The consideration of threshold concepts is offered in the context of biological education as a theoretical framework that may have utility in the teaching and learning of biology at all levels. Threshold concepts may provide a mechanism to explain the observed punctuated nature of conceptual change. This perspective raises the profile of periods of conceptual stasis from "failure to learn" to necessary periods within the normal pattern of student learning. Threshold concepts are seen as points where segmental/linear knowledge structures and cumulative/hierarchical knowledge structures are integrated, resulting in a transformation in understanding. "Evolution" and "dynamic transformation" are nominated as candidate threshold concepts within biology. (Contains 2 figures.)