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A New Synthesis or Just The New Synthesis?

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1249
DOI: 10.1126/science.1098543

The Speciation and Biogeography of Birds

by Ian Newton

Academic Press (Elsevier), San Diego, 2003. 680 pp. $75, £49.95. ISBN 0-12-517375-X.

The sweeping reviews of speciation and biogeography produced by the architects of the “new synthesis” (or “modern synthesis”), particularly Ernst Mayr, in the mid-20th century provided detailed summaries of ecological, distributional, and historical phenomena related to the evolution of biological diversity. The numerous recent advances in theory and technology, as well as the vastly improved information resources related to this subject, would seem to make possible fresh insights into this field. In particular, there has been much new thought about the process of biological diversification and its geographic dimensions, yet genuinely synthetic reviews of this fertile field have been few.

The appearance of a book boldly titled The Speciation and Biogeography of Birds brings hope of just such a review. Birds are the focus of intensive research in phylogeography, alpha systematics, distributional biology, and ecology, and would thus seem to be an ideal group on which to found such a new synthesis. Ian Newton, an ornithologist at the National Environmental Research Council's Institute of Terrestrial Ecology, Cambridgeshire, took on the big challenge of summarizing a truly massive literature into a coherent account. The book is attractive and illustrated amply with maps that—although simple and not particularly information-rich—quite aptly illuminate many of the concepts. It is also rich in examples illustrating the ideas and concepts that Newton treats.

Nonetheless, the book falls disappointingly short of achieving a new synthesis regarding the diversification of birds. On a number of points, in fact, it feels quite dated. Newton bases much of his discussion of phylogenetic history on DNA-DNA hybridization results (1) that have largely been left behind in the face of newer sequence-based findings (which are only scarcely cited); indeed, he falls into the trap of believing Sibley and Ahlquist's argument that their phenetic analyses were phylogenetic analyses. The author more or less rejects alternative species concepts, which many ornithologists believe have great potential to elucidate patterns of avian diversity by achieving a more consistent species catalog (2). Instead, he postulates that the biological species concept and alternative species concepts will converge on the same patterns. Newton places enormous confidence in molecular clock-based measurements of divergence times, in spite of accumulating evidence that such approaches to dating are likely to be fraught with inaccuracies and biases (3). He bases much of his discussion on such dated concepts as anagenetic (within lineage) speciation, competition as a major force structuring distributions of species and even faunas, species-to-genus ratios as indicators of colonization and speciation histories, and the adaptationist program. Readers will find that many exciting results from recent research have been dismissed, downplayed, or ignored, including the impressive avifaunal richness of Oceania before the arrival of humans (4), the role of pre-Pleistocene speciation in generating present-day diversity (5), the utility of climate-envelope approaches to understanding the effects of climate change on species' distributions (6), and alternatives to the Pleistocene forest refuge hypothesis (79).

I would also have preferred a different organization of the book. Long chapters on dispersal, barriers to movement, and migration come after the core accounts of endemism and major diversity patterns, when they would seem much more appropriate as introductory material. My other complaints are minor: The book's binding may not stand up to heavy use, and though overall the editing is good, there are a few typographical errors and dated place names or assertions (e.g., Ceylon for Sri Lanka and the comment that Oceanodroma macrodactyla “may be extinct”).

There remains a great need for a comprehensive, up-to-date account of avian evolution. Recent findings on the origin, diversification, Pleistocene and current diversity, phylogeography, and ecological biogeography of birds could be integrated into a profoundly novel view of the history of the class, which would offer an important perspective on the process of biological diversification in general.

Newton's eminently readable book probably provides the best available overview of the origin, distribution, and extinction of bird species on a global scale. But The Speciation and Biogeography of Birds fails to live up to the promise of a new synthesis and instead remains firmly embedded in the ideas of the new synthesis.

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