Hand of Man, Wing of Bat, Fin of Porpoise

The Evolution of Developmental Pathways By Adam S. Wilkins Sunderland, MA: Sinauer Associates, Inc. (2002). 603 pp. $54.95Biologists have always been fascinated by the astonishing diversity of metazoan life that has evolved on Earth. It is now evident that extant species have evolved from common ancestors through genetic changes that are acted upon by natural selection. In The Origin of Species (1859), Charles Darwin discussed the “law of embryonic resemblance.” He and others before him had noted that plants and animals within the same great classes, though morphologically diverse in their adult forms, were remarkably similar in their embryonic forms. For example, the limbs of vertebrates, including “the hand of a man, wing of a bat, and fin of a porpoise,” are morphologically and functionally distinct, yet they all develop from morphologically identical limb buds in their embryos. Darwin suggested that the embryos of different species provided a glimpse of a common parent for the different classes of organisms, supporting his concept of descent with modification. Thus was born the field of evolutionary developmental biology.Since its birth, evolutionary developmental biology has generally been split into separate fields of study, including evolution, systematics, comparative anatomy, embryology, and population genetics. Relatively recently, the so-called “Evo/Devo” field has emerged again, reintegrating these disciplines. At this moment in time, the Evo/Devo field appears to have come of age. There now seems to be a critical mass of scientists focusing on this fascinating area of biology who are utilizing modern approaches to understand how developmental pathways evolve to create plant and animal diversity. In the last ten years, a subset of developmental biologists have begun moving beyond the few primary model organisms, realizing the inherent limitations of studying a handful of organisms, to understand how developmental mechanisms are modified to achieve diversity. By necessity, this has led to new and productive collaborations between developmental, evolutionary, and comparative biologists. Some may have noticed that within the last year there have been at least three international scientific meetings (Chicago, York, and Cold Spring Harbor) that focused exclusively on the evolution of developmental mechanisms. In addition, there are now multiple scientific journals including Genes, Development, and Evolution (Springer), Evolution and Development (Blackwell Science), and genesis: The Journal of Genetics and Development (Wiley) that actively solicit Evo/Devo manuscripts. Even constrained journals such as Development (The Company of Biologists) and Developmental Biology (Academic Press) have become more active in publishing studies utilizing diverse organisms. The National Science Foundation in America has consciously nurtured (funded) the Evo/Devo field (see http://www.nsf.gov/pubs/2001/bio012). It now appears that biomedical agencies and disease-focused foundations are also funding Evo/Devo studies, presumably because they recognize that such studies provide unique insights into human biology and disease. These are very encouraging times for the Evo/Devo field.The Evo/Devo field necessarily requires the study of developmental processes between species. Obtaining embryos from diverse species can be a limiting factor. In contrast, it is relatively easy to obtain DNA from plants and animals, and a number of recent DNA-based technologies have helped open up the Evo/Devo field. Thus, DNA can be used to generate genomic libraries (phage- or BAC-based) or to clone orthologous genes by PCR. Automated sequencing projects then allow for comparisons between coding and intergenic regions. These DNA-based projects are beginning to reveal how plant and animal genomes have evolved. New molecular technologies also make it possible to generate cDNA libraries from relatively few embryos. This, in turn, can lead to sequencing projects generating expressed sequence tags (ESTs) to isolate and compare reference developmental control genes between species. Certain descriptive methods that are very familiar to developmental biologists can also be readily applied to the embryos of other organisms, including whole-mount RNA in situ hybridization and whole-mount immunostaining. Encouragingly, the Evo/Devo field is responding to valid criticism that it is primarily a descriptive science by beginning to push beyond description toward functional tests.Tests of gene function can be potentially very difficult, especially if the organisms under study are not amenable to genetic manipulation. Two recent technologies have facilitated gene knockdown approaches in diverse species. Morpholinos are modified antisense DNA oligonucleotides that inhibit gene expression at the level of translation. Morpholinos have been shown to work well in zebrafish and Xenopus, and apparently now in other diverse species. Double-stranded RNA also known as RNA interference (RNAi) has been shown to eliminate gene activity in C. elegans and, more recently, in other invertebrate species, including flies and planaria. In many cases, experimental embryological methods established in model organisms can also be applied to related species.Recently, a number of Evo/Devo books have been published, including The Shape of Life by R.A. Raff, (1996), Cells, Embryos, and Evolution by J. Gerhart and M. Kirchner (1997), Evolutionary Developmental Biology by B.K. Hall (1999), Genomic Regulatory Systems by E.H. Davidson (2001), and From DNA to Diversity by S.B. Carroll, J.K. Grenier, and S.D. Weatherbee (2001). The new The Evolution of Developmental Pathways by A.S. Wilkins is another addition to the text resources available to individuals interested in the re-emerging field of evolutionary developmental biology.The Evolution of Developmental Pathways is a thick (603 pages), densely written, admittedly zoocentric, scholarly book that is well suited for use as a text for upper level undergraduate and graduate level courses in evolutionary developmental biology. This book is clearly not intended for non-biologists. Those who will benefit most from reading this book are evolutionary and comparative biologists wanting to learn more about developmental biology and developmental biologists wanting to learn more about evolutionary and comparative biology.The Evolution of Developmental Pathways is divided into three major sections. The first section, Context and Foundations, provides introductory material to help bring the reader up to speed on the history of the Evo/Devo field, the fossil record and molecular comparisons of developmental evolution, the concept of genetic pathways and networks, and the evidence demonstrating the existence of conserved genes and functions in animal development. The second section, Case Studies in Pathway Evolution, focuses upon well-characterized “reference pathways” in developmental biology, including sex determination in flies and mammals, insect segmental patterning, and organogenesis of the nematode vulva and tetrapod limb and compares these processes with other species. This section may be particularly interesting to those wanting to enter the Evo/Devo field because it highlights the value of using reference pathways to analyze the evolution of development in related species. The final section, Conundrums, focuses upon the current questions and problems in the field. This is perhaps the most interesting part of the book. Notably, the author presents the information “in the spirit of a postgraduate student seminar” to spark ideas and discussion. Readers will also appreciate the extensive literature cited in the references.A disappointing aspect of The Evolution of Developmental Pathways is the lack of any color illustrations. Colorfully illustrated books in this field such as Cells, Embryos, and Evolution and From DNA to Diversity reveal the intrinsic beauty of organisms and their embryos and create an excitement about this area of research. The lack of color illustrations in The Evolution of Developmental Pathways, however, makes it somewhat dry and may not inspire excitement in the reader. I imagine that the decision not to include color illustrations was to keep the price of the book low in the range of comparable Evo/Devo books. While this is an admirable sentiment, the design of this book places it squarely in the category of traditional college textbooks and therefore might limit its appeal to a larger audience. All in all, I suspect that I will be revisiting the chapters of The Evolution of Developmental Pathways many times to learn more about this field.Finally, the current movement in the Evo/Devo field suggests a bright future. That future may be driven by those children who watched natural history programs on television and have been inspired to pursue studies and careers in biology. I predict that these young biologists will not be satisfied studying a handful of primary model organisms. I suspect that these enlightened individuals will have broader interests and will be naturally attracted to the Evo/Devo field to reveal the “hidden bond” described by Darwin that exists between common ancestors and current species.