Philosophical Transactions of the Royal Society B: Biological Sciences
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Sympatric, parapatric or allopatric: the most important way to classify speciation?

Roger K Butlin

Roger K Butlin

Department of Animal and Plant Sciences, University of SheffieldWestern Bank, Sheffield S10 2TN, UK

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,
Juan Galindo

Juan Galindo

Department of Animal and Plant Sciences, University of SheffieldWestern Bank, Sheffield S10 2TN, UK

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and
John W Grahame

John W Grahame

Institute of Integrative and Comparative Biology, University of LeedsLeeds LS2 9JT, UK

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    The most common classification of modes of speciation begins with the spatial context in which divergence occurs: sympatric, parapatric or allopatric. This classification is unsatisfactory because it divides a continuum into discrete categories, concentrating attention on the extremes, and it subordinates other dimensions on which speciation processes vary, such as the forces driving differentiation and the genetic basis of reproductive isolation. It also ignores the fact that speciation is a prolonged process that commonly has phases in different spatial contexts. We use the example of local adaptation and partial reproductive isolation in the intertidal gastropod Littorina saxatilis to illustrate the inadequacy of the spatial classification of speciation modes. Parallel divergence in shell form in response to similar environmental gradients in England, Spain and Sweden makes this an excellent model system. However, attempts to demonstrate ‘incipient’ and ‘sympatric’ speciation involve speculation about the future and the past. We suggest that it is more productive to study the current balance between local adaptation and gene flow, the interaction between components of reproductive isolation and the genetic basis of differentiation.

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