There are many natural questions that arise when considering the evolution of plants: Why did seeds evolve? How did the three separate genomes evolve in plants? How and why did plants evolve from aquatic habitats to terrestrial ones? Why do leafy plants have the leaf arrangements that they do? What is the average time scale needed for the evolution of a new plant species? What are the largest plant species that have yet evolved? How common is horizontal gene transfer in plants? What evolutionary advantages are there in pollination? From the standpoint of molecular biology, why do plants have the particular morphology that they do, as contrasted with other forms that seem plausible with respect to physical laws but do not occur? How extensive is the plant fossil record? Can the evolution of plants, indeed of living organisms in general, be simulated on a computing machine?
These questions, among many others, are addressed in this superbly written book, which despite being targeted towards readers with an advanced knowledge of botany can still be read by anyone curious about the subject matter. Unless the reader is an expert in evolutionary biology (which this reviewer is not), it would be difficult to assess the accuracy of the subject matter as compared to other works. The author does include however many references that can be consulted if readers find it necessary to gain more details on a particular topic. In addition to the quality of the writing, there are numerous diagrams and figures that illustrate the important principles. The inclusion of diagrams in any book on botany is of course a must, given the diversity of plant morphology. For readers with a background in modeling and simulation, the author includes a highly interesting discussion on how to simulate plant evolution by using computer-generated "adaptive walks" on "fitness landscapes". Simulations of course are not a replacement for sound and painstaking experimentation and scientific hypothesis building, but they can serve as a guide to understanding, at least in a general sense, of what is possible in biological evolution. In order to really appreciate the discussion on adaptive walks, the reader will need a fairly strong background in modeling and simulation, even though the discussion is purely descriptive, with no explicit mathematical formalism put down on paper.
The book is dense, being packed full of interesting information, demands the reader frequently back up and take pause so that the information can be assimilated more effectively. But the author's writing style is concise enough to keep the book at a manageable size. The different views on evolution, most of these coming down to the time scales over which changes are occurring, find their place in the book. The Darwinian view, which of course is the predominant one in the scientific community, is referred to as 'phyletic gradualism' in this book, and encapsulates the view that evolution is essentially an adaptive walk over a fitness landscape, driven by natural selection. One other view, called 'punctuated equilibrium', is at first glance a somewhat radical hypothesis, for it allows one to drop the requirement for intermediate phenotypes and view evolutionary change as "hopscotching" (in the author's words) from one fitness peak to another. The view of punctuated equilibrium is no doubt attractive to those who are wondering why the intermediate phenotypes are frequently missing observationally. Whichever of these viewpoints is closer to the truth, the wide variability in plants is quite amazing, over and above the case for other biological lifeforms in the opinion of the author. He refers to this as 'phenotypic plasticity' in the book, and alludes to the high rate of phenotypic innovation in some time periods. The concept of phenotypic plasticity is interesting for it allows a more quantitative measure of the degree to which changes are possible, i.e. a measurement of the impediments to evolutionary changes.
When contemplating the mechanisms of evolution it is easy to fall into the trap of believing that the morphology and functioning of an organism is the result of some sort of optimization process. The marvelous ingenuity of plants in dealing with their environments and their ingenious methods of reproduction sometimes begs for an explanation that is purposeful or goal-directed. There is no reason to believe however that the current morphology and functioning of a plant is the result of adaptation through natural selection. The author's view of adaptations is that they are specific to particular environmental contexts, namely that they are features that allow biological organisms to survive under very specific environmental conditions. In addition, any benefit that an organism obtains from an adaptation must assessed in relative terms. It would not be appropriate therefore to view a particular adaptation for a particular organism in a particular environment as being appropriate to another organism in another environment, even though the environments to both are similar enough that they tempt one to believe that the adaptations can be compared meaningfully. Of course, adaptations can only work by genetic transmission from one generation to the next, and there is no guarantee that they will remain efficacious for all future generations of the organism. An adaptation the author argues, is only a set of features that increases the probability that the organism will survive or reproduce successfully for a specific environment. It is natural to ask at this point whether if given a particular plant one can ascertain whether a certain feature is adaptive or not. The author is aware of this difficulty, since it requires the identification of the selection pressures that underly the functioning of the proposed adaptation. The resolution of this problem requires years of careful experimentation and observation, a course of activity that has characterized and will continue to characterize sound science.