Annals of Botany 90: 153-154, 2002
© 2002 Annals of Botany Company
Ayala, F.J., Fitch, W.M. and Clegg, M.T. Variation and evolution in plants and microorganisms: toward a new synthesis 50 years after Stebbins
Variation and evolution in plants and microorganisms: toward a new synthesis 50 years after Stebbins.
Ayala FJ, Fitch WM, Clegg MT, eds. 2000.
Washington, DC: (US) National Academy Press.
£14.95 (softback). xi + 340 pp.
In January 2000, an impressive cohort of evolutionary biologists convened in Irvine, California, to celebrate the 50th anniversary of the publication of G. Ledyard Stebbins’ Variation and evolution in plants. The brief introductory appreciation of Stebbins by Peter Raven describes Variation as ‘the most important book on plant evolution of the 20th century’ (p. 5). This strongly worded claim is not entirely without justification. Among the ‘New Synthesists’, Stebbins achieved for botanists what Theodosius Dobzhansky (later to be a colleague of Stebbins at UC Davis) had previously achieved for geneticists, Ernst Mayr for zoologists and G. G. Simpson for palaeontologists. Admittedly, the time lag of 6 years between Simpson’s magnum opus and Stebbins’ treatise continued the long tradition that, within the confines of evolutionary theory, botany follows, and is strongly influenced by, other more ‘senior’ branches of biology.
Ayala et al. contains 16 scientific chapters, each spanning 14–24 pages; they are grouped in sets of two to four under five headings, each section beginning with a brief but valuable introductory page. Under ‘Early evolution and the origin of cells’, an up-to-date but fundamentally historical account of Precambrian bacteriology by Schopf precedes the fascinating inference of five putative origins of the free nucleus following endosymbiosis, one line of which included the mitochondriate ancestor of the eukaryotes (Margulis et al.). Palmer et al. describe the inequitable exchange of introns (incoming) and functional genes (outgoing) between mitochondrion and nucleus, which leads to remarkable genome contrasts among even closely related species. In ‘Viral and bacterial models’, Moya et al. demonstrate that ‘model’ RNA viruses exhibit a phenomenal mutation rate, thus generating vast numbers of quasi-species, and suggest that the key to viral evolution is the most frequently encountered mutant ‘master sequence’. Bush et al. use the flu virus to caution against the medically relevant perils of sampling bias, and Levin and Bergstrom consider the importance of low rates of homologous recombination and of viral/plasmid-mediated gene transfer among distantly related bacteria.
‘Protoctist models’ explores mechanisms of two intriguing frame-shifting mechanisms in trypanosome mitochondria (Simpson et al.) and the very recent ancestry of the malarial vector, which interestingly suffers more rapid indels than base substitutions other than in the highly positively selected antigenic regions (Rich and Ayala).
‘Population variation’ begins with a review of the relationship in plants between transposons and repressive chromatin structures (Federoff); the inevitable grass-centred nature of this account grades into the following chapter by Gaut et al., who discuss the many extraordinary features of the maize genome. Polyploidization and transposon proliferation together make one wonder where this plant lies on the continuum between a truly ‘model’ organism and a bizarre evolutionary cul-de-sac. The case study of flower colour evolution by Clegg and Durbin raises many important points, especially regarding transposon-mediated gene redundancy, but cannot fully explain the apparent uncompetitiveness of white- vs. purple-flowered morning glories. Schaal and Olsen explore our rapidly expanding (although still technically constrained) knowledge of the molecular basis of alleles, emphasizing the increasingly important con tributions of phylogeographic studies of allele frequencies.
The final section, enticingly headed ‘Trends and patterns in plant evolution’, begins with Dilcher’s palaeobotanical update of the origin and early diversification of angiosperms, which focuses on the co-evolutionary importance of closed carpels, floral zygomorphy and fleshy nutritious fruits. Holsinger considers the complexity of reproductive control reliably invoked by authors to help explain the remarkable diversification (or, more accurately, species-richness) of the angiosperms, noting that in the short term allogams face an uphill struggle against the costs of outcrossing and reproductive assurance. Ellstrand and Schierenbeck examine another aspect of angiosperm success, specifically their predisposition to weedy invasiveness, convincingly invoking inter- or intraspecific hybridization as a frequent cause. Lastly, Soltis and Soltis review the oft-cited contributions of polyploidy to plant diversification, emphasizing the importance of high heterozygosity, low inbreeding depression, genome rearrangements, and multiple origins and subsequent functional divergence between copies of specific genes.
The overall impression gained from these chapters is one of authorities in their respective disciplines each exploring one, or at most a few, molecular–genetic mechanisms that we are able to understand at increasingly deep levels. It is therefore a difficult read that repays re-reading.
Given that no book is perfect, what elements are missing from this patchily cutting-edge and often provocative volume? A trivial answer could be an attractive, error-free format. A less trivial answer could be the current rapid expansion of evolutionary–developmental genetics, which is increasingly undermining the population focus and determinedly gradual phenotypic transitions of the mostly neoDarwinian New Synthesists, and threatens to replace them with a macroevolutionary paradigm altogether more staccato in tempo and more profound in phenotypic effect.
But perhaps the simplest answer to this question is that it is Stebbins himself who is missing from the volume, since he had the great misfortune to die a scant 8 days before his festschrift conference commenced, at the age of 94. The best of the New Synthesists became genuinely interdisciplinary scientists long before that much-abused term became over-popularized. Although their broader credibility depended heavily on the prior mathematical–genetic genius of J. B. S. Haldane, R. A. Fisher and Sewall Wright (a triumvirate who could justly claim to be at the forefront of the essential precursor programme of the ‘New Genetics’), the New Synthesists were well versed in comparative biology and each applied diverse concepts and techniques to wide ranges of organisms.
The authors of the 16 scientific chapters of this book have provided much of interest, and their collective molecular–genetic focus accurately represents the current emphases of evolutionary biology. Most of the chapters provided me with fascinating new insights, often refreshingly supported by data, and on this basis the book definitely merits recommendation to a wide spectrum of evolutionary biologists. But none of the authors has made a concerted attempt to resurrect the grandiose, integrated evolutionary vision of Stebbins and his contemporaries—the kind of vision that might at last project botany (and its own Cinderella disciplines, mycology and microbiology) beyond the long shadow of the zoologists, palaeozoologists and mathematico-zoologists who, in practice, set the evolutionary agenda throughout the 20th century. It would be good to see specialists in the other kingdoms respond even more positively to this challenge in the new millennium.
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