Evolution is a central but challenging topic in biology education. The research literature is rich in examples of how learners construal of evolutionary processes such as natural selection is inadequate. While a lot of attention has been drawn to how central facts of natural selection (variation, selection and inheritance) is comprehended by students, less attention has been payed to important concepts such as randomness and probability. These are suggested to be so called threshold concepts, which are more abstract in nature.  We believe that the random origin of novel traits is counter-intuitive and therefore less likely to picked up by students during teaching and learning. In an effort to address this, we work with the hypothesis that visualizations of evolution can be an effective way to make the complex and counter-intuitive nature of evolution less challenging for learners. In this study we report how a visualization of the evolution of antibiotic resistance seem to be interpreted by upper-secondary students. The focus of the visualization was to show that new traits arise by chance and not due to the “pressure” from the environment. Our preliminary results show that even though the students began to include mutation as a source of variation, this model was often mixed with their original ideas of the trait being caused by the pressure from the environment.