This work was supported by Genome Canada (L.H.R.), Genome BC (L.H.R.), the Natural Sciences and Engineering Council (NSERC) of Canada (NSERC-CANPOLIN publication 63), and a NSERC (Natural Science and Engineering Research Council of Canada) postgraduate scholarship to S.R. We thank members of the Rieseberg lab for constructive criticisms on earlier versions of the manuscript.