The purpose of this study was to explore science teachers' conceptions about micro-and macro-evolutionary processes. In this study, a case study was employed to compare and contrast novice and experienced middle school science teachers' evolution conceptions. The data were collected using four micro-and macro-evolutionary scenarios alongside cognitive interviews. The findings illustrated that while the novice teacher mostly provided non-Darwinian explanations, the experienced teacher predominantly applied key concepts of natural selection in her causal explanations, but cognitive biases regarding evolution were also found in her explanations. Therefore, the study suggested that the reasons for the coexistence of evolution concepts and cognitive biases be worthy of further exploration. The findings also indicated that even though the experienced teacher used several principles of macroevolution to explain the evolutionary changes above the species level, she based her causal explanations mainly on natural selection concepts. In this regard, the study recommended that teacher education programmes place special emphasis on the areas that are shown to be problematic for teachers like teaching and learning evolution, especially the concepts of macroevolution. The study also suggested that it may be worth the effort to develop a comprehensive qualitative measure in order to uncover learners' conceptions concerning macroevolution.