ENDLESS FORMS MOST BEAUTIFUL: THE NEW SCIENCE OF EVO DEVO AND THE MAKING OF THE ANIMAL KINGDOM

This timely and pleasingly illustrated summary of the field of the "evolution of development" (evo-devo) by University of Wisconsin-Madison, Professor of Genetics, Sean Carroll will likely be favourably received by most evolutionary biologists and paleontologists.

This useful review begins with ruminations on our 'attraction to and fascination with animal form [which] has motivated the greatest naturalists for centuries." This desire for explanation and insight into essential organising principles of morphology undoubtedly represents a perennial goal of biology and the main objective of evo-devo and this book. My reservations about the book, which I shall touch on later, are that it overemphasises "the evo-devo revolution" with exaggerated heroic rhetoric, and insufficient acknowledgement of precursor biological disciplines such as embryology and heterochrony. It also over-employs mechanistic metaphors about tool kits, master genes, control mechanisms and switches.

The main messages is that biologists have been 'surprised and delighted to find generalities' about form that extend deeply into the 'genetic machinery of development.' With this Carroll proceeds to share his conviction that it is all 'ultimately' about genetics and the 'power of natural selection.' He reviews the genetic tinkering that created mutant fruit flies, and introduces the homeobox which has shown us that clusters of homeotic genes (that code for 'similar' morphologies) are arranged along chromosomes in anterior to posterior sequences corresponding to the relative order of body parts. (As in the macrocosm so in the microcosm)! This discovery while significant simply underscores the relatedness of all life, as demonstrated in previous evolutionary studies. Homologous genes, identified in groups as different as mice and fruit flies, create the paradox of striking genetic similarity hidden by morphological differences (as so often cited in the case of chimp and human). Carroll discusses master genes (those, including hox genes with key functions in relation to building critical organs like eyes and heart) and 'tool kit' genes: e.g., transcription factors (proteins) that affect morphological organisation, not merely the building of components, by switching on and off of developmental processes. The strangely named "sonic hedgehog" which causes abnormal bristliness in mutant fly larvae but also exists in vertebrates, is one example.

Carroll casts brief philosophical nods towards reductionism and explanatory systems but his focus is on "building babies" and " the geography of embryos" and how development can be mapped into various anterior-posterior (proximal-distal) and dorsal-ventral zones, and how staining or colouring techniques, make the invisible precursors of physical structures, such as segments, visible in vivid colour. By chapter 5 we approach the mystery 'behind' genetics with talk of the dark matter of the genome which like the dark matter of the universe purportedly 'governs the behaviour of visible bodies.' In short, much DNA does not code for specific genes but acts as a constellation of switches (hotspots of evolution) that are part genetic computer, part artist…fantastic devices displaying the power and glory of combinatorial logic and wiring of the sort used in electrical circuits. After praising this mysterious, invisible, dark matter (Barfield's unrepresented?) that brings form to light it is suddenly described as instruction-less 'space-filling "junk" accumulated over the course of evolution.' Ironically this combination of enthusiastic rhetoric and veiled references to dark, hidden mysteries boils down to describing a constellation or cornucopia of switches that operate at a distance with mysterious subtle energies. Carroll again acknowledges the paradox of similar tool kit genes building quite different animals, but identifies 'switches' as the ultimate cause.

Carroll convinces us that evo-devo insights apply to evolution and allow new dialogs with paleontologists. Genetic similarities between modern animals show that we need not postulate new genes to explain the appearance of new prehistoric species. Switching creates sufficient combinations. Particular genes are systematically expressed in particular body regions: (e.g., Hoxc6 expresses at the boundary between neck and dorsal vertebrae where the fore limb bud grows). Thus, it plays a role in why snakes have no limbs and may help explain T. rex had puny arms.1 Most evolutionary innovations, from arthropod body plans, variable stripes in zebras, to eye spots in butterflies link to shifts in gene expression influenced by untold subtle factors such as seasonal temperature during development. If everything influences everything else, as Carroll indirectly suggests, then the 'ultimate' cause(s), behind genes and switches must remain elusive.

This useful summary of evo devo, certainly conveys understandable enthusiasm that will inspire students and professional biologists. I missed reference to discussion of traditional biological concepts like "convergence" and "heterochrony" which already point to many of the conclusions that Carroll proffers. Wolfgang Schad's masterful work on Man and Mammals: towards a biology of form, offers a deep understanding of morphological complexity in relation to development, physiology, coloration and behavior, that while avoiding genetics (the special province of evo-devo) takes our comprehension of complex organisation in mammals in every other field to a new level. Likewise Verhulst, (Network 82), offers greater insight into human morphology and behaviour than Carroll's penultimate chapter, over-ambitiously subtitled "the making of Homo sapiens." Bold claims that "the ultimate cause of development and physical change in the evolution of humans is genetic" are hardly supported by identifying a single gene affecting speech and another involved in the evolution of jaw muscles. We should, as Carroll very rightly points out, resist the 'tendency to treat new discoveries- … of a particular gene- as "the" solution to the puzzle of human evolution' rather than 'pieces of a more complex mosaic.'

Carroll ends with quoting disturbing statistics about American ignorance on subject of evolution, the role played by creationism and the challenges faced by evo-devo in desiring to create a new evolutionary synthesis that will counteract this educational misadventure. Although this final chapter is really an afterthought it clearly raises matters of genuine concern for evolutionary scientists of all hues. Darwinism is problematic in the minds of many because of its adherence to vague terms like selection, fitness and random mutation, which are often inadequately explained and inherently suggestive of greater mysteries, which science is still unable to address. Carroll need not fear that evo-devo has secured its place in the history of biology or that its 'brave' heroes will be acclaimed. The question is: how will the average student or citizen understand, and ultimately interpret the reductive minutia of sonic hedgehogs popping randomly on the scene as a result of mysterious, genetic, dark matter behind the already invisible and mysterious gene?

1 This T. rex interpretation is my own.