This article focuses on students' discursive moves and reasoning practices while engaged in a task that requires making explanatory links between sickle cell disease and malaria. Both diseases pertain to key areas of the biology curriculum, namely, genetic variability and natural selection, and are connected to the theory of evolution of living organisms. Specifically, this study examines the intersections among rhetoric, argumentation and epistemic actions in supporting students' understanding of complex biological dynamics, which are interlinked across time and space, but are often addressed separately in the curriculum. Data were collected over the course of two school years (2014-2016) with a group of 20 15-17-year-old students and their biology teacher. The findings indicate that while rhetorical moves helped students mobilize data, the use of evidence to support claims remained limited. Conversely, the type of epistemic actions enacted by the students appears to be directly related to the type of data being analysed. Hence, rhetorical moves in combination with argumentation practices appear to account for students' differential performances in building more complex explanations of evolutionary topics. We conclude that further understanding of reasoning practices and how these are shaped by discursive moves is required in biology education, in order to help students view biological processes in a wider context, and thus gain a better understanding of evolutionary phenomena.