Despite being a fundamental concept in biology, evolution continues to be one of the most challenging topics to teach in science education. Ideas of evolution emphasising anatomical or behavioural features of individuals, as opposed to the interplay between genetics and the environment, are reinforced through language and culture, making them robust and persistent in the student population at all educational levels. Model-based reasoning has been reported to be useful for students to make sense of process-based science content, combining epistemological with linguistic and value dimensions. However, there is a dearth of evidence in biology education showing how modelling can instigate epistemological maturity, specifically about issues of agency and design in evolution by natural selection. Drawing on this perspective, this study focuses on describing the nature of students' ideas while modelling the resistance developed by a population of mosquitoes in a lagoon after an insecticide is introduced. Data collection includes students' written reports and drawings, which were analysed with content and discourse analysis. The findings show that, at first, students believed adaptation to feature at will was a behavioural characteristic instigated by a pre-existing design. After modelling the process of natural selection, the explanations appeared to improve (from Lamarckian to Neo-Darwinian views), and most groups showed accurate explanations about adaptation.