A working understanding of natural selection is vital for addressing contemporary challenges, e.g., combatting antibiotic resistance, conserving biodiversity, ameliorating effects of climate change, and feeding a growing population. However, acquiring a basic understanding of the process is notoriously difficult for students. Recent research has highlighted the importance of so-called threshold concepts (randomness, probability, spatial and temporal scales) in addition to frequently taught key concepts of natural selection. While visualizations have potential utility for teaching natural selection, many visualizations have limitations in their representation of key and threshold concepts. In a previous study we found that visualizations of antibiotic resistance had several advantages but also shortcomings, especially regarding students’ understanding of random mutations. Here we report the design of two novel interactive visualizations intended to address those shortcomings by focusing on representation and role of random mutations in natural selection (antibiotic resistance and fur color of mice). Analysis of data from two pilot studies with secondary students revealed that randomness is often perceived by students in the representations. However, one of the studies revealed that students may have difficulties integrating the role of random variation in natural selection. Implications for visualization design and learning natural selection are discussed.