In philosophy, essentialism is a view that, for any specific unit of existence such as an animal, a group of people, a set of physical objects, a concept, etc., there is a set of characteristics which distinguishes the unit from other entities, and gives it a unique identity and function. The presence of common characteristics defines that object, unit or entity.
The concept of essentialism is found in the works of Plato and Aristotle. Some examples given in the article Unclassified projects bacterial behavior as part of essentialism. One common characteristic that is found only in bacteria is that they do not interbreed at all; instead they reproduce asexually- and also swap genes in cryptic ways. Another example of biological essentialism is that certain species of fishes do not produce males, only female fishes are found in that species.
Operationism is established when a concept that has manifestations in the form of observable actions, behaviors or operations (that can be seen and repeated by anyone). The concept of species mentioned in the article can be subjected to the application of operationism. One of the researcher points out that it is very difficult to establish the concept of species or to define it. He suggests it is futile to attempt to bring consensus on the meaning of species; instead, the scientists must use a pragmatic approach. The researchers can become more effective if they follow the path of operationism. For example, in the study of species, morphology might be useful for mammals but not for bacteria, and similarly, genetics suits well for bacteria but not for most fossils. Instead of searching for an absolute concept, the researchers must settle for what is observable and what is measurable.