The field of `evo-devo', the study of evolutionary developmental biology,is one of the most exciting in modern biology. Its appeal lies in the promise of delivering answers to the age-old question of where did we come from? The answers come from not one but two perspectives, which are intimately related but at two entirely different time scales. How did we, and the fascinating myriad of organisms we share this planet with, develop from embryos? And how did we reach our dramatically different morphological forms and functions via evolution? That evolutionary forces must act on developing embryos to produce differing body plans seems obvious, and yet the modern synthesis of evolutionary and developmental biology is a relatively recent phenomenon sparked by the discovery of genetic conservation across taxa. The evo-devo field is currently moving forward in leaps and bounds, and although there are limitless questions left to answer, some general principles have already emerged. The time is thus ripe for an accessible text on evo-devo that will be appealing and informative to a wide readership.
This challenge has been taken up by Sean Carroll, an evolutionary developmental biologist from the University of Wisconsin, whose own work has played a central role in putting evo-devo studies on the map. Carroll's new book is titled Endless Forms Most Beautiful, a quote from the final paragraph of The Origin of Species(Darwin, 1859) and it nicely sums up Carroll's attitude to his subject. The book is easy to read with a well-balanced combination of personal anecdote and hard science, is not too long, has beautiful color plates and is very reasonably priced. It is enjoyable because of the palpable enthusiasm and excitement that Carroll brings to his subject. His appreciation for the beauty of the natural world in all its infinite variety is contagious, and his desire to explain to us how this variety arose keeps the pages turning rapidly. Although we did find some aspects of Endless Forms frustrating, because of an overly narrow focus that led to missed opportunities, we nevertheless felt that, on the whole, it is a successful book with much to commend it.
Endless Forms is essentially a popular science book with a unique emphasis on developmental biology and the genetics of evolutionary change. It has been some time since any books that fall into this general niche were published: Kenneth McNamara's Shapes of Time(McNamara, 1997) and Carl Zimmer's At the Water's Edge (Zimmer, 1998) being the most obvious contenders. Carroll's book gives the reader an exciting picture of the resurgence of comparative developmental biology following the discovery of the unexpected conservation of Hox genes across taxa in the 1980s. The potential of developmental biology for solving evolutionary conundrums is enthusiastically conveyed, and Carroll gives a concise, informative history of the modern merging of these two disciplines. However, he defines evo-devo narrowly as `the comparison of developmental genes between species', thus giving his description of this new synthesis of evolution and development a decidedly genetic focus. As a consequence much of this book deals with the respective roles of gene products (`tool kit genes') versus cis-regulatory regions (`switches') in generating diversity.
The book is organized around four main questions: (1) what are the rules for generating form; (2) how is the information encoded; (3) how does diversity evolve; and (4) what explains large-scale evolutionary change? In the first half of the book, questions 1 and 2 are addressed through a fast-paced tour of developmental and molecular biology. It is difficult for us to gauge exactly how easy these chapters would be to follow for a complete novice, but they are certainly written in an accessible way, and Carroll tries hard to use lay-terms and analogies, rather than to introduce too much specialized terminology. His embryos have longitude and latitude rather than animal-vegetal and dorsoventral axes, and regulatory elements are termed`switches' that act as `GPS integrators'. For the most part these devices work well, although just occasionally they seem more likely to obfuscate than clarify: a description of limb axes that includes `top (back)' and `back(pinkie)' in a single sentence had us tying our fingers in knots. Remarkably,in just a couple of short chapters the book manages to effectively summarize all the major themes that a standard undergraduate developmental biology course is likely to touch on. This is followed by `The Dark Matter of the Genome', a chapter using the pleasing cosmological analogy of non-coding DNA as dark matter of the universe to lead into a fairly detailed discussion of transcriptional regulation. Developmental biologists should not expect any revelations from these chapters, but they do provide some excellent fodder for teaching development to students who are not majoring in biology. This first part of the book on the making of animals succeeds well, and sets up the reader for the evolutionary biology to follow.
