Testing Darwin's postulates

Adaptive Genetic Variation in the WildEdited by T.A. Mousseau, B. Sinervo and J. EndlerOxford University Press, 2000. £40.00 hbk (x + 265 pages)ISBN 019 512 183 XDarwin's theory of evolution by natural selection is the logical outcome of three postulates: (1) individuals within species are variable; (2) some of these variations are heritable; and (3) individuals that reproduce the most are those with the most favorable variations. Thus, adaptive genetic variation forms the very basis of evolution, and explaining the generation and maintenance of genetic variation for adaptive traits in natural populations has long been a central concern in evolutionary biology. In the 1970s and early 1980s, much of this attention was focused on the fitness consequences of allozyme variation, but increasing attention is being paid to quantitative traits in the past two decades 1.xEndler, J.A. See all References, 2.xRapid evolution as an ecological process. Thompson, J.N. Trends Ecol. Evol. 1998; 13: 329–332Abstract | Full Text | Full Text PDF | PubMed | Scopus (22)See all References. Adaptive Genetic Variation in the Wild, the result of a symposium at the Ecological Society of America in 1998, provides a progress report of this topic by some of its leading contributors.The central premise of the book is that a complete understanding of adaptive genetic variation can only emerge from careful, long-term studies of natural populations. Each chapter in this multi-authored volume focuses on a different ‘model system’ that provides insights into the origin and maintenance of adaptive genetic variation in the wild. As with most symposium volumes, the contributions are heterogeneous in style and substance, and the authors interpret adaptive genetic variation in a variety of ways. However, several common themes do emerge. One theme is evolution on complex fitness surfaces. Sinervo presents an elegant experimental analysis of the fitness saddle relating egg size and clutch size in fence lizards, and explores how the orientation of this saddle relates to genetic covariation between these two traits. Grant and Grant, Robinson and Schluter, and Smith and Girman each describe how resource polymorphisms associated with feeding can generate multiple fitness peaks, and how this has led to adaptive diversification in different study systems. Grant and Grant emphasize the important role of asymmetric hybridization in maintaining the high levels of genetic variation that have contributed to rapid adaptive diversification in Darwin's finches. Robinson and Schluter combine ecological, genetic and phylogenetic evidence to make a compelling case for parallel phenotypic divergences associated with resource polymorphisms in sticklebacks and other freshwater fishes.A second theme is the spatial and temporal structure of genetic variation. Mopper and colleagues present an intriguing analysis of how genetic structure in a metapopulation of leafminers results from the combination of gene flow among populations and genetic adaptation to local host types within populations. Mazer and Meade provide an interesting, cautionary tale in their discussion of evolution of flower size in wild radish. There is substantial genetic variation within and genetic differentiation among populations in flower size in radish, as well as variation in pollinator types and behavior related to radish flower size. However, the observed patterns of selection on radish flower size by pollinators cannot account for the population differences of that feature.Finally, a third theme that emerges is how heritability might vary among types of traits or environmental contexts. Of particular note is Hoffmann's thoughtful reanalysis of heritability studies in Drosophila. Hoffmann illustrates how measurement error and other methodological issues can confound apparent differences in heritabilities among life history, morphological and other traits.A major shortcoming of this volume is that ‘genetic’ in Adaptive Genetic Variation is largely restricted to the quantitative genetic level of heritabilities and genetic correlations. The fitness consequences of allozyme 3xMitton, J.B. See all References3 or major histocompatibility complex (MHC) variation 4xEvolution and ecology of MHC molecules: from genomics to sexual selection. Edwards, S.V and Hedrick, P.W. Trends Ecol. Evol. 1998; 13: 305–311Abstract | Full Text | Full Text PDF | PubMed | Scopus (221)See all References4 are not represented and single-locus traits are largely ignored. Analyses of molecular markers are prominent only in the brief methodological contribution by Ritland; recent studies using QTL and related tools to dissect the genetic architecture of adaptive variation 5xGenetic mapping of floral traits associated with reproductive isolation in moneyflowers. Bradshaw, H.C et al. Nature. 1998; 376: 762–765CrossrefSee all References5 are missing.No symposium volume can be comprehensive, and the above omissions do not detract from the excellence of many of the chapters. However, the heavy emphasis on traditional quantitative genetics seems curious at a time when molecular genetic tools are increasingly important in studies of quantitative traits and adaptation. In this sense, the book is more successful in presenting a compelling snapshot of our current understanding rather than in illuminating potential future directions for the study of adaptive variation. Despite this concern, this book is a valuable guide on testing Darwin's postulates in the wild.