Evolutionary transitions and reversions in individuality

Biological individuality exists in different forms-unicellular, multicellular, colonial, etc.-which have arisen through evolutionary transitions in individuality. These involve bundling separate, lower-level individuals (particles) into higher-level ones (collectives) that transition to being individuals in their own right. What it is that "transitions" in the process is an open question inviting different answers, based on distinct conceptual accounts of biological individuality. We argue that evolutionary transitions in individuality always produce "paradigmatic" individuals, i.e., those identified as individuals under any concept. This renders distinguishing among individuality concepts moot, yielding two negative consequences. First, it has let evolutionary biologists refrain from declaring what kind of individuality they speak of in the evolutionary transitions in individuality, which has led them to talk past one another. Second, it has made them overlook the possibility that transitions may revert. Drawing on different individuality concepts, we identify two conceptually-different ways for collectives to "lose" their individuality. Paradigmatic reversions involve the complete undoing of a former evolutionary transition in individuality, e.g., a shift from multicellularity to unicellularity. Agential reversions do not involve such an organizational shift-a multicellular organism remains multicellular-but rather see the level at which selection and adaptation prevails change. Whereas paradigmatic reversions could also be caused by ecological shifts, agential reversions can occur only through internal conflict, where different particles within a collective have mutually-exclusive evolutionary interests. We conclude by discussing how reversions help create a more elaborate and accurate understanding of individuality and its evolution.