Threshold concepts are fundamental to a discipline and, once understood, transform students’ understanding and perception of the subject. Despite the value of threshold concepts as a learning “portal” for heuristic purposes, there is limited empirical evidence of threshold crossing or achieving mastery. As a threshold concept, biological variation within species is fundamental to understanding evolution and provides a target for analyzing threshold crossing. We aimed to 1) examine student understanding of variation using four dimensions of a threshold concept (discursive, troublesome, liminal, and integrative), 2) measure “threshold crossing,” and 3) investigate the utility of the threshold concept framework to curriculum design. We conducted semistructured interviews of 29 students affiliated with a “variation-enriched” curriculum in a cross-sectional design with precurriculum, current, and postcurriculum groups (Pre, Current, and Post) and an outgroup of three postbaccalaureate advanced learners (Outgroup). Interview transcripts revealed that Current students expand their “variation discourse,” while the Post group and Outgroup displayed conformity in word choice about variation. The Post and Current groups displayed less troublesome and more integrative responses. Pre, Post, and Outgroup explanations’ revealed liminality, with discomfort and uncertainty regardless of accuracy. When we combined all four threshold concept dimensions for each respondent, patterns indicative of threshold crossing emerged along with new insight regarding curricular design.
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