1. ¹Department of Biological Sciences, Ecology and Evolution Research Group, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
2. ²Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK

Correspondence: J Silvertown, Ecology and Evolution Research Group, Department of Biological Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. E-mail: j.silvertown@open.ac.uk

³Current address: National Botanic Garden of Belgium, Meise, Belgium

⁴Current address: Ecology and Evolution Research Group, Departmetn of Biological Sciences, The Open University, Milton Keynes, UK

Received 18 November 2004; Accepted 31 May 2005; Published online 6 July 2005.

Abstract

It has been debated, ever since Charles Darwin and Alfred Russell Wallace disagreed about the matter, whether natural selection plays a role in reinforcing reproductive isolation during the earliest stages of speciation. Recent theory suggests that it can do so, but until now the empirical evidence has conspicuously lacked a case in which reinforcement has actually been observed to split a population. We show that this has occurred at least once in populations of the grass Anthoxanthum odoratum growing in the Park Grass Experiment where flowering time has shifted at the boundaries between plots. As a consequence, gene flow via pollen has been severely limited and adjacent populations that had a common origin at the start of the experiment in 1856 have now diverged at neutral marker loci.