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. Demonstrations of 'parallel speciation', or assortative mating by selective environment, link ecology and isolation, 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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Acknowledgements We thank L. Bauers, V. Braithwaite, K. Faller, S. Foster, S. Gray, M. Ishikawa, P. Jacobsen, P. Katz, R. King, B. Kristjansson, M. Nemethy, H. Ogawa, W. Paulson, J. Poole, E. Sassman, S. Shell, S. Skulason and R. Snyder for help collecting fish and/or data. M. Blows, H. Rundle and A. Hendry provided useful comments on the manuscript and the University of Queensland hosted J.S.M. during writing. This project was supported by an NSF research grant, REU supplements and a Putnam grant (J.S.M.), the Howard Hughes Medical Institute (D.M.K.) and an NSERC grant (D.S.).
Acknowledgements WewouldliketoacknowledgethecaptainandcrewoftheF/SMeteorandthe scientists aboard the Meteor 55 SOLAS cruise, especially D. Wallace, K. Lochte, H. Bange, P. Croot, M. Voss, F. Malien, R. Langlois and P. Fritsche. We thank D. Wallace for insightful comments on this manuscript. Additionally, we acknowledge W. Balzer for loaning the clean container used during M55. This work was supported by the Deutsche Forschungsgemeinschaft’s Meteor Schwerpunktprogramm, a Natural Environment Research Council grant to R.J.G., and a Marie Curie Post Doctoral fellowship to C.R.