[Paleopsych] Sigma Xi: Evolution's Many Branches

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Evolution's Many Branches

    [28]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

    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

    [29]click for full image and caption
    [30]Ernst Haeckel's tree of life

    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

    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.

Reviewer Information

    Douglas H. Erwin is a senior scientist and Curator of Paleobiology at
    the National Museum of Natural History of the Smithsonian Institution,
    and an external faculty member at the Santa Fe Institute. His new book
    on the end-Permian mass extinction, Extinction: How Life Nearly Died
    250 Million Years Ago, will be published by Princeton University Press
    in the fall.


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