Defining organisms and boundary cases

Meaning of organism and its definition

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.

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Etymology of the term organism

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