We draw on insights from the philosophy of biology to show how individuals may fall apart in different ways, depending on how individuality becomes compromised. Different types of individuality in biology have been shaped by evolutionary transitions in individuality [2/8]

e.g. from unicellular to multicellular organisms. But what is that “transitions” in the process? This can be answered in different ways, drawing on different individuality concepts. As it turns out, these transitions actually involve changes along all such axes of individuality! [3/8]

So, we dub the product of an evolutionary transition in individuality a “paradigmatic” individual, something that ticks all the boxes, leaving no reason to worry about what an individual is, exactly. However, this ignores the possibility for specific components of individuality to be eroded. [4/8]

Such erosion may bring about an evolutionary reversion, effectively undoing an evolutionary transition in individuality, if only in part. We identify the complete dissolution of the “collective” individual, e.g. from multicellular to unicellular organisms, as the “paradigmatic” case. [5/8]

This involves loss of all physiological integration, collective fitness optimization, etc., basically every aspect of individuality is reverted. We also argue that reversions may happen in part, eroding the capacity for “collectives” to function as adaptive evolutionary agents. [6/8]

We dub these “agential” reversions, and identify two subtypes of reversions that may occur. Agential reversions are uniquely caused by internal conflicts, where parts of an individual (genes, cells, members of a colony) become embroiled in conflict with each other over the course of evolution. [7/8]