The organization of the book
is logical and fairly traditional, with the first two chapters
providing an historical context and clarification of species concepts.
Certainly not all biologists concur on how species are defined,
but Arnold argues that some of our definitions serve to impede 
rather than illuminate the role of hybridization as a process
in natural populations. I don't fully agree that "all species
concepts are based on the viewpoint that this process is somehow
'bad,"' (p. 12), but it is true that hybridization and the
novel genotypes that result are often viewed as maladaptive rather
than as creative forces. In examining hybridization and introgression
in Asclepias many years ago (Kephart et al., 1988), we speculated
that hybrids that are capable of invading ecotones between parental
species are in effect "exploiting" alternate habitats
which are not only spatially defined but may vary temporally during
succession (e.g., in light gaps or in secondary environments that
result from anthropogenic events). Although the evolutionary
significance of such colonization depends on both the frequency
of such events in time and space, and the relative fitness of
hybrid and parental populations, this may be one of many questions
that "are rarely if ever addressed" (p. 21), perhaps
because of the biases that Arnold sees as inherent in our articulation
of species concepts. Thus, while some might argue that Arnold
overstates his case in places (e.g., that most "crossing
experiments were merely an exercise in demonstrating the dogma
that hybridization is maladaptive"), such assertions do have
a considerable basis for some taxa, and this position may encourage
additional investigation in these groups.
The book also guides the reader to numerous classic and contemporary studies of hybridization, highlighting both the strength of recent phylogenetic tests of reticulation in plant species (e.g., Rieseberg et al. 1990) and the relative dearth of population-based studies of hybrid zones by botanists. Moreover, the case studies involving fossil or contemporary floras and faunas (e.g., Hawaiian silver swords, cladocerans) will help us to identify more readily the effects of hybridization and introgression, and to understand their heterogeneous distribution among taxa. However, our evidence of these processes is often dynamic, particularly as new molecular techniques emerge. For example, a diploid hybrid speciation model initially discounted by Wolfe and Elisens based on the available allozyme and DNA markers (i.e., Arnold's discussion, p. 43), has recently been supported using an ISSR-based microsatellite approach (Wolfe et al, in review).
Another strong feature of the book are the helpful discussions of incongruities among data sets, and the integration of animal and plant examples in ways that illuminate general patterns. The inclusion of numerous figures and tables is a real asset, particularly in the chapter on reproductive barriers. The treatment of differing perspectives is also balanced (e.g., Grant versus Chase & Raven on Aquilegia), but a cautionary note on the vagaries of interpreting pollination syndromes would have been useful (i.e., Waser et al., 1996). Also, I would have enjoyed more discussion of the difficulties of interpreting gene flow and reproductive patterns from population versus species level perspectives. The potential for useful discussion is considerable in lpomopsis aggegata (i.e., contrast the varied discourse by Grant, and Paige & Whitham), and the linking of molecular, phylogenetic and pollination data has been excellent (e.g., papers by Soltis et. al, and by Wolf). Unfortunately there is very little discussion of a vast literature on ferns and bryophytes, particularly given the pervasiveness of hybridization in the former group. Perhaps future editions can address this omission.
Theoretical models of hybrid zone evolution are also reviewed in the book, with the overlying theme: What is fitness of hybrids relative to parental types and how aoes that information influence the role of hybrids in plant and animal evolution? Arnold documents in tabular form the variability in hybrid fitness (low to high) for numerous taxa including the complex of irises he and his coworkers have studied. He argues cogently that hybrids are not only sometimes more fit (e.g. Artemesia) or variably less fit (e.g. Iris) than their parents but they may be heterogeneous with respect to fitness as a result of differing patterns of environment-dependent selection. Arnold also reviews recent experiments designed to examine the difficulty of initial hybrid formation; he then incorporates such rarity into a revised model of hybrid zone evolution, one which includes a role for exogenous and endogenous selection and for the establishment of "fit" hybrids in novel habitats.
One of the real strengths of this treatise on the evolutionary role of hybridization lies in Amold's ability to link differently configured studies across historical and contemporary periods into a cohesive unit. This feature is especially evident in the final chapters which summarize the evidence that hybridization and introgression have indeed led to the development of new evolutionary lineages via diploid and polyploid speciation. Included are controversial cases such as the red wolf, and notably missing are examples of pteridophyte evolution. Yet the reader is still provided with an insightful re-examination and summary of how hybridization and introgression (sensu Edgar Anderson; Lewontin and Birch) may lead to the replacement of species by recombinant types or to the extension of introgressants and hybrid derivatives into novel habitats. Little new ground is traversed here that is not already known in the literature or alluded to in previous chapters, but the synthesis is clearly effective. Moreover, Arnold's discussion of the importance of introgressive hybridization in conservation biology adds a significant dimension to our understanding of how this process may both enrich the gene pool and enhance the fitness of rare taxa.
Overall, the book is an excellent and engaging analysis of the evolutionary role of hybridization in natural populations, well exceeding the objectives established in the preface. It is not an exhaustive summary, partly because the published literature is voluminous, and perhaps because the compact, paperback form lends itself so well to the goal of stimulating further study and to synthesis of a controversial, yet highly important topic. The prose is well-written and the chapter summaries are helpful and usually reflect the major points discussed therein. I highly recommend the book to advanced undergraduates, graduate students, and professionals alike. Most of us with an interest in hybridization will likely find our copies well-used, and a source of ideas for future study. - Susan R. Kephart, Department of Biology, Willamette University, Salem OR 97301