Research that contributed in part to this paper was made possible through an NSF Transforming Undergraduate Education in STEM grant (NSF no. 1323258) and an HHMI professors grant originally awarded to Jo Handelsman. We thank our collaborators on this grant, including Xinnian Chen, Brian Couch, Mary Durham, Jennifer Frederick, Monica Hargraves, Claire Hebbard, Jennifer Knight, Jessamina Blum, Michelle Withers, Clarissa Dinks, Jane Buckley, and William Trochim, and we acknowledge Jonathan Waterhouse for contributions to Figure 4.