way, competitively mediated selection may cause interacting species and populations to evolve different resourceacquisition traits (for example, morphologies and behaviors). darwin argued that this PREfaCE x • P R E f a C E process might even cause interacting conspecific populations to diverge from one another to such a degree that they might eventually become entirely new species. thus, according to darwin (1859 [2009]), competitively mediated selection could explain two important features of the living world: why there are so many different species, and why even closely related species typically differ in ecologically relevant traits. although these ideas about how competition generates biodiversity were crucial to darwin, they are often misunderstood and under-appreciated today. moreover, darwin’s ideas have equally profound implications for ecology, which are sometimes overlooked. indeed, competition potentially explains why species are found where they are, why they specialize on the particular resources that they do, and why they (ultimately) go extinct. in short, competition is crucial to any general theory concerning the origins, maintenance, abundance, and distribution of biodiversity. trait evolution that arises as an adaptive response to resource competition or deleterious reproductive interactions between species is now known as “character displacement ” (sensu Brown and Wilson 1956). this book explores the causes and consequences of this process. a central premise of our book is that, by understanding these causes and consequences, we can gain crucial insights into some of the most fundamental issues in evolutionary biology and ecology, including how new traits and new species arise, why species diversify, and how they are able to coexist with each other. indeed, character displacement is not only central to ecology and evolutionary biology, it can also unify these sometimes disparate fields. Before proceeding, however, we need to answer an important question: why do we need a book on character displacement? after all, the basic idea has been around for 150 years. moreover, several books have appeared recently that highlight character displacement to at least some extent (for example, schluter 2000; Coyne and Orr 2004; grant and grant 2008; nosil 2012). What is the justification for an entire book on character displacement? Our motivation in writing this book was threefold. First, most prior discussion of character displacement has concentrated on how to detect it, and on whether or not it even occurs. given that ample evidence now indicates that character displacement does indeed occur (chapter 1), an integrated discussion of the factors that fuel character displacement and its consequences beyond mere trait evolution is needed. therefore, in the first half of the book, we discuss how competitively mediated selection often (but not always) promotes character displacement (chapter 2), the factors that determine when character displacement occurs (chapter 3), and the proximate (that is, genetic and developmental) mechanisms of character displacement (chapter 4). some of the topics covered in this section have received scant attention. For example, in chapter 4, we consider whether phenotypic plasticity (the capacity of a single genotype to produce multiple phenotypes in response to variation in the organism’s environment) plays a role in character displacement. in fact, facultative niche shifts induced by competitors might often be a significant...

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