This paper presents Frog Pond, an interactive code-first learning environment about biological evolution. We deployed Frog Pond as part of a six-day curricular unit on natural selection implemented in six 7th grade science classes. Here we describe a case study of two students, Charlie and Aaron who participated in the unit. Comparing pre- and post- interviews in which they were asked to design a program for a hypothetical simulation of evolution, we found that both students shifted from an event-based programming approach to a rule-based approach. Both students also drew upon their experience with Frog Pond to explain an evolutionary phenomenon. However, the level of sophistication of the two students' explanations varied along with the aspects of Frog Pond they drew upon. These findings have implications for design improvement to better support students' understanding of evolution.