Speciation: Why, how, where and when?

Species are one of the few fundamental units in biology. Yet, study of how they form has had an erratic history. Despite the title of his book, Darwin gave the process of speciation little attention in the 'Origin of Species' — even though his earlier notebooks show that he thought deeply about many issues that are still current. (Darwin's apparent neglect of speciation may have been due to his wish to emphasise the evolutionary continuity between 'varieties' and species.) The late 19th century saw substantial work on the geography of speciation (especially and in mid-twentieth century, the 'Modern Synthesis' between Darwinian evolution and Mendelian genetics developed our modern ideas of what species are, and how they form. However, the luminaries of the field, Wright, Fisher and Haldane, barely considered speciation in their development of population genetics, and more recently, evolutionary biology has concentrated on other issues: what maintains molecular variation, the description of phylogenetic relationships, and measurement of natural selection in the wild. Over the last decade, interest in speciation has revived, yet until publication of Coyne and Orr's new book 'Speciation', there had been no comprehensive treatment since M.J.D. White's 'Modes of Speciation' in 1978. Coyne and Orr begin by showing that species are real biological units, recognised in essentially the same way by diverse cultures; but there is continuing argument over how they should be defined. The next chapter discusses the various ways by which we can study speciation, and gives a nice overview of the whole book. Coyne and Orr go on to discuss the geography of speciation: after 150 years, the questions of whether species form in geographic isolation (in allopatry), and the extent to which gene flow impedes divergence, remain central. The following chapters examine the different kinds of reproductive isolation that separate species — ecological, behavioural and postzygotic — and their genetic basis. Finally, Coyne and Orr consider how species can form rapidly, through hybridisation and polyploidy; how selection can reinforce mating isolation; and how rates of speciation can be measured. Thus the book covers essentially the whole field: the only significant omission is the relation with the large ecological literature on species diversity: how do mechanisms and rates of speciation influence the observed numbers and distribution of species? Coyne and Orr have little sympathy with the " vast and stupefying literature " on species definitions. They adhere to the traditional 'biological species concept', …