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Book Review |
Department of Integrative Biology, University of California, Berkeley, California 94720 USA
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INTRODUCTION |
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 TOP INTRODUCTION CONCLUSIONLITERATURE CITED |
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), fossils do offer a unique perspective on the temporal and spatial scales of evolution, while framing these in the context of a dynamic Earth. However, study of the "dead" is only one line of inquiry, because no matter how well-preserved a particular fossil is, the past plant it marks cannot be greened and coaxed to grow from the rocks. To fully understand the mechanisms driving evolution, life's genealogy, development, as well as ecology must be infused into the geological pattern.
The field of paleobotanical research has recently accelerated on several fronts, fueled by advances in rock dating methods (Knoll and Carroll, 1999
), mega-analyses to study patterns of biotic replacement (Lupia, Lidgard, and Crane, 1999
), micro-scale and macro-scale biochemical analyses of fossils (Arens, Jahren, and Amundson, 2000
; Briggs, Evershed, and Lockheart, 2000
; Boyce, Hazen, and Knoll, 2001
), plus new approaches to taphonomy (Behrensmeyer, Kidwell, and Gastaldo, 2000
) and clarification of phylogenetic patterns as well as developmental rules and functional biology of extant groups (Mathews and Donoghue, 1999
; Chaw et al., 2000
; Cronk, 2001
; Pryer et al., 2001
; Boyce and Knoll, 2002
). And all the while, the fossil record of plants grows richer and richer by the year, exposing new interfaces for the application of techniques and opportunities for addressing questions. Taken by this spirit, and a belief in the importance of combining studies of the dead plants with living ones, K. J. Willis and J. C. McElwain, two players in the paleobotanical renaissance, have fashioned a new, undergraduate-targeted text on the evolution of plants.
The content of the text, The Evolution of Plants, is colored by both Willis' and McElwain's interests in paleoclimatology and the functional biology of extinct plants. Willis' research, based out of the School of Geography and Environment at Oxford University, has largely focused on the long-term relationships between vegetation dynamics (largely Holocene processes) and environmental changes, as well as possible coevolution between early angiosperms and dinosaurs. At the Field Museum, McElwain's work centers on using structure–function relations of fossil plants to reconstruct geologic-time scale changes in global carbon cycle dynamics. In particular, her pioneering work exploring the links between stomata structure and the interpretation of past climates has had significant impacts on our understanding of plant responses at times of animal mass extinction.
Summarizing the evolutionary chronicle of plants is a difficult task by any measure. Crafting a plant evolution course into a format enticing enough to woo undergraduates away from other courses is even more daunting. Paleobotany can seem particularly far-removed from day-to-day affairs because of the strong attachment to terminology and study of bits of seemingly insignificant plant detritus. But in everyday life, plants loom large, providing the raw materials for most of the clothes we wear and filling our stomachs and lungs with life-giving energy and oxygen. But it's more than that, most of us will see more plants in our lives than animals, in forests to weeds growing at the bus stop—you can't easily get away from plants. To enrich our understanding about the natural world, just about everyone needs to know a thing or two about plants. So how did the particular cohort of plants that pervade our world today come to be? Was the world always like this? It turns out that things get even more interesting, when the layers, or rather the rocks, are peeled back, and we begin to examine the 90% of all the plant species that ever lived. The text by Willis and McElwain effectively serves as a good introductory analysis of plant evolution, to get neophytes generally set off on the right foot.
Several recent texts have touched on the broad topic of plant evolutionary biology, in particular covering aspects of the fossil record. Most notable is K. J. Niklas' book Evolutionary Biology of Plants (1997)
, which is well written and broad in scope. This text does an excellent job at marrying insights from the living and the dead to examine plant evolution. Also, there is L. E. Graham's Origin of Land Plants (1993)
, which is particularly fun to read and focuses on the transition of plants onto land and into air from green algal ancestors. Finally, W. N. Stewart and G. W. Rothwell's Paleobotany and the Evolution of Plants (1993)
as well as T. N. Taylor and E. L. Taylor's The Biology and Evolution of Fossil Plants (1993)
both showcase how deep the rabbit hole of plant evolution goes, providing all the details one could want about what groups lived when and where and who may have been related to whom. Besides extensive and clear coverage of the rogues gallery of plant evolution and review of the major mechanisms underlying evolutionary change in plants, what are the real advances made in The Evolution of Plants by Willis and McElwain? There are several.
