APS Box — What Is a Biological Individual?

A biological individual is not always synonymous with a physically separate organism.

Biologists have proposed many criteria for identifying individuals, including physiological integration, developmental coordination, reproductive continuity, evolutionary persistence, and behavioural organisation. Different criteria often identify different entities as biologically significant.

This creates difficult cases. Is a lichen one individual or several? Is a coral colony an organism or a collection of organisms? Should a host and its microbiome be treated separately or as an integrated system?

Rather than revealing a failure of biology, these cases show that individuality itself can be context-dependent. Biological individuals are often best understood as organised systems whose components function together in ways that sustain a coherent pattern of persistence and activity.