Mechanisms of Life History Evolution the Genetics and Physiology of Life History Traits and Trade-Offs
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OUP CORRECTED PROOF – FINAL, 04/23/2011, SPi Mechanisms of Life History Evolution The Genetics and Physiology of Life History Traits and Trade-Offs !"#$!" %& Thomas Flatt Group Leader at the Institute of Population Genetics at the Vetmeduni Vienna, Austria Andreas Heyland Assistant Professor at the Department of Integrative Biology at the University of Guelph, Canada 1 OUP CORRECTED PROOF – FINAL, 04/23/2011, SPi 3 Great Clarendon Street, Oxford ox2 6dp Oxford University Press is a department of the University of Oxford. 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Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Library of Congress Control Number: 2011920657 Typeset by SPI Publisher Services, Pondicherry, India Printed in Great Britain on acid-free paper by CPI Antony Rowe, Chippenham, Wiltshire ISBN 978–0–19–956876–5 (Hbk.) 978–0–19–956877–2 (Pbk.) 1 3 5 7 9 10 8 6 4 2 OUP CORRECTED PROOF – FINAL, 04/23/2011, SPi “…integrating an understanding of mechanisms into life history theory will be one of the most exciting tasks facing evolutionary biologists in the 21st century.” Barnes & Partridge (2003 ) OUP CORRECTED PROOF – FINAL, 04/23/2011, SPi Foreword: Harvey’s legacy Graham Bell “Ex ovo omnia” wrote Harvey ( 1651 ) , and more trematodes. It is often dif' cult to work out how than three centuries later his dictum still stands. and when one stage gives rise to another, espe- Many examples of vegetative reproduction in ani- cially when this involves reproduction by eggs. In mals have been described since Trembley (1744) other cases, however, the developmental sequence astonished the world by describing the asexual is clear even though it passes through larval stages budding of Hydra , but all lineages, so far as we that are very different from the adult, by virtue of know, pass through a single-cell stage sooner or the physical continuity of macroscopic individu- later. The only more certain observation is that the als. There is no profound difference between the individual that develops from the egg will eventu- two, however. The juvenile star' sh, for example, ally senesce and die. The journey between the two develops as a miniature version of the adult from a ' xed points of egg and corpse has no prescribed small patch of tissue within the body of a small route, however, and biologists have struggled to ciliated-band animal living in the plankton. As the discover and interpret the lives of animals, plants, star' sh animal enlarges, the ciliated-band animal fungi, and seaweeds since the beginning of scien- shrinks, and its remnants are eventually discarded. ti' c biology in Harvey’s time. This process is called “development” because the In very broad terms, two main approaches have physical continuity of larva and adult is clear, but been followed. The ' rst is concerned largely with it is manifestly the same kind of process as the juvenile forms, and speci' cally with discrete devel- transition between polyp and medusa, a succes- opmental stages such as larvae. This is the older sion of morphologically distinct phases within a school, and its subject is usually called “life cycles.” single lineage. The second is concerned largely with adults, and The study of life cycles has been predominantly speci' cally with the quantitative schedule of repro- descriptive, a tradition that has continued, in the duction. This has developed over the last 50 years, form of discovering the regulatory genes responsi- and its subject is usually called “life histories.” The ble for the evolution of body plans, down to the two have been developed to a large extent inde- present day. The possibility of a theoretical account pendently of one another, and the linkages between of the life cycle was raised by Garstang (1928) nearly them remain few and weak. a century ago: given that an animal developing The study of life cycles is the older tradition, and from an egg must grow steadily larger, it must be dates back to the time when it was ' rst demon- functional ' rst as a small individual and subse- strated by Steenstrup (1845) that very dissimilar quently as a large individual. A ciliary-band animal individuals could be produced, one from another, living at low Reynold’s number in the plankton is as a succession of forms belonging to the same lin- one possible route to a large hydraulically powered eage. The only example that is familiar to everyone animal living on the sea ( oor, whereas the reverse (now that classical zoology has withered from the route is impracticable. Nevertheless, a systematic curriculum) is the succession of polyp and medusa theoretical framework capable of interpreting the in some cnidarians, but much more complex life succession of phases in development has yet to be cycles have evolved in groups such as digenean constructed. OUP CORRECTED PROOF – FINAL, 04/23/2011, SPi FOREWORD: HARVEY’S LEGACY vii The parallel effort to understand the sexual stages such as sporophytes and gametophytes are phases of life cycles is a similar but more compli- not distinguished. Freed from zoology, botany, and cated story, largely because of the great dif' culty of genetics a highly abstract and general theory of the establishing the site and timing of the crucial events life history could be developed. of fusion and reduction, even after the advances of Much of this theory was based on a principle of microscope design in the later nineteenth century. It optimality: quantities such as the rate of reproduc- was not until the closing decades of the century that tion and the probability of survival are negatively the essential distinction between spore and gamete correlated, such that intermediate values of both was unequivocally established, and by the end of maximize the overall rate of increase of a lineage. the ' rst decade of the twentieth century a correct This enabled the schedule of reproduction to be pre- account of the alternation of generations in the life dicted from the costs of reproduction, in terms of cycle of seaweeds and land plants had been suc- reduced growth or survival. This approach has had cessfully accomplished (see the review by Farley some brilliant successes, beginning with David 1982). A theoretical basis for understanding the Lack’s classical interpretation of clutch size in birds alternation of generations in terms of the funda- (see Lack 1966). It could also be extended to the mentally different requirements of spores and gam- puzzling phenomenon of senescence, which could etes, and thus the different structure and behavior now be interpreted as a non-adaptive side effect of of spore-producing and gamete-producing individ- selection for early reproductive maturity ( Williams uals, was provided at the same time (Bower 1908). 1966a ). Moreover, the generality of the predictive Before it could be extensively developed, however, framework made it possible to contrive experimen- the ' eld was largely abandoned, swept away by the tal tests in laboratory model systems, which had ( ood of research into transmission genetics, which never been possible for life cycles. had just been provided with the ' rm theoretical The very generality that gave the theory such framework of Mendelism. The endlessly varied power was also a source of weakness, insofar as the sexual cycles of eukaryotes have continued to pro- sources of the costs of reproduction on which the vide rich material for extending our knowledge of theory was based did not need to be speci' ed and natural history, but we cannot yet interpret them therefore could not be investigated within the con- within a consistent theoretical framework. In recent ' nes of the theory itself. For this reason, attention years there has been a modest revival of interest in began to shift towards the nature of the costs them- the theory of phenomena such as gamete dimor- selves. This is not as straightforward as it might phism, but this has not yet become ' rmly coupled appear. It seems obvious that producing larger off- to ' eld studies and experimentation. spring will mean producing fewer, and almost The ' eld of life histories has developed more equally obvious that allocating a greater share of recently and in a very different fashion, being highly resources to reproduction must deplete the stock theoretical from the outset. Simplifying history (as available to support maintenance and defense. One one must in a foreword) it was galvanized by Cole’s function necessarily interferes with the other.