Intraspecific competition and niche expansion

Theory has long suggested that intraspecific competition can drive the evolution of increased niche diversity. While a substantial body of theory has developed around this idea, empirical support is largely circumstantial. This dissertation combines empirical and theoretical approaches to further explore the relationship between intraspecific competition and niche diversity. Chapters 1 and 2 describe experimental tests of this relationship. The Chapter 1 experiment was conducted in a tightly controlled laboratory setting, and shows that adaptation to unfamiliar (and suboptimal) resources is faster in populations experiencing intense competition. The experiments for Chapter 2 were carried out in a natural setting by manipulating the population density of natural populations. These field experiments show that intraspecific competition generates disruptive selection, while observational field studies suggest that such disruptive selection occurs naturally. The generality of these results depend in large part on how often natural populations experience frequency-dependent competition. Such frequency-dependence occurs when individuals within a single population rely on different subsets of the population's resource base. Chapter 3 reviews the literature to examine the incidence and implications of such individual specialization. Chapter 4 describes the different methods available to quantify the degree of individual specialization. Chapter 5 examines alternative evolutionary outcomes to the kind of competitive disruptive selection observed in Chapter 2. Both speciation and ecological sexual dimorphisms are thought to evolve in response to such stable fitness minima. Using both analytical theory and numerical simulations, I show that these two alternative outcomes can share identical ecological causes, and that which outcome occurs depends on the genetic basis of assortative mating and the genetic covariance between male and female traits. Chapter 6 explores the effect of one parameter in this speciation model, previously assumed to have a set value. This analysis suggests that extant models of sympatric speciation can only explain rapid adaptive radiation by invoking unrealistically intense assortative mating. Taken as a whole, these chapters provide empirical confirmation that intraspecific competition favors increased niche diversity, that the conditions for such divergence are widespread, and that these stable fitness minima are less likely to lead to sympatric speciation than current theory suggests.