The unfounded assumption that there must be an underlying reason or purpose for something. In other words: the conflation of purpose and effect.
For example, in this statement:
“The purpose of predators in an ecosystem is to control the populations of their prey and to cull out sick and weak animals.”
Nature is not a system in which some species have been assigned specific “purposes”. A better way to think about it is that over time some species have developed the ability to consume food in the form of other animals, which had the effect of establishing a balance between predators and prey. However, terms such as “intention”, “goal” or here: “purpose” imply that there is a deliberate act behind their role, which is of course not the case here.
The Greek word télos [τέλος] can be translated as “goal”, “aim” or “result”. It gives its name to teleology, a doctrine or school of thought, according to which everything that happens has a goal and therefore also a meaning. The most recognisable teleological question is that of the “meaning of life” – and it is certainly worthy of discussion whether this question is not from the outset based on the fallacy described in this article.
Evidently, not everything has a purpose. Much of what happens is simply random coincidence – such as radioactive decay, or a dice roll - but these events can still have life-changing and even history-changing effects: If you lose your possessions by gambling, for example, or if radioactive radiation causes a genetic mutation that then leads to cancer, the effects can go far beyond the actual random event.
Nevertheless, the fundamental idea that there must be a “meaning” or a “purpose” for everything is widespread. Even in scientific publications, this mindset is repeatedly incorporated, often unspoken and in the form of a choice of words that merely implies such “meaningfulness” (loaded language).
As in the initial example, in biology (or even more so: ecology), living organisms are often ascribed a peculiar “purpose” in their biotope community. However, this implies that there has been a planning intervention in the composition of the biotope, which is not the case in nature – unlike in e.g. an aquarium or vivarium, where the composition of the animal and plant world is (ideally) actually based on proper functional planning.
To avoid this fallacy, it helps to stay aware of the difference between “purpose” (also “meaning” or “function”) and “effect” (also “result”, “impact”, etc.), and to used them consciously when formulating statements such as this:
The effect (not: “purpose”!) of the presence of predators in a biotope is that the population of herbivores is limited by their hunting behaviour, and thus the plant resources are not over-exploited.
There are also many things that were indeed created of controlled with a purpose. To speak of them as having an “aim” or “meaning“ is justified, if there was indeed a willful planning intervention involved in creating them for the ascribed purpose.
“The purpose of the steering wheel on a car is to allow the driver to control the vehicle.”
In this case, the steering wheel was indeed created for a deliberate purpose and with a specific aim, and did not just appear “by chance” and then prove to be useful. The situation is similar with the aquariums/vivariums mentioned above. In these cases, someone has made a more or less conscious selection of species, which are to live together in their biotope. Ideally, these were selected so that they harmonise with each other and are ideally indeed supposed to take on certain functions in the community. Different to wild habitates, it makes sense to attribute a “purpose” to each species in this context.