BackgroundTo teach evolution efficiently teachers must be able to diagnose their students’ ideas and understanding of the phylogeny of organisms. This encompasses different facets of content-specific professional knowledge, that is, knowledge about core ideas and theories, as well as knowledge about respective misconceptions. However, as findings from the field of psychology have shown, diagnostic activities comprise a further facet, namely, teachers’ judgment accuracy. This refers to the question of whether achievement-irrelevant information about the student influences teachers’ diagnoses. Against this background we conducted a study (1) to assess trainee teachers’ abilities to diagnose (a) the scientific correctness of students’ written answers, (b) students’ misconceptions about evolution, and (2) to investigate the interplay of evolution specific and generic facets of professional knowledge during the diagnosis. For this purpose, we applied a digital instrument, the Student Inventory (SI). Using this instrument, the trainee teachers (N = 27) first diagnosed written answers (N = 6) from virtual students regarding their scientific correctness and regarding students’ misconceptions about the natural selection of the peppered moth. Second, to test for judgment accuracy, the trainee teachers received—via the SI—achievement-irrelevant information about each virtual student, that is, the previous result of a multiple-choice questionnaire about evolution, before diagnosing the written answers.ResultsThe trainee teachers were able to distinguish between scientifically correct (90.8%) and scientifically incorrect (91.7%) written answers. Trainee teachers faced problems when diagnosing specific misconceptions categories. Anthropomorphic misconceptions were diagnosed significantly more often (61.1%) than teleological misconceptions (27.8%). The achievement-irrelevant information influenced the trainee teachers’ assessment of written answers (F [1,26] = 5.94, p < .022, η2 = .186) as they scored the written answers higher if the performance in the questionnaire was good and vice versa.ConclusionThe findings indicate that the diagnosis is easier or more difficult depending on the particular misconception category. However, the findings also reveal that, besides the evolution-specific facets of professional knowledge, generic facets interrelate with the quality of the diagnosis result. We conclude from these findings that an integration of evolution-specific and generic knowledge into the education of biology teachers is critical.