Etymologically the term individual means that which is not divisible because if an organism is physically divided it ceases to be the organism. However, a problem with this definition is that there are colonial organisms that can be divided and live such as sponges. In addition, there are mutual symbionts such as lichens therefore some species do not fit this simple definition based on vertebrates.
To define an individual two problems must be tackled: identity and individuation. This is as to define an individual we must know both what makes it one thing and also what causes it to be different from others.
Peter Godfrey-Smith Professor of History and Philosophy of Science at the University of Sydney stated that the distinction between the growth and reproduction lies at the core of defining biological individuality. Thomas Henry Huxley believed defining an individual based on its capacity to self sustain would lead to contradictions. Therefore, in line with Smith’s ideas he singles out sexual reproduction as the criteria for a new individual. This is as he argues it separates growing larger via asexual reproduction from growing new by sexual reproduction. However, this raises an issue which can be demonstrated in the case of bacteria. Bacteria reproduce by asexual division, dividing to produce clones. This would mean that by Huxley’s definition an entire population would be considered an individual.
It could be considered that an individual is something which is contextual and that maybe there’s no simple way of picking out the true nature of biological individuals instead there may be many kinds of individual relevant to different purposes.
American philosopher David Hull disagreed with this and also Huxley. He saw no scientific need to link the concept of an individual to sexual reproduction. Hull instead argued that the theory of evolution by natural selection was the only theory that accounted for individuation. He believed that individuality comes from what is needed for selection. This is as individuals vary and it’s this variation that causes certain individuals to be fitter than others which drives evolution. He stated that these individuals can be thought of as ‘units of selection’ upon which natural selection operates. The selective pressure acting on a species explains why organisms are well adapted to their environment, form from unicellular eggs and why they function cohesively. These properties as touched on earlier can be used to define an individual. The definition of an individual should be based on the strongest theories and at present I believe that is natural selection.
Although as discussed here there are may complex cases in the argument for what truly defines an individual I believe that a fairly simple statement of ‘a single separate organism (animal or plant) distinguished from others of a same kind’ is mostly satisfactory. There are some unique cases such as the symbiosis of some bacteria with aphids neither of which would survive without the other due to years of dependency however in terms of the above definition they are two separate individuals. This negates the problem of stating an individual by its ability to self sustain which as Huxley said leads to absurdities and contradictions.
There are many unique cases which complicate the task of determining an exact definition of an individual based on organisms and therefore biological philosophers still ponder this topic. A view of biological individuals centered on organisms provides a useful way to recognize core structures to the world of heterogeneous biological individuals. Therefore the question of what exactly constitutes an individual is crucial to understanding life over time which is a keystone of geology.
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