NTS 33 34 We thank the students of UBC for their participation, NSERC for its financial support, Y. Kondo 35 36 for her statistical advice, and M. Bailey and L. Wardil for affable assistance with the 37 38 39 experiments. 40 41 42 43 44 REFERENCES 45 46 1 Boyd, R. & Richerson, P.J. 2009 Cu