An introduction to phylogenetic theory and methods in this vein remains to be written — the closest we have is perhaps a textbook like Page and Holmes' Molecular Evolution — but Avise's Evolutionary Pathways in Nature offers a broad survey of phylogenetic applications. Summarising recent research and moving from general context to focus on results, it is pitched at biologists working outside phylogenetics, but it is also a good fit for students, potential students, and generalists like myself.
Evolutionary Pathways explores applications of comparative phylogenetics to natural history, ethology, biogeography, taxonomy and other areas of biology. Each of its seventy essays follows a similar pattern, giving some background on a group of organisms, posing a question about their evolutionary history, and attempting to answer that question using phylogenetic analysis. The essays are independent, with just the occasional cross-reference, and can be read in any order.
At the heart of each essay is a phylogenetic tree, taken from the most recent studies and illustrated with a figure drawn for this book. How these trees were obtained is not covered, however: the captions offer only such information as "from mtDNA gene sequences" or "based on DNA sequences from four nuclear genes" and there is no discussion of phylogenetic methods. Nor is there any detail about the matching of phenotypic characters to extant species and the estimation of the most likely states at ancestral nodes, though an appendix explains the basic theory of phylogenetic character mapping.
The substance ofEvolutionary Pathways is not in phylogenetic theory but in its applications. The sections into which the essays are grouped give a feel for its reach: anatomy and morphology, body coloration, sex and reproduction, behavior and ecology, cell biology and physiology and genetics, and biogeography. And it ranges from theoretical insights in these domains to more isolated facts that are nevertheless striking.
There is one essay on bacteria, exploring the origins of magnetotaxis, one on the origins and spread of the HIV virus, one on lichens, and one on the base of the tree of life, looking at DNA repair mechanisms. And there are three essays on plants: on the coevolution of monkeyflowers with hummingbirds, on overseas plant dispersal in the Australian genus Scaevola, which appears to have reached Hawaii four times independently, and on lateral gene transfer between parasitic Rafflesia plants and their hosts. The other sixty essays span a huge zoological range, from shrimp to whales and from Australia to the Arctic.
Some of the conclusions are of relatively narrow taxonomic interest. New Zealand's giant extinct Haast Eagle was more closely related to the small Hieraaetus eagles of Eastern Asia rather than the more obvious candidate, the large and geographically closer Australian Wedge-tailed Eagle. And toucans are not just related to barbets, but are actually embedded within the barbet clade. Other taxonomic questions are broader: one essay reviews the latest work on the relationship of microbats, megabats and primates.
Evolutionary Pathways covers more than taxonomy, however, with essays that range right across biology, often touching on broader theory as well as addressing specific questions. Sexual dichromatism and monochromatism in bird colouring. Dollo's law, which says that complex adaptations once lost are never regained in the same form. The possibility of a non-monophyletic species, resulting from chirality-controlled mating in Japanese land snails Euhadra. A possible case of Müllerian mimicry in the poisonous Pitohui birds of New Guinea. Oviparity and viviparity in lizards. Placentas in fish. Convergent evolution in the stomach lysozymes of vertebrate foreguts. And much, much more.
A few excerpts to whet the appetite:
"At least some parthenogenetic vertebrates can experience considerable ecological success despite their extinction-prone clonal nature. However, molecular phylogenetic analyses have confirmed that, in nearly every case, any ecological good fortune that a unisexual biotype might enjoy is evolutionarily fleeting."
"the category 'lichen,' although ecologically meaningful, is not a coherent unit in terms of phylogenetic origins. ... the lichen lifestyle is a derived condition that emerged from multiple types of ancestral association, and ... the progression is not always from aggressive parasitism to friendly mutualism."
"Jamaican land crabs evolved their diverse adaptations for non-marine life and complex brood care over the relatively short evolutionary span of just a few million years. By contrast, pairs of marine crabs in the Atlantic and Pacific Oceans, isolated by the Isthmus of Panama for roughly the same amount of time, have remained ecologically and morphologically very similar to one another. Thus, newly available terrestrial habitats in Jamaica ... must have opened novel ecological opportunities that provided an evolutionary impetus for rapid diversification"
Evolutionary Pathways in Nature is not popular science in the narrow sense. It makes no explorations into biography and touches only very occasionally on the history of science. It is concise, focused, and not at all repetitive. Neither the title nor the term "phylogenetic character mapping" are attention-grabbing and there is no attempt to simplify technical terminology, though a glossary is provided. And, perhaps most significantly, Evolutionary Pathways is not published as popular science: it is a large format work printed on high quality paper, with a recommended retail price that puts it outside popular reach.
On the other hand, such brief, independent essays obviously can't go into any technical detail and there's nothing in Evolutionary Pathways that a keen reader of popular biology or a bright high school student couldn't follow. Avise's enthusiasm is obvious and the material is highly accessible, with most of the essays on vertebrates and many on "charismatic megafauna" — elephants, polar bears, yetis, and so forth. The background information is often intriguing in its own right and many of the conclusions have a strong "wow, did you know that ..." element. And, though there are no photographs, there are attractive line drawings of a few of the animals discussed. Evolutionary Pathways in Nature can be appreciated simply as an exploration of "the marvelous workings of the natural world".
So in some ways Evolutionary Pathways sits awkwardly between two stools: general readers may find it daunting or dry, while specialists may find it lacks depth and detail. There are, however, plenty of potential readers in between those groups.
Many readers of popular science want more science and less digression. In the hands of a Stephen Jay Gould, each of Avise's topics would have become an essay a dozen pages long, offering insights into the history and philosophy of science with literary aplomb. That would have made a fine volume, but it would also have been three times the length without offering any more actual science. And in the hands of less skilled and knowledgeable popularisers this kind of expansiveness can easily descend into vague, unfocused and inaccurate meandering.
It is still possible to do a degree in biology without being exposed to much phylogenetics. And increasing specialisation means that even research scientists need non-technical reviews, summaries, and overviews to keep up with work outside their fields. Here, as a showcase of the ways in which "comparative phylogenetic perspectives can contribute to the process of biological discovery", Evolutionary Pathways may help biologists understand what phylogenetics has to offer their own research. There are no notes in the text, but the bibliography lists the sources for each essay separately; there are also pointers to works on theory and methods.
Avise's approach is not necessarily appropriate for every subject, but it is one I think should be considered more often. If scientists can step back a little, distill key ideas and results, and write clear and lucid prose, then they can make science accessible to a broader audience without having to take on literary ambitions, become biographers and historians, or dumb down their material.
Note: this review was commissioned by and first appeared in Systematic Biology. Thanks to review editor David Morrison for suggestions.
May 2007
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