Overall, the book is lucidly written and produced fairly well, with copious use of well-chosen and clearly labeled figures. There are all of the usual reconstructions (although rendered with new drawings) of many of the paleobotantist's dearest friends including Archaeopteris, Glossopteris, Medullosa, Williamsonia, and so forth. The texts also includes a few black-and-white plates of living plants for good measure. There is a lot to be learned here, especially in understanding the potential climatic and functional drivers of evolutionary change. Finally, the presentation of summary points, really "take-home messages," at the end of each chapter is laudable.
The book is organized broadly into three parts. This first part is a brief and clearly written opening discussion that introduces readers to the business end of paleobotany. Here questions such as, What can we learn from the fossils themselves? How do we ordinate fossils in space and time (e.g., dating methods)? What does the geologic time scale really mean?, take center stage. However, I was surprised that absent from this mix were brief introductions on the disciplines of sedimentology, which is our window on the physical settings where organisms lived, and taphonomy, which provides the sobering lenses for viewing what is really possible to reconstruct and test using the fossil record (Behrensmeyer, Kidwell, and Gastaldo, 2000
). The subject of taphonomy is briefly taken up near the end of the book (in a small section on biomolecules and the fossilization process) and would be better covered at the outset.
With the introductory remarks completed, Willis and McElawin move on to the second stage, which consists of a six-chapter tour drawing detailed portraits of plant life, starting with its earliest forms to flowering plants. Importantly, whole-plant reconstructions and phylogenetic placements of lineages provide the organizing principles. I think the liberal use of phylogenies is a particularly noteworthy advance of this text, encouraging students to think in terms of "trees." However, it is tacitly assumed that reader will be savvy and does not need review from background-related or introductory material. Phylogenetic hypotheses provide excellent organizational frameworks to hang hypotheses on and can find broad application in teaching. I just wish a few words, á la the clear and highly readable style of Brooks and McLennan (2002)
on the conceptual tools of phylogenetic reconstruction, were articulated somewhere in the book to keep students from quietly guessing what phylogenies mean and how much faith one should be put in a particular hypothesis. I truly sympathize with the difficulty of this next task, especially since when we live in a time when every other week a new phylogeny erupts to shake the tree of life. However, greater care should be taken in correctly pointing out who is related to whom (i.e., on p. 278, Nelumbo is not a water lily [Nymphaeales] but is in fact more closely related to Platanus, an observation in and of itself that could provoke a lively classroom discussion on the role that molecular phylogeny has played in redrawing pattern of plant evolution!).
Each chapter on the major phases of plant evolution also has a general underlying structure, starting with a mini-review on the environmental and geographic arena and how these features change over the salient time frame for the origin, diversification, and/or decline to extinction of a particular group. Following this are discussions on major trends in evolution of functional and reproductive characters, generally considered in relation to the repertoire of underlying extrinsic climatic events. And then each chapter ends with a paleobiogeographic picture for the distribution of how different biomes may have been stratified across the Earth. The synopsis of plant diversity over geologic time is succinct; in only 212 pages the reader is whisked from the origin of life to the emergence of our modern flora that bursts at the seams with flowering plants.
Finally, the third portion of text gets into the details of how we interpret the evolutionary forces shaping the pattern of plant evolution recorded in the rocks. This section consists of a heterogeneous group of topics, including mass extinctions, the punctuated equilibrium model of evolution, and a variety of empirical angles (applications of stable carbon isotopes to examine plant function and ancient DNA methods) to study the fossil record. Also there is an interesting discussion on the role carbon dioxide may have played in plant evolution, a nice topic to bring out, given the current rapid rise in atmospheric carbon dioxide, which students will be able to connect with. However, throughout this section, it is assumed that students will have lightning recall of basic plant physiological principles.
