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The term “organism” is commonly defined as a living individual, but defining “individual” in biology is challenging and leads to competing criteria.
An organism is generally defined as any living thing that functions as an individual. However, this definition is difficult to apply because the concept of an “individual” is itself not straightforward in biology. Several criteria have been proposed to distinguish organisms, such as autonomous reproduction, growth, and metabolism, or the idea that an organism cannot be divided without losing functionality. Yet many of these criteria face exceptions and lead to debate about whether a single, clear definition is possible or even necessary. Boundary cases highlight the problem. Viruses are often excluded because they typically lack autonomous reproduction, growth, metabolism, and homeostasis, even though they evolve and replicate using host machinery. Colonial and partnership-like entities (e.g., eusocial insect colonies, lichens, and siphonophores) can meet some organism-like criteria such as coordinated function and specialization, but they blur the line between a single organism and a group of cooperating units. Some researchers propose that “organismality” evolves socially through cooperation and suggest cooperation as a defining trait, which would allow certain partnerships and colonies to be treated as organisms at different levels of biological organization.
The term “organism” is commonly defined as a living individual, but defining “individual” in biology is challenging and leads to competing criteria.
Proposed organism criteria (e.g., autonomous reproduction, growth, metabolism, non-divisibility) do not cleanly resolve boundary cases like viruses and cooperative colonies/partnerships.
Organisms can be understood at multiple levels (unicellular, multicellular, colonial, superorganism), and some theories define organismality through degrees of cooperation and conflict.
A living entity that functions as an individual, though the exact criteria for individuality are debated.
The ability to reproduce without relying on another organism’s machinery.
A proposed criterion stating that dividing a system would cause loss of its functional organization.
The set of qualities or attributes used to determine whether an entity counts as an organism, sometimes framed in terms of cooperation.
A colony (such as an ant colony) that functions as a single integrated social unit made of many individuals.
A biological entity often excluded from organisms because it lacks autonomous metabolism and reproduction, despite evolving and replicating via hosts.
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