Review
Theory and speciation

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Abstract

The study of speciation has become one of the most active areas of evolutionary biology, and substantial progress has been made in documenting and understanding phenomena ranging from sympatric speciation and reinforcement to the evolutionary genetics of postzygotic isolation. This progress has been driven largely by empirical results, and most useful theoretical work has concentrated on making sense of empirical patterns. Given the complexity of speciation, mathematical theory is subordinate to verbal theory and generalizations about data. Nevertheless, mathematical theory can provide a useful classification of verbal theories; can help determine the biological plausibility of verbal theories; can determine whether alternative mechanisms of speciation are consistent with empirical patterns; and can occasionally provide predictions that go beyond empirical generalizations. We discuss recent examples of progress in each of these areas.

Section snippets

Allopatric speciation

Both pre- and postzygotic isolating mechanisms arise as inevitable by-products of genetic divergence in allopatry, and their evolution can be accelerated by divergent selection 18 (Schluter 15, this volume). The main problem with understanding the origin of isolating mechanisms during allopatric speciation is not theoretical but empirical: in most cases, we do not know which forms of reproductive isolation evolved first, and which forms evolved only after other forms had already prevented gene

Hybrid speciation and polyploidy

Many botanists have argued that hybridization provides variation that facilitates adaptation and that hybrids might even evolve into new species 56, 57, 58, 59. Such ‘hybrid speciation’ must, by necessity, begin in sympatry. We expect recombinant hybrid genotypes to be, on average, less fit than individuals from the parental species, simply because most have never been tested by natural selection. However, some hybrid genotypes might be fitter than the parentals: given some degree of gene

Conclusions and prospects

Because it often includes sophisticated mathematics, non-verbal theory has a special air of authority among biologists. Many experimentalists are unequipped to judge the limitations or weakness of such theory, which has occasionally been misleading. For example, the results of Spencer et al. 94 convinced many that reinforcement was nearly impossible to obtain, even under optimal conditions. Renewed empirical interest emerged only when new data indicated that reinforcement was plausible 4, and

Acknowledgements

We thank D. Bolnick, B. Fitzpatrick, S. Gavrilets, R. Haygood, C.D. Jones, M. Kirkpatrick, A. Kondrashov, J.B. Mullet, S.V. Nuzhdin, H.A. Orr, T.D. Price, T. Prout, D.W. Schemske, D. Schluter, M.R. Servedio and P.S. Ward for discussion and comments. Some of these reviewers disagree with our conclusions. This work was supported by US National Science Foundation grants DEB 9527808 and DEB 0089716 to MT, grants from the Darwin Trust of Edinburgh and the Biotechnology and Biological Sciences

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