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Evolution's Many Branches

Douglas Erwin

Assembling the Tree of Life. Edited by Joel Cracraft and Michael J. Donoghue. xvi + 576 pp. Oxford University Press, 2004. $59.95.

Big problems often require big science. For example, synchrotrons, cyclotrons, linear accelerators and interplanetary spacecraft all cost too much for single investigators. Thus high-energy experimental physicists and planetary geologists have been forced to come up with communal approaches to research and to acquire the political skills needed to get their projects funded. The advent of farms of DNA sequencers and of the necessary computational power to make them useful has brought molecular biology into the era of big science as well with a variety of efforts to sequence whole genomes, including the Human Genome Project. Related developments include various proteomic and structural biology projects and the sprouting of new departments of systems biology, in which computer scientists and underemployed physicists mix freely with biologists. But big science is not driven exclusively by problems requiring large, expensive instruments. Innovation in instrumentation can also be a factor, opening up new avenues for exploration and reopening old questions that had been abandoned because progress was so difficult.

The effort to produce a Tree of Life—a "correct and verifiable family tree" of all life, both living and extinct—is unquestionably big science. This formidable undertaking will require the mobilization of vast numbers of systematists to acquire and analyze reams of morphological, behavioral and molecular data, and the informatics involved in the unenviable task of classifying and storing phylogenetic information is complex. The end result will show the evolutionary relationships between all organisms, from those whose diversity is still poorly explored—such as crenarchaea (sulfur-metabolizing organisms that thrive at high temperatures) and heterokonts (which include water molds, diatoms and brown algae)—to nematodes, probably the most species-rich group of animals.

What might be the justifications for constructing a Tree of Life? E. O. Wilson nominates simply having a complete accounting of life on Earth, promoting conservation, searching for new biological products and improving our understanding of community assembly (how species coadapt to live together in a given spot). Not surprisingly, Wilson makes an effective case that a Tree of Life will revolutionize ecology by marrying NASA-like technology with old-fashioned fieldwork to allow rapid characterization of broad swaths of the members of a community.

Assembling the Tree of Life grew out of a 2002 symposium (sponsored jointly by the American Museum of Natural History, Yale University, The International Union of Biological Science and the international biodiversity science program DIVERSITAS) that produced a synthesis of knowledge about evolutionary relationships among the major branches of the Tree of Life. The book is the first comprehensive scientific attempt to consider the tree of life since the publication in 1989 of The Hierarchy of Life (the proceedings of a Nobel Symposium, edited by Bo Fernholm and others).

Although we are not yet within reach of Wilson's dream, we are much closer to it than one might have expected in 1989. The "debate" between molecules and morphology, which was a centerpiece of The Hierarchy of Life, has vanished with the recognition that no one source of information can provide an infallible guide; rather, a variety of combined-evidence approaches are required. Editors Joel Cracraft and Michael J. Donoghue have two ambitious and at times contradictory aims: to demonstrate to readers outside the field of systematics the broader significance of building the Tree of Life (that is, to explain why systematists need big science) and to provide a current assessment of phylogenetic efforts across the tree.

The contributors, who include nearly 100 systematic biologists, are more successful in achieving the latter goal. There is no question that individual chapters will be useful for those seeking a meaty overview of a particular clade. Most of the systematic chapters are written by the premier experts in the area and include a brief synopsis of the morphology of the group, some anatomical highlights and comments on diversity. The better chapters include detailed and critical commentaries on previous phylogenetic analyses and discuss where they might have gone wrong; the chapter by Maureen A. O'Leary and colleagues on mammalian phylogeny is particularly noteworthy in this regard.

The early chapters on microbial phylogeny provide a wealth of information on the problems of phylogenetic reconstruction in the face of an unknown degree of lateral gene transfer and serve as an excellent primer on the evolutionary history of these groups. The differing perspectives on phylogenetic relationships offered by Sandra L. Baldauf and colleagues, by Norman R. Pace and by W. Ford Doolittle illustrate the magnitude of the problem. The chapter on early algal evolution by Charles F. Delwiche and others provides an outstanding illustration of how critical a phylogenetic perspective is to unraveling the evolutionary history of a clade. It is particularly unfortunate that the authors of many of the chapters on animal groups did not make a similar effort; many of those authors appear to view a phylogeny as an end in itself, rather than as a tool to advance other questions.

