Willis, K.J., McElwain, J.C. The evolution of plants

Abstract

This book, aimed at undergraduates, deals mostly with the information that can be gleaned about plant evolution from the fossil record. In some sense, its broad title is slightly misleading because other topics central to plant evolution such as genetic change in populations and mechanisms of speciation are hardly covered. After a brief introduction to the geological timescale, types of fossil and how they are dated, the central six chapters forming the bulk of the book follow a chronological journey. This takes the reader from the appearance of the first photosynthesizers, to the colonization of the land, through to the evolution and dominance of angiosperms and the development of the ecosystems that clothe today’s planet. The final section comprising three chapters is less descriptive: two chapters deal with whether the plant fossil record tells us different stories of mass extinction and macroevolution than the animal record, and another discusses what we can learn from DNA extracted from plant fossils. The main section of the book providing an overview of the evolution of plant groups and their morphology through geological time is successful. I am not a palaeobotanist, and have not read any wide review of this topic since I was an undergraduate in the 1980s. I found the account easy to read—in general, the language of the book is very accessible. It is also up‐to‐date, for example in dealing with the implications of new molecular phylogenies for our understanding of early angiosperm evolution. One very useful aspect of this section of the book is its overview of the biogeographical distribution of global vegetation from the Carboniferous to the present day. In each chapter, maps are presented showing the extent of the various biomes that have covered significant areas of the earth. Viewed as a series, they provide a summary picture of how various biomes have waxed and waned as continents have moved and global climates have changed. My only criticism would be that it is very difficult to distinguish between the subtly different grey shades representing different biomes, making the maps difficult to interpret. However, the maps are available in full colour from the accompanying website (http://www.oup.co.uk/best.textbooks/biology/plantevol/) as downloadable PowerPoint files. It seems that the authors are less comfortable when dealing with topics away from the central theme of the fossil record, making the final section of the book perhaps less convincing. For example, the basic descriptions of hybridization and polyploidy in Chapter 8 seem to be taken largely from other textbooks, and are perhaps superfluous in this book about macroevolutionary patterns. There is some weakness in the discussion of phylogenetic topics, such as the cladogram in Fig. 6.17b (p. 185), which purports to depict Bennettitales as most closely related to angiosperms but is, in fact, unresolved. Least satisfactory of all is Chapter 9 dealing with ancient DNA. This topic probably merited no more than an aside rather than a substantial section; perhaps the authors’ rationale was to use this ‘Jurassic Park’ theme to grab the attention of students. Nucleotide sequences extracted from plant fossils are regarded as probably artefactual by the majority of the botanical community. The authors acknowledge (making the inclusion of this chapter even stranger) the good biochemical evidence showing that strands of DNA longer than a few hundred base pairs are unlikely to survive for a few thousand, let alone a few million years. Furthermore, even if the published studies were not controversial, there are so few of them that they have very little potential for revolutionizing anything that we know about plant or DNA evolution, especially across the wide geological timescale covered by this volume. The authors’ final appeal for the utility of ancient DNA is a study of the domestication of maize using DNA extracted from 4500‐year‐old cultivated plants. However, the microevolution of plant domestication is hardly relevant to the broad macroevolutionary sweep of this book. If the authors and publishers are considering a second edition of this volume, this chapter might usefully be replaced by an account of exciting new studies where dated fossils can be used to assign minimum ages to nodes in molecular phylogenies of extant plants (e.g. Bremer, 2000; Lavin et al., 2001). New techniques, which need not assume that DNA substitution have been clock‐like (Sanderson, 1997, 2002), can use these calibrations to infer dates of all nodes in a molecular phylogenetic tree, thereby providing an indication of the age of events that have not been recorded in the fossil record. The other final chapters, one placed either side of the ancient DNA account, are better. The first discusses how the plant fossil record does not show the same degree of periodic mass extinction as the animal record. It argues (reasonably) that this reflects the greater ability of plants to survive environmental stress via losing leaves or even whole branch systems, or by perennation as underground stems, rhizomes or seeds. Polyploidy and hybridization are also invoked as factors that enable plants to survive by means of rapid evolution in the face of environmental change. The final chapter continues the comparison with the animal record and considers whether the plant record also supports the macroevolutionary pattern of punctuated equilibrium, or whether it is more consistent with the competing concept of phyletic gradualism. The lack of mass extinction in the plant record is used to argue that plant macroevolutionary patterns were different. However, the authors conclude that on the broadest scale, plant evolution was punctuated, with major changes concentrated in short periods. They provide evidence that these periods coincide with pulses of global plate spreading and increased tectonic activity, and suggest that increased global CO2 concentrations may have been a critical factor. In the sections where it plays to its strengths—the plant fossil record and macroevolutionary patterns—this book is well written and informative. All the illustrations are available as downloadable files from the website, which will be very helpful to lecturers and students. The book itself is well produced, though some of the large tables spread across double pages are poorly typeset and hard to follow. I think that The evolution of plants will find a wide student market, not least because in an age where academic books are often extortionately priced, it is available for just over £20.