Several researchers and scientific institutions argue that evolution should be explored from the first school years. However, few studies have analysed primary school students’ understanding of evolutionary processes or evaluated the impact of educational activities on such knowledge. The available data: i) suggest that primary school students can learn about evolution; and ii) identify differential reproduction as the key evolution concept less often used by students to make and justify evolutionary predictions. In the present study, we evaluate the impact of an educational programme on primary school students’ level of understanding of evolution by sexual selection and on their ability to employ differential reproduction to propose and justify evolutionary predictions. An evaluation framework was applied to estimate primary school students’ level of understanding of evolution by sexual selection in third- and fourth-grade classes, before and after the students were exposed to the educational programme. A significant increase in the level of understanding of evolution by sexual selection was observed in the target classes, but not in the control classes. This result was primarily driven by a significant increase in the students’ justifications employing the concept of differential reproduction. The results suggest that activities that model and simulate biological evolution through sexual selection can contribute to primary school students’ understanding of evolutionary processes.