The volume is a treasury of unexpected information. Who knew that in the 1830s France imported 50 million leeches annually for medicinal bloodletting (and that the government collected a tax of one franc per thousand leeches)? And who would have guessed that the key to unraveling lepidopteran phylogeny lies in "an almost infinite variety of small, drab moths from multiple evolutionary lineages"?

Curiously, different authors in the volume appear to mean different things by the Tree of Life. Most appear to be principally concerned with the topology of the tree—with the relationships between various subclades and species that the tree depicts. Others, including Wilson, seem more concerned with taxonomic descriptions, databases of images of types and the like—an effort that has also been described as the Encyclopedia of Life. The distinction between these disparate views is critically important for identifying the scope and likely cost of the project as well as for deciding how it should be carried out. If a tree alone is the goal, the new effort at DNA bar-coding might be all that is required. One can even imagine the whole process being automated, with organisms fed into a hopper at one end and the critical sequences isolated, sequenced and added to GenBank (the genetic sequence database of the National Institutes of Health) as the biological exudates are heaped on a growing recycling pile. Of course most of the contributors to this volume are not really interested in topology alone,which would provide us with none of the critical information that accompanies proper systematic treatments—information about functional adaptations essential for understanding evolutionary pattern and process, for example.

What is missing from the volume? Understandably, most clades are not treated in much detail, with the exception of one subclade of aberrant, highly encephalized primates. Cnidarians (such as jellyfish and corals) get short shrift, which is rather unusual given that significant advances have been made recently in understanding the group. But Douglas J. Eernisse and Kevin J. Peterson, in their detailed discussion of metazoan phylogeny, do discuss the recent evidence that the calcareous and siliceous sponges arose independently. Happily, arthropods and their ecdysozoan relatives have been allotted just five chapters (some enthusiasts will doubtless be disappointed).

Ernst Haeckel's tree of lifeClick to Enlarge Image

There are two more telling omissions. Although the editors' introduction provides a very brief historical synopsis of tree-building, complete with Darwin's canonical figure from The Origin of Species and Ernst Haeckel's 1866 Tree of Life, a contribution by a historian of science on evolving approaches to the subject would have been most welcome. The second omission is more procedural or methodological: Few contributions explicitly address our current abilities to actually produce a rigorous, well-substantiated tree of life. This is far from a simple matter, as the most serious discussion of this shortcoming (in the chapter by O'Leary and others) makes clear. Building supertrees is more complicated than adding up previously published trees or building a massive character matrix. The initial steps in resolving this problem have been quite positive, but ultimately the viability of the Tree of Life enterprise requires addressing these and related methodological issues. Some might suggest that it would have been inappropriate to include such dirty laundry in this volume, but I would argue that to have done so might have gotten additional computer scientists and mathematicians interested in these problems.

Reviewers are expected to offer some platitudinous comments on the appropriate readership for a volume, although whether this is for the edification of librarians or the gratification of the publisher continues to elude me. The question is particularly apt in the case at hand, for the intended readership is as poorly resolved as some of the topologies. In their introduction, the editors note the impact of Fernholm's The Hierarchy of Life. My observations suggest that it is one of the more widely stolen library volumes, which is a sort of impact metric, and perhaps Assembling the Tree of Life will also disappear from shelves. It should probably be required reading for first-year biology graduate students, but otherwise the effort to demonstrate the significance of the Tree of Life project is largely preaching to the converted. The first three chapters (by Terry L. Yates and others, Rita R. Colwell and Douglas J. Futuyama) provide some stimulating insights into how the Tree of Life could be useful in human health, conservation and agriculture. The 26 systematic chapters are most likely to appeal to specialists in related areas and to students and teachers seeking a solid, tree-based introduction to specific clades.

So why do we need to construct the Tree of Life? Because ultimately it is, like a synchrotron or a spaceship, a tool that will allow future generations of scientists to address a whole new set of questions—about the ecological and evolutionary processes that have produced the diversity of life on Earth.

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