Understanding natural selection requires mentally linking various key concepts and processes. Many students find it difficult to construct scientific explanations about natural selection and might display misconceptions. Our study aimed to investigate the understanding of natural selection and the existence of teleological or other misconceptions on the part of Basque secondary school students. A diagnostic test was completed by 106 Basque secondary students. Students' responses to knowledge questions and application questions addressing evolutionary trait gain situations were assessed for consistency. Results varied between education levels without showing a clear pattern. Students in the highest grades performed better than 10th graders in knowledge questions but not in all questions requiring them to apply their knowledge. Although teleological expressions addressing intentionality tended to be common in student's explanations, most students stated that mutations occurred randomly, suggesting that teleological wording might reflect a linguistic issue rather than an actual lack of knowledge. Our results indicate that that caution should be exercised when inferring teleological thinking from open-ended application contexts.