Also in this section (i.e., in the chapter on mass extinctions and persistent populations), it was generally assumed that the early fossil appearances for a clade directly linked up with a particular genus living today. A way to considerably clarify how fossils fit into the extant phylogenetic pattern would have been to make a clear distinction between crown-group (i.e., the clade consisting of all derivatives of the most recent common ancestor of the living members) lineages and stem group lineages (i.e., all extinct side-branches leading to the crown-group, possessing some of the shared derived characters of extant taxa; de Queiroz and Gauthier, 1990
). For example, it was implied that the Winteraceae genus Takhtajania has a fossil record extending back 120 million years before present. Instead, this date is the approximate origin of the Winteraceae stem lineage, represented by Walkeripollis pollen grains from Early Cretaceous sediments of Gabon and Israel, which were not assignable to any modern clade (including Takhtajania, which clings to life in a few remaining cloud forests in Madagascar). Instead, the first bona fide evidence for origin of the Winteraceae crown group are pollen grains occurring some 20 to 30 million years ago from Australia that have the hallmarks of living Tasmannia (Doyle, 2000
). Similarly, the extant genus Chloranthus (Chloranthaceae) lacks a fossil record extending to 120 million years before present as well (Friis et al., 1997
).
Overall, a distinct difference between this text and previous efforts at documenting the unfolding of plant diversity over geologic time is that major events of plant history are framed in a paleobiogeographic and paleoclimatic contexts. This segues nicely into consideration of the roles that climate change and mass extinction have played in triggering or applying the brakes to bouts of evolutionary diversification. Notable examples include the environmental factors surrounding the rise of flowering plants to ecological dominance and the role of oxygen as an environmental gatekeeper of eukaryotic evolution.
Another noteworthy facet of this book is that the long-simmering partnership of paleobotany and functional biology heats up. Many examples are provided on how the evolution of plant features may have been associated with particular physiological demands. Vascular plants are well-suited for the integration of functional biology and paleontology because on land nearly all plants make their living in similar ways. There are various specializations to deal with limitation of water, light, nutrients, symbionts, and substrates, but with the exception of a few parasites, all plants gather sunlight, water, and carbon dioxide to conduct photosynthesis. As a result, there is, in general, far less uncertainty about the interpretation of functional morphology in fossil plants than there is with fossil animals.
An illustrative example of how this partnership comes into play is a discussion on the Late Devonian appearance of megaphylls from microphylls. Reviewing a study by Beerling, Osborne, and Chaloner (2001)
, Willis and McElwain point out that in the high CO2 world of the Early Devonian (when microphylls prevailed), if megaphylls had evolved, these would have been lethal to plant survival, because foliar temperatures would have been driven into fatal overheating. However once CO2 amounts decreased drastically in the Late Devonian, atmospheric conditions permitted the development of large laminae from microphylls (i.e., webbing of the space between vascular bundles) because leaf temperatures would have been pushed below fatal limits. Although I was pleased to see an appreciation of the functional domain of plant evolution, discussions like this are naked and leave the reader begging for more. What are the specific "biophysical principles of plant physiology" (p. 92) that underlie this transition? For instance, it is not discussed that these thermal limits are based on modern analog relationships (from flowering plants) between leaf size, boundary layer conductance, stomatal density, and leaf temperature. However, the appearance of megaphylls is a controversial topic and could reflect a host of other factors that are not discussed, including innovations in xylem vascular capability that enable a greater transpirational surface to be hydraulically and biomechanically supported, as well as ecological interactions. Did microphyllous plants entirely die out in the Devonian? If not, then did megaphyllous taxa evolve in the shade of microphyllous dominants such that the increase in leaf surface area reflects the evolution of greater low-light harvesting ability? The point is that by not emphasizing which topics are particularly contentious in textbooks like this, an important educational opportunity is lost. Issues like these can be used as tools to enliven discussions in the classroom and get students asking questions themselves and figuring out what kinds of evidence they need to test their ideas.
