The Model Organism

The widely accepted view is that scientific models are used to represent parts of the world. Models help us reason about the parts of the world that they represent. Any part of the world that we’re concerned with is known as the model’s target system, original, prototype or parent system. The term prototype is most commonly used in engineering sciences. Prototype refers to an operating version of a solution. For example, an engine is an instance and material embodiment of one solution type that is possible within the domain of physics (a particular computer program is also one possible solution within the domain of computation). In the biological sciences, one can speak of model organisms, such as the common fruit fly. In such instances, the fruit fly is classed as a model because it is a secondary independent system, independent from the target system. Two systems are independent if a change in one does not cause a change in the other.

Carrying out potentially dangerous or lethal experiments on humans is legally and morally prohibited, so fruit flies are used to stand in for us. Fruit flies, then, are the model organism while humans or some species-general biological phenomena are the target system–the very thing we want to know more about. The US National Institutes of Health recognises thirteen species as model organisms for research use in the biomedical sciences, including the fruit fly, nematode worm, zebrafish, thale cress, mouse, and rat. As genetic and physiological mechanisms are discovered in model organisms, scientists then transfer and impute these claims to humans, other animals with similar genetic and physiological make ups, and to phenomena, the objects of scientific inquiry. Exploiting similarity, scientists then make a move such as if x is in y and y is sufficiently similar to z, then x is probably in z. Of course, in order for a fruit fly to prove useful in any crucial way, it needs to be similar to us. Beyond heritage in the tree of life [3], one significant and useful similarity is in the fact that 60% of genes identified in diseases in humans have a parallel in the fruit fly [4]. Reciprocal genes further serve as explanation for the existence of system parallels–skeletal and muscular, excretory system, digestive tract, nervous system, circulatory system–between humans and the fly.

Model organisms are studied in order to generate data and theories, so that a broader range of complex biological phenomena can be understood. Unlike complex target systems, most model organisms have the benefit of being easy to breed and raise in large numbers in laboratories. In virtue of having less complexity, model organisms are subject to standardisation. The features of the world that we take them to represent–directed by our research question of interest, e.g whether we want to find out about their mechanism of flight or circulatory system–are less varied, as the standardisation process veers the model organisms away from “natural” or “wild” organisms. The aforementioned organisms are not the only ones to have been used, or even thought of as particularly well-suited, for biomedical research. A variety of historical discussions point to the use of the tobacco mosaic virus (the first known virus) in the study of RNA; the sea urchin for understanding development; the flatworm in inheritance studies; Chlamydomonas reinhardtii, a unicellular green algae, for the study of photosynthesis; slime mold (Dictyostelium discoideum) for the study of cellular differentiation and communication; Aplysia, the sea slug, for studies in neurobiology; the breast muscle of pigeons in order to understand oxidative metabolism, or how cells use oxygen to make energy; the dog to study physiology and blood transfusion; the rat for understanding nutrition, neurology and behavioural psychology; the mouse in physiology, immunology and oncology; the frog, its use dating back to Aristotle, for understanding muscle action and electricity, circulation and respiration; and the guinea pig for understanding anatomy, the development of germ theory, the discovery of vitamin C, and the development of diphtheria and cholera vaccines [6]. These are just some examples amongst many others [for more, see 7, 8, 9, 10].

In a 2018 experimental study of the fruit fly [11], scientists discovered that fat cells can move. In particular, they discovered that fat cells can migrate to wound sites for purposes of aiding in repair and the further prevention of infection. Curiously, fat cells migrate by exploiting a peristaltic mode of motion, or alternate contraction and relaxation motion. This same type of motion is exploited by the aesophagus, stomach and the intenstines in pushing food through the digestive system. Now, while countless models pervade science, the fruit fly as model is just one such example. It’s no exaggeration to say that models exist in every domain of scientific inquiry.