The concept of natural selection serves as a foundation for understanding diverse biological concepts and has broad applicability to other domains. However, we know little about students' abilities to transfer (i.e. apply to a new context or use generatively) this concept and the relation between students' conceptual understanding and transfer ability. Consequently, the purposes of this study were to describe the patterns of transfer displayed by high school biology students learning about natural selection over time, evaluate their overall level of success at transferring the concept across knowledge domains and examine the relation between conceptual understanding and level of transfer of the concept. Transfer ability and conceptual understanding were assessed using open-response items administered to 138 students in the United States. Based on the responses to these items, we identified particular patterns of surface and deep-level transfer and found that deep-level transfer was uncommon. Further, we found that deep-level transfer and conceptual understanding shared a small but significant relation; surface-level transfer was unrelated. Based on these results, we recommend that teachers explicitly focus on fostering transfer of the concept of natural selection and use specific teaching for transfer strategies, in addition to teaching for conceptual understanding.