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CONCLUSION |
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TOPINTRODUCTIONCONCLUSIONLITERATURE CITED |
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500 million year history of "dead" plants and the other in functional biology of living ones, The Evolution of Plants introduces students to a research world filled with considerable prospects and does so in a readable way. As a whole, the book provides a novel perspective and good overview of the fundamentals of plant evolution. By considering head-on the roles of climate in the evolutionary diversification of plants, it is a thought-provoking book, and I suspect that it will find broad application in undergraduate courses in plant evolution. Depending on the flavor that a particular instructor may want to achieve on their own, this text would benefit from small supplemental inputs from the primary literature, in particular from the domains of plant physiology, phylogenetics, and evolution.
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FOOTNOTES |
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2 Tel.: 510-528-4070; feild{at}botany.utoronto.ca
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LITERATURE CITED |
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TOPINTRODUCTIONCONCLUSIONLITERATURE CITED |
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Beerling D. J. C. P. Osborne W. G. Chaloner 2001 Evolution of leaf-form in land plants linked to atmospheric CO2. Nature 410: 352-354[CrossRef][Medline]
Behrensmeyer A. K. S. M. Kidwell R. A. Gastaldo 2000 Taphonomy and paleobiology. Paleobiology 26: 103-147
Boyce C. K. R. M. Hazen A. H. Knoll 2001 Nondestructive, in situ, cellular-scale mapping of elemental abundances including organic carbon in permineralized fossils. Proceedings of the National Academy of Science (USA) 98: 5970-5974
Boyce C. K. A. H. Knoll 2002 Evolution of developmental potential and the multiple independent origins of leaves in Paleozoic vascular plants. Paleobiology 28: 70-100
Briggs D. E. G. R. P. Evershed M. J. Lockheart 2000 The biomolecular paleontology of continental fossils. Paleobiology 26: 169-193
Brooks D. R. D. A. McLennan 2002 The nature of diversity: an evolutionary voyage of discovery. University of Chicago Press, Chicago, Illinois, USA
Chaw S. M. C. L. Parkinson Y. Cheng T. M. Vincent J. D. Palmer 2000 Seed plant phylogeny inferred from all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Science (USA) 97: 4086-4091
Cronk Q. C. B. 2001 Plant evolution and development in a post-genomic context. Nature Reviews Genetics 2: 607-619[CrossRef][ISI][Medline]
de Queiroz K. J. Gautheir 1990 Phylogeny as a central principle in taxonomy: phylogenetic definitions of taxon names. Systematic Zoology 39: 307-322[CrossRef][ISI]
Doyle J. A. 2000 Paleobotany, relationships, and geographic history of Winteraceae. Annals of the Missouri Botanical Garden 87: 303-316[CrossRef][ISI]
Friis E. M. P. R. Crane K. R. Pedersen 1997 Fossil history of magnoliid angiosperms. In K. Iwatsuki and P. R. Raven [eds.], Evolution and diversification of land plants, 121–156. Springer-Verlag, Tokyo, Japan
Graham L. E. 1993 Origin of land plants. Wiley, New York, New York, USA
Knoll A. H. S. B. Carroll 1999 Early animal evolution: emerging views from comparative biology and geology. Science 284: 2129-2137
Lupia R. S. Lidgard P. R. Crane 1999 Comparing palynological abundance and diversity: implications for biotic replacement during the Cretaceous angiosperm radiation. Paleobiology 25: 305-340[Abstract]
Mathews S. M. J. Donoghue 1999 The root of angiosperm phylogeny inferred from duplicate phytochrome genes. Science 286: 947-950
Niklas K. J. 1997 Evolutionary biology of plants. University of Chicago Press, Chicago, Illinois, USA
Pryer K. M. H. Schneider A. R. Smith R. Canfrill P. G. Wolf J. S. Hunt S. D. Sipes 2001 Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature 409: 618-622[CrossRef][Medline]
Stewart W. N. G. W. Rothwell 1993 Paleobotany and the evolution of plants. Cambridge University Press, New York, New York, USA
Taylor T. N. E. L. Taylor 1993 The biology and evolution of fossil plants. Prentice Hall, Englewood Cliffs, New Jersey, USA
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