Speciation in birds – a complete picture?
Version of Record online: 25 OCT 2002
Journal of Evolutionary Biology
Volume 15, Issue 6, pages 1095–1096, November 2002
How to Cite
Björklund, M. (2002), Speciation in birds – a complete picture?. Journal of Evolutionary Biology, 15: 1095–1096. doi: 10.1046/j.1420-9101.2002.00453.x
- Issue online: 25 OCT 2002
- Version of Record online: 25 OCT 2002
The Birds of Northern Melanesia. Speciation, Ecology & Biogeography. By Ernst Mayr & Jared Diamond. Oxford University Press, 2001. £45.00. ISBN 0-19-514170-9.
One of the most cited and pervasive books in evolutionary biology during the last century is Mayr's ‘Animal Species and Evolution’ from 1963. Among all the different theories and suggestions, two stand out as still being heavily debated and on which considerable research is currently being undertaken, namely the issue of sympatric speciation, and the role of founders in the formation of new species (Mallet, 2001). In the case of sympatric speciation the conclusion is well known: given the evidence, the plausibility of the models and the realism of the assumptions, sympatric speciation was not a viable alternative. This has also been the majority opinion among evolutionary biologists until recently; data and models now show that sympatric speciation might work.
The second controversial issue was the role of peripheral isolates. Mayr was clear in the 1963 book: ‘…most peripheral isolates do not evolve into new species, but when a new species evolved, it is almost invariably from a peripheral isolate’ (italics original). This conclusion has not reached general acceptance, far from it, and the models in disfavour of this idea outnumber those in favour.
Many of the examples in the 1963 book were taken from the avian fauna in East Asia where Mayr had carried out extensive field work since the 1930s. Years later Jared Diamond worked in the same area, and Mayr and Diamond agreed to produce a single book, more extensive than any other previous treatment. Now, some 30 years later, the book is completed. Two questions came to my mind when I got this book: given the recent wealth of models of sympatric speciation and the availability of much more data, has Mayr (and Diamond) changed his mind with regard to this issue? Secondly, are peripheral isolates still the most likely source of new species?
The book is divided into seven parts, distributional maps and seven appendices with background data. The first two parts relate to the environmental background and human impacts whereas the third part is a description of the avifauna of the area and the fourth a description of possible colonization routes. These four parts are short and serve as a general background to the analyses that comprise the bulk of the book which are divided into a taxonomic and a geographical analysis. The text ends with a synthesis and some suggestions for future research. Many different taxa and their geographical variants are presented as beautiful colour plates.
The analysis is an in-depth analysis of every bird species in the archipelago, their degree of geographical variation with regard to plumage and, if data was available, ecology. Without having complete knowledge of the avian literature, I do not hesitate to consider this book the most comprehensive and detailed account of any avian fauna in the world, and probably among the most detailed ones for any fauna. What makes this book different from most others is the geographical, rather than taxonomic, perspective, which is rare among avian books, and the book should perhaps not be seen as a bird book, but as a book about speciation and geography using birds as an example. I am afraid that many potential readers will discard the book by only looking at the title that indicates that this is just a fauna of an unknown area. However, the book is so much more than that. Most readers will probably skip the very detailed accounts of the separate taxa, and go directly to the Synthesis, and they will miss little (unless one is unwilling to accept the inferences drawn by the data). The Synthesis in itself provides few surprises but will serve as excellent material for use in teaching, going from detailed accounts of each taxa to broad generalizations about the process of geographical speciation, with all the various stages being well represented in the data.
So, what about the two questions posed in the beginning of this review? With regard to sympatric speciation nothing is new, or as phrased by the authors: ‘We cannot recognize any case that could plausibly be interpreted as a stage in sympatric speciation’ (p. xxii), and given the data presented, I am inclined to agree. However, the conclusions are almost entirely based on morphological data using museum specimens. As has become apparent recently, the use of molecular techniques may lead to surprises concerning the phylogenetic relationships of various taxa, even in birds. The authors mention briefly that this might be the case, but the important point to make is that given all these detailed accounts concerning morphology, ecology at a broad level and distribution are available and collected into one single book, this can serve as an excellent background to detailed molecular work which certainly can give rise to very important new insights. If one disputes the conclusion about sympatric speciation, the data is there for reinterpretation!
What about founder events then? Is the 1963 conclusion still valid? Not really; in the section ‘The evolutionary significance of peripheral isolates’, the authors bring the message we might have suspected: ‘…it is rare for peripheral isolates to expand into central locations and become important evolutionary novelties, because of the problems of upstream colonization and faunal dominance…’ (p. 297). Accordingly, the role of vicariance events and dispersal into larger islands are stressed in favour of peripheral isolates. Thus, of the two conclusions drawn by Mayr in 1963 only one still stands, whereas the second probably has fallen in popularity (Mayr, 1963). However, the last words have not yet been said in these matters.
In conclusion, despite the slightly misleading title, this book is probably one of the major books in recent times dealing with geographical differentiation and speciation in a vertebrate using a wealth of indepth data to show the major routes of speciation.