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From DNA to Diversity, Molecular Genetics and the Evolution of Animal Design

Posted on: Tuesday, 1 July 2003, 06:00 CDT

Sean B. Carroll, Jennifer K. Greiner, and Scott D. Weatherbee. From DNA to Diversity, Molecular Genetics and the Evolution of Animal Design. 2001. Blackwell Science, Malden, Massachusetts, 214 + xv p.

From DNA to Diversity serves as an Evo Devo Intro for Paleo-an accessible, accelerated introduction to the field of developmental biology. The typical paleontologist today did not learn the concept of Hox genes in his or her undergraduate introductory biology course (let alone a campy vocabulary of hedgehogs, zinc fingers, gurkens, and knirps). In their book Carroll, Griener, and Weatherbee review the exciting rejuvenation that evolutionary biology has experienced in the past decade. This colorfully illustrated book provides easy access to the fundamentals of rapidly changing components of evolutionary biology, genetics, and development. This is especially welcome to a paleontologist who wishes to expand his or her knowledge of the field without delving into the depths of the primary literature.

The subject matter flows from an introductory overview of the diversity and fundamental divisions of life, through a history of genetics, the chronology of which naturally moves from simple to more complex concepts. A key success of the book for communicating to nonspecialists is in the figures, which are presented in a manner somewhere between a cartoon and a technical drawing. This style is not only helpful in creating a perception of accessibility (i.e., inviting), but the figures are invaluable for the information that they convey in support of the text. The glossary is an essential resource, but assumes a fair degree of prior knowledge of biology and is often self-referential. Geologists may find several minor errors in application of geologic lime units (e.g., the Ordovician Period "immediately preceding" the Devonian) distracting.

The leaping strides that have been made in our understanding of the functional relationships between developmental biology and genetics are largely the result of recent advances in laboratory methods. The expansion of our conceptual understanding of this continuum (molecules to morphology) is dynamic and ongoing. It is evident from examples in this book that today's broadly visioned geneticists and developmental biologists not only welcome, but depend on, insights from paleontology, systematics, and morphology. Beyond simply providing an understanding of how factors and mechanisms involved in relationships among genes, development and the phenotype, the integrated approach outlined in this book yields new, fundamental principles for our understanding of body plan organization, diversity, and disparity from the lowest to the highest taxonomic levels.

Using now-classical examples from the body plans of model organisms, Drosophola (arthropod), mice and zebra fish (vertebrates), yeast (monerans), and C. elegans (nematode), Carroll and his co-authors chronicle one of the most important advances in the field of evo-devo: the recognition that hierarchical processes were involved with the implementation of the genetic code. For example, the genes that encode proteins required for all of the basic functions in cells are known as "housekeeping genes." Their origin predates the origin of multicellular life (c. 1 Ga). Such genes are not, however, responsible for macroevolutionary differences in body plans among metazoans. Hox genes regulate the identity of body regions within an organism. The concept of regulation is important because Hox genes do not "create" body parts: they serve to control and regulate the timing, placement, and degrees of development. Thus, Hox genes are able to establish anterior-posterior gradients of activation and represser proteins. When this is combined with independently controlled dorsal-ventral gradients, the combined gradients provide a three-dimensional coordinate system for the placement of organs and structures at early stages of development.

Small changes (mutations) in the regulatory Hox genes can produce dramatic change to body plan in a single generation. Several examples are summarized by Carroll et al., including the now- classic manipulations of fruit flies that produce chimera such as legs in the place of antennae, extra sets of wings, and inducement of eye cell formation on various parts throughout the body. Thus major changes in body plan (phenotype) are possible over very short lime intervals when homeotic, regulatory genes are involved. The authors do an admirable job of summarizing the complex process involved in gene expression, e.g., multiple genes controlling the same function: hierarchical control processes with one gene dependent on the activity of another, which itself may serve as an "on-off" switch for several levels of a genetic hierarchy.

Advances in this field allow Carroll and his co-authors to address classical questions in evolutionary biology. For example, why there is little correlation between the absolute amount of DNA in a metazoan and the apparent complexity of that organism? Why is there so much redundant or nonfunctioning genetic code in each organism? What does this shared (or divergent) DNA tell us about the timing of major evolutionary events such as the origin of bilaterians and chordates? Why do seemingly distantly related forms share so much DNA (in the fruit fly Drosophola, at least 38 of the 44 known proteins that regulate transcriptions of genes have homologues in vertebrates)? What are the relationships between genetic diversity and body plan complexity? (e.g., expansion of the genetic tool kit through geologic time has increased complexity but not taxonomic diversity).

The only major disappointment that I encountered with this book arises more from a gap in our knowledge than from a weakness of treatment by the authors. The question of how genetic diversity related to body plan complexity (its significance or lack thereof) is left wanting an answer from the paleontological perspective. In the past 15 years, paleontologists have instilled specific biological meaning in the difference between the concepts of diversity (a measure of the number of different species or other taxonomic level) and disparity (a measure of the degree of morphological difference among members within a clade, defined by distance in morphospace or number of steps within a phylogenetic tree). What is the evolutionary significance of a clade/fauna with high diversity-low disparity relative to a clade/fauna with low diversity-high disparity? Fundamental questions that deal with evolutionary constraints and limits on body plans (genetic, developmental, and constructional) are common concerns to all students of biological evolution, regardless of whether the investigation originates from the morphological or molecular side. The answers are important to our perception of biodiversity and evo lutionary rates. To their credit, Carroll and his co-authors clearly acknowledge the importance of the disparity-vs.-diversity question, but seem unsure what to do with it in regards to integrating these concepts with the fundamental idea of the Genetic Toolkit. This should alarm paleontologists. The disconnect between paleontologists highlighting the macroevolutionary significance of diversity vs. disparity, and the inability of leading developmental biologists to understand their message, let alone to integrate it into a postmodern synthesis, cries out for increased dialogue among all evolutionary (paleo)biologists. The time is ripe for concepts of diversity and disparity to be integrated and reconciled with the rapid advances in developmental biology and paleontology.

From DNA to Diversity is not a "Developmental Biology for Dummies." The subject matter is dense, especially for one playing catch-up. However, even with only a superficial understanding of the biochemical details involved, paleontologists will be engaged by the fundamental processes of evolutionary biology that are addressed. Even without directly incorporating the details of this book into paleontology courses or research programs, the ideas that this book conveys will freshen any paleontologist's perspective on the natural world.

J. Paleont., 77(3), 2003, pp. 597-598

Copyright (C) 2003, The Paleonlological Society

0022-3360/03/0077-597$03.00

STEVEN J. HAGEMAN

Department of Geology

Appalachian State University

Boone, NC 28608

Copyright Paleontological Society May 2003

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