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Michael Doebeli is a professor in the departments of zoology and mathematics at the University of British Columbia.

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"Presenting the theoretical background to evolutionary theory in the context of adaptation and speciation, this book fills an important niche and provides a versatile approach to a range of evolutionary phenomena. It will inspire a new generation of evolutionary theory students and have a significant impact on future research directions."--Diethard Tautz, Max-Planck Institute for Evolutionary Biology

"This logically structured, clearly written, and easy-to-follow book provides a systematic framework for understanding how and why frequency-dependent selection impacts the evolution of diversity. I am certain this book will motivate researchers to take up adaptive dynamics theory."--Michael Travisano, University of Minnesota

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Adaptive Diversification

PRINCETON UNIVERSITY PRESS

Contents

Acknowledgments........................................................................................................................................xi1. Introduction........................................................................................................................................12. Evolutionary Branching in a Classical Model for Sympatric Speciation................................................................................93. Adaptive Diversification Due to Resource Competition in Asexual Models..............................................................................384. Adaptive Diversification Due to Resource Competition in Sexual Models...............................................................................745. Adaptive Diversification Due to Predator-Prey Interactions..........................................................................................1136. Adaptive Diversification Due to Cooperative Interactions............................................................................................1397. More Examples: Adaptive Diversification in Dispersal Rates, the Evolution of Anisogamy, and the Evolution of Trophic Preference.....................1638. Cultural Evolution: Adaptive Diversification in Language and Religion...............................................................................1959. Adaptive Diversification and Speciation as Pattern Formation in Partial Differential Equation Models................................................21710. Experimental Evolution of Adaptive Diversification in Microbes.....................................................................................262Appendix: Basic Concepts in Adaptive Dynamics..........................................................................................................279Bibliography...........................................................................................................................................306Index..................................................................................................................................................323

Chapter One

Evolution occurs when organisms reproduce so that their offspring inherit certain characteristics, or traits. Variation in heritable traits, together with variation in reproductive success, generates evolutionary change in trait distributions. If the correlation between heritable variation and reproductive variation is (close to) zero, evolutionary change is neutral, and the trait distribution performs an evolutionary random walk. In contrast, evolution is adaptive if the correlation between heritable variation and reproductive variation is significantly different from zero.

Adaptive evolution is generally thought to be of central importance for the history of life on earth. The process of adaptation, whereby types that are better adapted to the prevalent circumstances leave more offspring than types that are less well adapted, is, for example, believed to have been the main driving force generating major evolutionary transitions (Szathmáry & Maynard Smith, 1995). By far the most widespread view of adaptation, both among experts and laymen, is that of an optimization process: Given a set of environmental conditions, the type that is best adapted to these conditions prevails. Determining the optimal type in a given situation, and understanding how genetic and developmental constraints impinge on the evolutionary trajectory toward such optimal types, have been among the main objectives in evolutionary theory.

One of the problems with viewing evolution as an optimization process is that this perspective leaves little room for diversity: the optimally adapted type has more offspring than all other types, and so eventually, all other types will go extinct, leaving the optimal type as the single type present. Of course, recurring mutations may constantly introduce genetic variation into a population, but optimization essentially generates uniformity. In particular, evolution of distinct ecological types out of a uniform ancestral lineage at the same physical location is precluded under the tenet of evolutionary optimization.

Yet understanding the evolution of diversity is one of the central and most fundamental problems in biology. To explain the evolution of diversity in the realm of the traditional optimization perspective, one needs to invoke geographical heterogeneity: if environmental conditions differ between different geographical locations, then different optimization problems must be solved, and hence different adaptations evolve in different locations. The process of diversification due to local adaptation to different environments is usually called ecological speciation (Schluter, 2000, 2009), but different local adaptations can also be generated by sexual selection (e.g., Lande, 1981). After their formation in separate geographical areas, different types may migrate to and coexist at the same location due to a plethora of genetic and ecological mechanisms, which have been the subject of intense study. However, physical separation, and hence an intrinsically nonbiological ingredient, is necessary to explain the emergence of diverse life forms if one views evolution primarily as an optimization process. Note that geographical isolation is also necessary for diversity to arise due to neutral evolution, but such a neutral theory of diversification has become less popular among evolutionary biologists (e.g., Hendry, 2009; Schluter, 2009), partly because it runs contrary to the generally accepted notion that diversity is paramount in nonneutral traits (i.e., in traits in which heritable variation and variation in reproductive success are significantly correlated).

Optimization theory has proved to be useful for gaining many evolutionary insights. However, it misses out on a class of ecological and evolutionary mechanisms that are intuitively appealing, and that opens up a whole new perspective on the problem of the evolution of diversity. These mechanisms operate whenever the relevant components of the environment determining selection pressures on a given focal type not only consist of abiotic, physical ingredients that may remain constant over evolutionary time, but also comprise other organisms that may be present in the environment. Whether these other organisms are individuals of the focal type's species, or part of other species with which the focal type interacts, it is often obvious that an individual's survival and fecundity generally depend on the ecological impact of other organisms. For example, if organisms with different traits eat different types of food, then whether a given trait confers a high food intake will depend on the traits of the other organisms currently present in the population (with food intake low if the other organisms have similar traits, and hence eat similar food). Moreover, the food intake of a given organism may change as the distribution of traits in the population changes. As a consequence, adaptation to constant conditions may rarely occur: as the population evolves, the biological environment changes, and hence the optimization problem changes as evolution unfolds.

The phenomenon that the evolving population is part of the changing environment determining the evolutionary trajectory is usually referred to as frequency-dependent selection....