Defining the Organism

Organisms are central to biological evolution. Organisms reproduce. Organisms have heritable traits that they pass on to their offspring. Organisms behave and can survive or die in the struggle for existence. Despite the central place of organisms in evolution, there is no agreed definition of the term. In practice, this may not be a serious problem. Usually it is clear whether something is an organism. A cow is an organism. So are plants and bacteria. But other examples are less straightforward. What about a lichen, for example? Or a virus? Would a lichen and a virus be organisms because they reproduce and evolve? And what about the habit of some people to distinguish between a single cell and a multicellular organism? To deal with the more difficult examples, and as a basis for a more detailed discussion of evolution, it helps to start with a definition of the term organism. Here we will build on previous work by the author.