Shapes of Time. The Evolution of Growth and Development

American Zoologist, Dec 1998 by Reilly, Stephen M

Shapes of Time. The Evolution of Growth and Development. KENNETH J. MCNAMARA. John Hopkins University Press, 1997, 342 pages, index. (ISBN: 0-8010-5571-3, $34.95.) Heterochrony (change in the timing or rate of development) is a major explanatory concept to quantify how development is modified to produce phenotypic and phyletic variation. The aim of this book is to "tell the story" of how heterochrony provides the all-important link between genetics and natural selection. It argues for heterochrony as an evolutionary force by discussing numerous examples related to a series of issues presented in twelve chapters. Chapters I (The Evolving Embryo) and 2 (The Topsy-Turvy World of Dr. Haeckel and Dr. Garstang) review the rise, fall, and re-emergence of heterochronic thinking and the ontogeny-phylogeny relationship, and briefly explain the heterochronic terminology used in the book. Chapter 3 (Ihe Shape oJ Things to Come) discusses various developmental mechanisms that influence the timing and rate of development, from box genes, to hormones and growth factors, to epigenetic cascades. The fourth chapter (It's a Dog's Life) describes numerous examples of intraspecific heterochrony and allometry (all incorrectly termed paedo- or peramorphic), pointing out that much of the variation found within species arises from tinkering with developmental mechanisms. A Time .for Sex (chapter 5) focuses on heterochrony and the evolution of sexual dimorphism in size and traits, which. as in the previous chapter, mostly involve intraspecific patterns.

From chapter 6 onward, the book deals primarily with interspecific patterns of heterochrony. Chapter 6 (Birds, Bratlhiopod and Bil.shE>uc-ks: Heterochrony in Speciation) discusses heterochronic shifts among species, explaining the idea of ontogenetic gradients or clines (i.e., so-called paedo-peramorphoclines). These are used to argue that shifts along ontogenetic gradients have allowed species to evolve as they invade new environments or respond to environmental changes on geological time scales. Chapter 7 (The Peter Pan S!ndrome) asks if peramorphosis is as common as paedomorphosis, and why some lineages increase in complexity by peramorphosis while others become more simplified by paedomorphosis. The author never really answers these questions, but from the examples given, one would have to conclude that paedomorphosis is more common than peramorphosis. Chapter 8 (lima/hes ot the Past, Shaoes of the Future) explains that traits, not organisms, are heterochronic; thus, there are many examples of organisms with a mixture, or "cocktail," of peramorphic and paedomorphic features. Therefore, different traits often are developmentally dissociated and under different selective pressures. In this chapter, McNamara introduces the term "sequential" heterochrony which is applied to portions of ontogenies that are changing relative to other portions. The term relates to the apparent paradox that the offset of one portion may influence the onset of a following portion. However, the need for this new term and the corresponding paradox disappear if one simply plots ontogenetic trajectories (trait i's. age) for the ancestral and descendant species and describe what is different about them. Evolving the Shale,s Beyond (chapter 9) is essentially a chapter about allometry, it also illustrates one of the most frustrating aspects of the book-that allometry (trait r's. size) is not distinguished from heterochrony (trait irs. age/time). This is a real problem for paleontological comparisons and one that is often ignored in neontological studies of heterochrony. Chapter 10 (Evolving a Way of Life) describes the effects of heterochrony on the evolution of life history and behavioral traits-one of the most important and least appreciated consequences of heterochrony. As such, it is one of the most novel and interesting parts of the book. Fueling the Biological Arns Race (chapter I 1) discusses the impact of heterochrony in producing variation for evolutionary changes driven by predator-prey relationships. The final chapter (The Baby-faced Super Ape) covers the mosaic of truncated and extended characters in humans, and reviews how changing attitudes towards heterochrony over the last 200 years have colored our views of the evolution of human anatomy. This is perhaps the best chapter.

As the first book in two decades to formally portray heterochrony as a major mechanism of evolution, Shapes of Time is a fair attempt. However, I am not sure of the intended audience. Only selected references are given as footnotes, and most of the figures are general in nature (not a single ontogenetic trajectory is presented). The book parallels the field of heterochrony in general in lacking empiricism and a lucid terminology. Without trait-by-age data (true ontogenetic trajectories) or phylogenies, I found it difficult to evaluate or polarize heterochronic patterns for the species being compared. In addition, although the distinction is made, the author often ignores the difference between intraspecific and interspecific patterns of heterochrony. The phyletic nature of the terms pera- and paedomorphosis is strongly entrenched in the heterochrony literature; yet McNamara, like some others, terms intraspecific variants as paedomorphs or peramorphs. A discussion and solution to this and other problems is presented in Reilly et al. ( 1997). Furthermore, many of the examples are not really examples of heterochrony but involve changes on the size axis as well. Allometry and heterochrony are different concepts that can share the trait axis. Two very recent papers (Rice, 1997: Klingenberg, 1998) attempt to make this distinction clearer and provide ways to study both concepts at the same time.