Evolution's Wedge: Competition and the Origins of Diversity

Chapter 1. Discovery of a Unifying Principle

1 in a frequently heard—and perhaps apocryphal—story, the evolutionary biologist J.B.s. Haldane, when asked to comment on what could be inferred about the Creator based on the creation, is reported to have said, “He must have had an inordinate fondness of beetles” (Farrell 1998). Why are there so many species of beetles (Figure 1.1a)? For that matter, why are there so many different kinds of living things generally? as it turns out, living things are amazingly diverse. as one measure of this diversity, conservative estimates place the number of species alive today at 8 to 10 million (stork 1993; Hamilton et al. 2010; Wilson 2010, p. x; mora et al. 2011). Yet, as staggering as these numbers are, they vastly underestimate life’s true diversity: all species consist of individual organisms that are themselves unique. indeed, each species comprises a bewildering array of morphological, physiological, ecological, and behavioral traits (for example, see Figure 1.1B, C). in short, the total inherited variation of all organisms— “biodiversity,” as coined by Wilson and Peter (1988)—is truly astounding. Biodiversity demands explanation. Why are there so many different species? Why do species typically express different traits, especially when these traits influence resource acquisition or reproduction (for example, see Figure 1.1B)? Why do species harbor so much trait variation within them, which, in some cases, is as pronounced as the variation normally seen between different species (for example, see Figure 1.1C)? What factors trigger the formation of new traits and new species in the first place? Finally, once new species do arise, why do they sometimes coexist with other species and sometimes not? One hundred and fifty years ago, Charles darwin (1859 [2009]) offered a scien1 disCovERy of a unifying PRinCiPlE 2 • C h a P t E R o n E tific explanation for biodiversity—natural selection. although natural selection is an evolutionary process, darwin argued that at natural selection’s core is an ecological process: competition. (throughout this book, the term “competition” refers to any direct or indirect interaction between species or populations that reduces access to vital resources or successful reproductive opportunities and that is therefore deleterious —on average—to both parties; see table 1.1.) according to darwin (1859 [2009]), all organisms face recurring competition for scarce resources, and this competition favors individuals that are least like their competitors in resource use and associated traits. Consequently, groups of organisms that compete should become increasingly different over time. selection driven by competition, darwin held, is the primary engine of diversification. this book explores how competitively mediated selection generates biodiversity. in particular, we examine how such selection—whether stemming from competition for resources or access to successful reproduction—can promote evolutionary diversification through a process known as “character displacement” (sensu Brown and Wilson figuRE 1.1. among biology’s most persistent challenges is to explain why there are so many different kinds of living things. Consider, for example, the beetles. (a) although scientists do not know how many beetle species there are (about 350,000 species have been described), they are thought to be among the most diverse group of organisms. (B) Even closely related species typically differ in traits associated with resource use and reproduction. For example, male stag beetles from southeast asia, Odontolabis cuvera (left) and O. mouhoti (right), differ in body size, pigmentation, and genitalia length (circled), which are features that are important in obtaining reproduction. (C) Pronounced trait variation can even be found within species. For instance, horned beetles, Onthophagus nigriventris, consist of minor males (left) and larger, major males (right), which possess a distinctive thoracic horn. Photo credits: (a): the natural History museum, London; (B): Kawano (2003); (C): alex Wild. reproduced with the kind permission of the photographers and publishers. a B C d i s C o v E R y o f a u n i f y i n g P R i n C i P l E • 3 1956). although evolutionary biologists have long recognized that numerous factors can act as agents of selection and thereby potentially promote diversification (these factors are reviewed in macColl 2011), competition may be the most common of all selective agents (vermeij 1987; amarasekare 2009). moreover, because competition (unlike, for example, predation) is uniquely mutually costly to both parties involved (table 1.1), it is a particularly potent agent of divergent selection, which can serve as a wedge...