A rich body of research has investigated students’ explanations of natural selection by focusing on a number of key concepts such as variation, differential survival, and change in population. Recent research has pointed out that other underlying abstract concepts (so-called threshold concepts) such as randomness, probability, spatial scale, and temporal scales are central for evolution understanding. However, no holistic account exists of students’ threshold concepts use in evolutionary explanations. This study therefore aims to explore how students use the abovementioned threshold concepts in their explanations of natural selection. We collected written answers to three natural selection items (i.e., evolution of 1) antibiotic-resistance in bacteria, 2) running speed in cheetahs, and 3) blindness in cave salamanders)  from 247 university students. To characterize and quantify the use of threshold concepts, content analysis and subsequent statistical analysis were carried out. Overall, students’ explanations of the three items differ in their expression of threshold concepts. Students most often phrased some kind of spatial scales but less often randomness or probability. A more-fine grained analysis indicated that randomness and spatial scale were most frequent in the bacteria item, while probability was most often expressed in the cheetah item. In contrast, temporal scale was used across all three items, although with rather unspecific expressions. Our work indicates that item context affects the expression and use of threshold concepts in evolutionary explanations.