The objective of this naturalistic study was to explore, model and visualise the learning progress of 13-year-old students in the domain of evolution theory. Data were collected under actual classroom conditions and with a sample size of 107 learners, which followed a teaching unit on Darwin's theory of natural selection. Before and after the teaching sequence, the students wrote texts that explained an evolutionary phenomenon. Their explanations for evolutionary change were analysed and categorised into nine different patterns. Furthermore, we contrasted these explanation patterns with the corresponding scientific conceptions. This resulted in five conceptual frontiers, each of them marking one major learning task. The actual learning progress of the sample group was visualised as learning trajectories on a conceptual landscape. Our findings indicate that learning to explain evolution is a very individual process where the students depart from several distinct ideas and take different trajectories. The method of mapping a content-specific learning progress within a mental landscape may be advantageous for other domains of science teaching, too. (Contains 2 figures and 1 table.)