My previous forays into botany have not been particularly successful: I have found it hard to swallow large doses of functional anatomy and descriptive taxonomy. But The Evolutionary Biology of Plants demonstrates the power of an evolutionary perspective to make sense of otherwise unconnected facts. Not only did I learn from it an immense amount of botany (including much of the anatomy and taxonomy that I had never previously retained), but I also found it a source of fresh insight into evolutionary biology. Indeed these things are woven together so tightly that I wonder now that anyone could try to make sense of them in isolation.
The Evolutionary Biology of Plants is a general review of central topics in plant evolution, which assumes no prior botanical knowledge and opens with a general introduction to evolutionary biology. Chapter one covers such basics as natural selection, variation, norms of reaction, fitness landscapes, adaptation, and the testing of adaptive hypotheses using cladistics. Chapter two covers species definitions and speciation: allopatric and sympatric speciation, reproductive barriers, bottlenecks and founder effects, monogenic speciation, regulatory genes and heterochrony, and punctuated equilibrium. This material is an excellent introduction to evolutionary theory, pitched at a level suitable for undergraduates with a general biology background. It is illustrated with botanical examples and stresses elements often neglected in primarily zoological approaches, such as reticulate evolution (hybridisation) and polyploidy.
Part two is an account of the key events in the history of plants. Chapter three deals with the origins of life, of photosynthesis, of the eukaryotic cell, of sexual reproduction and meiosis, and of multicellularity and the plant life cycle; and chapter four with the embryophytes and their invasion of the land, and the appearance of vascular conducting tissue, seeds, and flowers. Niklas presents phyletic hypotheses and taxonomic classifications alongside explanations of the relevant functional biology, without assuming the reader is familiar with either. Understanding of the biochemistry of photosynthesis or the function of an archegonium is not necessary, for example, and nor is knowledge of the taxonomic position of the mosses — these things are explained as necessary.
Part three combines models of fitness landscapes with computer simulations of adaptive walks to produce hypotheses about the evolution of plant morphology. Niklas focuses on simplified systems where the constraints come from biomechanics and biophysics. In chapter five he models unicellular morphology (size, shape, and geometry) and multicellular structure in aquatic environments, where light harvesting efficiency is critical, and explores the changing constraints imposed by semi-aquatic and then terrestrial environments. On land, chapter six models branching structure in vascular plants (balancing light interception, spore dispersal, and mechanical stability), the appearance of Fibonacchi sequences in the distribution of leaves, stem and stele structure (under mechanical constraints), and early seed structures (with selection for protection and wind dispersal).
Part four deals with the modes and tempos of plant evolution. Chapter seven explains divergence, convergence, and parallelism in evolutionary history, and the key concepts of analogy and homology. Niklas' major case study is convergence in arborescence (tree growth), with analagous stem structures appearing in different lineages, under similar selection pressures and the same biophysical constraints. More briefly, he also examines the evidence for convergence on heterospory and the seed habit. Chapter eight turns its attention to the tempos of evolution, and particularly the rate of change at the molecular level. Niklas describes the organisation and evolution of mitochondrial, chloroplast, and nuclear DNA and compares their rates of change across different taxa. He also touches on molecular clocks and the neutral theory. Finally he looks at extinction and speciation, both at their genetic foundations and at their long-term patterns in the paleobotanical fossil record.
August 1997
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