1. Biological Sciences, University of Wisconsin-Whitewater, Whitewater, Wisconsin 53190, USA
2. Biological Laboratory, Gifu-Keizai University, Ogaki, Gifu prefecture 503-8550, Japan
3. HHMI and Stanford University School of Medicine, Beckman Center B300, 279 Campus Drive, Stanford, California 94305-5329, USA
4. Department of Zoology and Centre for Biodiversity, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
5. Present addresses: Department of Biology, Indiana University, Jordan Hall Rm 325, 1001 E. 3rd Street, Bloomington, Indiana 47405, USA (B.K.B,); Department of Biochemistry, Stanford University School of Medicine, Stanford, California 94305, USA (L.D.)

Correspondence to: Jeffrey S. McKinnon¹ Email: mckinnoj@uww.edu

A principal challenge in testing the role of natural selection in speciation is to connect the build-up of reproductive isolation between populations to divergence of ecologically important traits^{1, 2}. Demonstrations of 'parallel speciation', or assortative mating by selective environment, link ecology and isolation^{3, 4, 5}, but the phenotypic traits mediating isolation have not been confirmed. Here we show that the parallel build-up of mating incompatibilities between stickleback populations can be largely accounted for by assortative mating based on one trait, body size, which evolves predictably according to environment. In addition to documenting the influence of body size on reproductive isolation for stickleback populations spread across the Northern Hemisphere, we have confirmed its importance through a new experimental manipulation. Together, these results suggest that speciation may arise largely as a by-product of ecological differences and divergent selection on a small number of phenotypic traits.

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