The second half of the book tackles the questions of diversity and evolutionary change from the new evo-devo perspective. We found this section of the book to be less consistent in quality. A chapter on human evolution owes much to a previous article of Carroll's, and is rather disappointing in its lack of a true evo-devo perspective. We think a discussion of heterochrony here could have helped to broaden the book as a whole. By contrast, the discussion of melanism works much better, and the chapter on butterfly spots is one of the most fascinating pieces of popular science writing that we've read; indeed, it can stand alone as a primer on mimicry.
The chapter focusing on the modification of appendages, an area where Carroll's own work has had a major impact, is nicely done and right up to date, including exciting ongoing work in the Kingsley laboratory on sticklebacks. However, one of our primary criticisms of this part of the book is that it tends to give us the evo-devo world according to Carroll. That is a world controlled almost exclusively by changes in cis-regulatory elements. Studies in this area, many from Carroll's own lab, have been both successful and enlightening; but they do not provide a full story. There is no reason to suppose that important evolutionary changes at the genetic level will not be uncovered wherever they are looked for. As our understanding of gene regulation and protein function becomes more sophisticated, we will find evolution at every level of gene activity. In addition, whereas Carroll downplays changes in coding sequences, because pleiotropy makes it difficult for proteins to change while remaining functional, he de-emphasizes the role of gene duplication and other mechanisms in releasing genes from these constraints to allow new functions to evolve.
Carroll's focus on `tinkering' with existing morphology via changes in cis-regulation, and on the somewhat obscure Williston's law of specialization and reduction in serial homologs, does not lead him to ignore larger scale changes and the acquisition of novelties. However, we finished the book feeling that these areas remained under-explored. For example, there is little to complain about in Carroll's description of the Cambrian explosion, but it neglects to discuss how the different phyla, and their already complex array of `tool kit genes', arose in the first place. Sadly, despite Carroll's consistent attempts to use evo-devo to dispel intelligent design (ID) ideas,this omission can even be misinterpreted as fuel for the ID `theory': a review of the book at Amazon.com from an ID proponent goes so far as to pose the question of `Who put the tools in the tool kit?'
In parts of this second section of the book, we felt that the reader is left to guess at how evolution works. Carroll pronounces that evolution always proceeds by incremental gradual change, and does not present the ongoing debate of the relative importance of gradualism versus punctuated equilibrium. It is difficult for the reader to reconcile the origin of novelties with the idea that change is always incremental. For evolution of human characters such as brain size, we are told that there was likely to be `selection for variants of many genes, responsible for small increments of differences...over sustained intervals of many thousands of generations'. Yet evolution of human brain size during the Pleistocene Epoch is described by Carroll as occurring through two rapid phases of expansion punctuated by a period of stability. The reader is left to try to connect the dots between the pattern seen in the fossil record and evolution proceeding in small increments. The concept of the hopeful monster is introduced, then quickly discarded as a specter vanquished by evo-devo. Here, we think, Carroll does Richard Goldschmidt a disservice. Although much of Goldschmidt's The Material Basis of Evolution has proved to be wrong, his discussion of macroevolution and its relationship to changes in developmental timing, embodied in the concept of the hopeful monster, has some enduring value. Unfortunately, Carroll doesn't mention Goldschmidt, and anachronistically discusses hopeful monsters in terms of Bateson's work on discontinuous variation(Bateson, 1894). Although Carroll shows us how evo-devo has identified genes that allow or cause large morphological changes, he then tells us there is no need for nature to make jumps to produce diversity. What does explain large-scale evolutionary change?Ultimately, this question is not fully addressed.
Despite our criticisms, we feel that this book fills a much-needed gap. If you are a developmental biologist tightly focused on a single model organism and wondering what evo-devo is all about, then we highly recommend the non-developmental chapters of Endless Forms. Carroll suggests that the book is appropriate not only for students and educators, but also for those interested in complexity and for the large number of natural history buffs. The book should appeal not only to these readers, but also to a wider audience, and its conversational tone, together with its clever use of photographs and diagrams, will augment their enjoyment, as well as their understanding.