Implications for plant evolutionary ecology

The Origin, Expansion, and Demise of Plant Species (Oxford Series in Ecology and Evolution)By Donald A. LevinOxford University Press, 2000. £24.95 pbk; £49.50 hbk (vi + 230 pages)ISBN 0-19-512729-3 (pbk); 0-19-512728-5 (hbk)It has been a while since a comprehensive book on the evolution of plant species has been written, and Donald Levin has filled the void admirably. In a 1977 paper 1xSee all References1, he suggested that ‘it is paramount that we explore the evolutionary implications of demographic features which typically reside in the domain of ecologists’. In Levin's new book, he has followed his own advice and put in one place a summary of the data accumulated on the systematics and ecological genetics of plant populations. An unusual effort has been made to incorporate polyploidy and relevant crop literature into the larger framework of evolutionary history. The book should be of interest to all plant ecologists, taxonomists, conservation biologists and germplasm collectors.The book follows the genetic and ecological interactions of species from their origin, expansion, differentiation, decline and ultimate extinction. G. Ledyard Stebbins pioneered this concept in his brief description of the birth and death of polyploid species 2xStebbins, G.L. See all References2, but Levin's treatment is the first really extensive coverage of the life history of all types of plant species. He starts with a wide-ranging discussion of the species concept, making it clear that he prefers a broad interpretation that encompasses both ecological and genetic divergence. This allows him to include the role of interspecific hybridization in the generation and demise of species. He views species as clusters of populations and warns ‘it is important to understand the biological properties and relationships of population systems rather than getting hung up on putting them into conceptual pigeonholes.’ He repeatedly reminds us that plant species, when faced with environmental challenges at a site, often track favorable environments and move, rather than adapt to the new conditions.Levin takes a largely intuitive rather than mathematical approach and richly documents all his points with relevant studies from the past 50 years of literature. Throughout the book he considers the metapopulation as being much more important in speciation than single isolated populations. He bases this outlook on the vulnerability of small populations to extinction, and provides an excellent review of the ecological genetics of extinction and rates of species loss.Overall, this book provides the most complete evolutionary synthesis on plants since Verne Grant's books 3xGrant, V. See all References, 4xGrant, V. See all References, which are now >30-years old. Levin carefully evaluates the existing data on how plants change as they speciate. He outlines numerous examples where changes in pollination systems, breeding systems and flowering time have been shown to have a genetic basis, but points out that far too few genetic studies have been conducted in the field. Most estimates of heritability have been done in the greenhouse, which often yield higher heritability estimates than in nature. Levin notes that although genetic change is commonly observed during speciation, the specific changes necessary in the formation of new species are still poorly understood. We really only know that the strength of reproductive isolating barriers (RIBs) is positively associated with the degree of genetic divergence, because once RIBs have formed, it is difficult to make the crosses necessary to determine their genetic basis.The factors influencing range expansion receive considerable detail in this book, including patterns of adaptive radiation, chromosomal causes of hybrid sterility, spread of neospecies and Holocene expansions. Levin views bottlenecks as playing an important role in divergence, but points out that not all such divergence is adaptive. Levin also reviews the literature on the genetic changes associated with species decline. He uncovers some surprising examples where levels of diversity are relatively high in declining species and geographically restricted ones, and finds that there is not always a positive correlation between genetic diversity and population size or rarity. This lack of association makes it difficult to predict which species are likely to become extinct.Levin has provided us with the first really comprehensive synthesis of traditional systematics and ecological genetics. He encourages the re-emergent belief that hybridization has played an important role in plant evolution 5xThe role of hybridization in evolution – old wine in new skins. Rieseberg, L.H. Am. J. Bot. 1995; 82: 944–953CrossrefSee all References5, and supports the growing sentiment that polyploidy plays a dynamic role in plant evolution 6xPolyploidy: recurrent formation and genome evolution. Soltis, D.E and Soltis, P.S. Trends Ecol. Evol. 1999; 14: 348–352Abstract | Full Text | Full Text PDF | PubMed | Scopus (632)See all References6. Along the way, Levin identifies several unanswered questions, but in many more cases provides unique clarity through his review of relevant studies. He has given us a clear idea of where plant evolutionary ecology is today and where it should go.