Michael Doebeli - Adaptive Diversification

Adaptive Diversification

MONOGRAPHS IN POPULATION BIOLOGY

EDITED BY SIMON A. LEVIN AND HENRY S. HORN

A complete series list follows the index

MICHAEL DOEBELI

PRINCETON UNIVERSITY PRESS

Princeton and Oxford

Copyright 2011 by Princeton University Press
Published by Princeton University Press, 41 William Street,
Princeton, New Jersey 08540
In the United Kingdom: Princeton University Press,
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Library of Congress Cataloging-in-Publication Data
Doebeli, Michael, 1961
Adaptive diversification / Michael Doebeli.

p. cm. (Monographs in population biology)

Summary: Adaptive biological diversification occurs when frequency-dependent selection generates advantages for rare phenotypes and induces a split of an ancestral lineage into multiple descendant lineages. Using adaptive dynamics theory, individual-based simulations, and partial differential equation models, this book illustrates that adaptive diversification due to frequency-dependent ecological interaction is a theoretically ubiquitous phenomenon Provided by publisher.

Includes bibliographical references and index.

ISBN 978-0-691-12893-1 (hardback) ISBN 978-0-691-12894-8 (paperback) 1. Adaptation (Biology)Mathematical models. 2. BiodiversityMathematical models. 3. Evolution (Biology)Mathematical models. I. Title.

QH546.D64 2011

578.4dc22

2011006879

British Library Cataloging-in-Publication Data is available

This book has been composed in Times New Roman

Printed on acid-free paper.

Printed in the United States of America

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To Gabriela and Carlos, my beautiful children

There are two camps. There always are.

Thats the worst thing about democracy: there have

to be two opinions about every issue.

Ross Macdonald, Black Money

I am indebted to Steve Stearns, who took me under his wings when all I had was a PhD in Mathematics and no clue about evolution. Steve had a tremendous impact on evolutionary biology in Europe, and he had a tremendous impact on me. Along the way, I have met a number of scientists who I greatly admire, and who greatly influenced me in one way or another. Among them are Hans Metz, the visionary inventor of adaptive dynamics, Ulf Dieckmann, whose technical versatility always awed me, and Martin Ackermann, a great scientist, and a great friend. I would also like to thank Tim Killingback and Christoph Hauert for many interesting collaborations, and Karl Sigmund, Alan Hastings, and Peter Abrams for their continued support. Many thanks also go to all the people who passed through my research group and made my daily life interesting. I was particularly impressed with Maren Friesen, who, as an undergraduate, single-handedly established my experimental evolution lab, and with Slava Ispolatov, who seems to understand everything. Slava made essential contributions to . Alistair Blachford and Erik Hanschen were a great help with many technical aspects of producing this book. At the University of British Columbia I have a great group of colleagues, and I am particularly grateful to Dolph Schluter, Martin Barlow and Ed Perkins for their support. My ultimate thanks go to Nelly Pante for staying the course.

Adaptive Diversification

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 (Szathmry & 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 types species, or part of other species with which the focal type interacts, it is often obvious that an individuals 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.