To test whether epistemically unproblematic ways of thinking and talking about science could be transformed during preservice teacher training, we designed a system of learning activities based on a set of "heuristics for progressive disciplinary discourse" (HPDD). The HPDD outline six design principles of learning environments where the aim is to foster learners' participation in material and discursive activities that characterize the work of scientists. After tracking participants through university coursework where the HPDD was employed and into their teaching practicums, we found that most came to reconceptualize the interrelated roles of models, theory, evidence, and argument. These ideas ultimately supported a shift in their goals for scientific investigation--from "proving" a hypothesis, to testing and revising explanatory models. Preliminary findings from teaching episodes with their own secondary students indicated that some participants took up "epistemically ambitious" classroom practices, pressing learners to develop testable models of natural phenomena and gather evidence to link observations with underlying explanatory processes. (Contains 6 tables and 2 footnotes.)