Evolution of Biological Diversity

. Anne E. Magurran and Robert M. May (eds). Oxford University Press, Oxford. 1999. Pp. 329. Price £24.99, paperback. ISBN 0 19 850304 0.

There has been a great expansion of studies of biodiversity and speciation recently. By my reckoning, at least six single-themed international conferences and four multiauthored volumes addressing speciation have appeared since Otte & Endler’s landmark volume Speciation and its Consequences (1989). The editors state that the aim of this latest volume is to explore the evolutionary origins of biological diversity in relation to population differentiation and population structure. This seems a narrower remit than it actually achieves because, in addition to population-level work, it includes chapters on the genetics of isolation and divergence and palaeontological perspectives on biodiversity.

Four chapters adopt a genetic approach to diversity and speciation. Coyne and Orr summarize knowledge of the genetics of pre and postmating reproductive isolation. Their table highlights the fact that few really informative studies are available, showing only 26 pairs or groups of organisms which they consider worth including. More could be added, but not many. Two chapters ask how genetic diversity within and between populations and species conform to theoretical expectations; not very well is the conclusion of both. Harwood and Amos speculate about the role of selective sweeps and odd mutation processes (pointing out that it is far too convenient simply to infer that ‘bottlenecks’ are responsible for low heterozygosity). Tregenza and Butlin also show that heterozygosity conforms rather poorly to neutral predictions (taking account of apparent population size). A potentially important point they make is that the heritability of traits influencing alpha diversity, notably those involved in premating isolation, may have little relationship to heterozygosity inferred from neutral markers. Tautz and Schmid describe an ingenious method to detect quickly evolving genes — presumably those under selection and contributing most to divergence. This neatly reverses the usual orientation of work in evolutionary developmental genetics; the most interesting genes may be the least conserved.

Four chapters are directly concerned with population processes and speciation. Two document speciation in freshwater fish. Skúlason et al. argue that resource polymorphism and phenotypic plasticity may rapidly lead to sympatric speciation, illustrated by arctic charr (and elsewhere by sticklebacks and phytophagous insects). Turner documents the remarkable radiation of cichlids in African lakes (a striking calculation appears to show evolutionary rates of one new species per 10,000–100 000 years, compared with one per 2–4 million years in Hawaiian Drosophila, for example). Turner’s favoured explanation is sexual selection, and the currently highly fashionable relationship between sexual selection and speciation is explored fully in another three chapters. Partridge and Parker, and Magurran, point out that strong sexual selection can in fact reduce speciation if strong intrasexual competition selects for males who circumvent female choice. The outcome of sexual conflict is therefore critical. A particularly thoughtful article by Price also challenges the simplistic view that high sexual dimorphism implies a direct role for sexual selection in speciation. He argues that rapid divergence of sexually selected traits in birds is unlikely to have a simple correlated response in sexual isolation, and that only later can sexual imprinting (or preference for traits diverging as a result of adaptive radiation) feed back to speciation. Clearly much more caution (and quantitative analyses) are required to assess the role of sexual selection in speciation.

Two chapters take a comparative approach. For me, Gaston and Chown’s study of range sizes and speciation was too vague about the predicted and observed relationship between the variables for meaningful conclusions to be drawn. In contrast, Barraclough et al. show how clearly testable predictions allow inference of modes of speciation from phylogenies. Again, this is a study area which is in its infancy.

The remaining three chapters view the ‘bigger’ picture. It is sometimes argued (not only by Gould) that speciation biologists observe the world at too low a level to understand patterns of biodiversity. Gould’s chapter encouraging a multilevel view of selection is both entertaining and thought provoking. Conway Morris and Sepkowski illustrate the historical patterns of biodiversity, showing how this is largely influenced by random factors such as mass extinctions and the delay in rebounding from them. Also, major taxa vary quite dramatically in the rate of divergence and apparent speciation. As Sepkowski argues, much of the pattern of biodiversity reflects these characteristic (and ‘heritable’) rates of extinction and speciation, yet the reasons remain ‘one of the great unsolved problems of macroevolution’.

The breadth of approach illustrated in this volume distinguishes it from similar recent publications. It is an excellent compilation of views of speciation and biodiversity (although the absence of any chapters on the plant or microbial kingdoms is remarkable). I would describe it as essential reading for anyone with an interest in speciation, were it not for the fact that more or less the same publications are also available in a volume of the Philosophical Transactions of the Royal Society (following a meeting in 1997). As I gaze at the pile of as yet unread, journals and books accumulating in a corner of my office, I am not very appreciative of any trend in publishing which artificially escalates this further. Either of these two publications is essential reading – to publish both independently seems unnecessary.