A life-cycle is a temporally organised trajectory of characteristic biological transformations through which a living system passes during its existence.
In conventional biology, life-cycles are often represented as ordered sequences of stages such as birth, growth, reproduction, and death. APS treats such stages as useful descriptions where appropriate, while emphasising that the processes connecting them may be gradual, overlapping, context-dependent, or dynamically regulated.
A life-cycle is therefore more than elapsed time or a list of successive states. It identifies an organised temporal pattern in the development and transformation of a biological system.
Depending on the organism and explanatory target, a life-cycle may include:
- developmental transitions,
- metamorphosis,
- physiological change,
- behavioural reorganisation,
- ecological role shifts,
- reproductive transitions,
- ageing,
- dormancy,
- migration,
- and senescence.
Not every life-cycle contains all of these processes, and their biological significance must be established for the case concerned.
Life-cycles can be relevant to organised persistence because a living system may remain organisationally continuous while undergoing substantial transformation. This relation is not built into the definition of a life-cycle, however. A life-cycle describes the temporally organised trajectory; whether particular processes within that trajectory contribute to viability, persistence, disruption, or loss is a further explanatory question.
The spatial and temporal extent required for a life-cycle explanation is likewise target-dependent. Some explananda concern the trajectory of an organism as a whole, while others may require attention to cellular, physiological, ecological, social, or other relations that materially affect that trajectory. APS does not therefore infer a hierarchy of nested life-cycles merely from the presence of processes operating over different extents.
APS also distinguishes life-cycles from simple stage models. Stage descriptions divide a trajectory into analytically useful phases, whereas the underlying biological processes may cross or blur those boundaries. Whether a stage model or a more continuous process description is appropriate depends on the biological case and explanatory purpose.
Life-cycles also differ from mere temporal duration. Duration records how long something exists; a life-cycle characterises an organised pattern of biological transformation through time.
Reproduction may form an important part of a life-cycle, but its explanatory role must be specified. At the organismic scale, reproduction may constitute a characteristic transition or activity within the trajectory. Relations among reproduction, inheritance, lineage continuity, population change, and evolution concern further explananda and should not be inferred merely from reproduction’s position within a life-cycle.
A life-cycle therefore provides a way of describing and explaining temporally organised biological transformation without assuming that every transition preserves viability, that biological identity is constituted by the life-cycle itself, or that organismic, intergenerational, and evolutionary continuity are the same explanatory relation.